Sunday, November 17, 2013

Jackson 2.3.0 released -- quick feature overview

Now that Jackson 2.3.0 is finally finalized and released (official release date 14th November, 2013), it is time for a quick sampling of new features. Note that this is a very limited sampling -- across all core components and modules, there are close to 100 closed features; some fixes, but most improvements of some kind.

So here's my list of 6 notable features.

1. JsonPointer support for Tree Model

One of most often asked features for Jackson has been ability to support a path language to traverse JSON. So with 2.3 we chose the simplest standardized alternative, JSON Pointer (version 3), and made Tree Model (JsonNode) allow navigating using it.

Usage is simple: for JSON like

"address" : { "street" : "2940 5th Ave", "zip" : 980021 }, "dimensions" : [ 10.0, 20.0, 15.0 ]

you could use expressions like:

JsonNode root = mapper.readTree(src);
int zip =root. at("/address/zipcode").asIntValue();
double height = root.add("/dimensions/1").asDoubleValue(); // assuming it's the second number in there

Also note that you can pre-compile JSON Pointer expressions with "JsonPointer.compile(...)", instead of passing Strings; however, pointer expressions are not particularly expensive to tokenize. JsonPointer instances also have full serialization and deserialization support, so you can conveniently use them as part of configuration data for things like, say, DropWizard Configuration objects.

2. @JsonFilter improvements

With earlier versions, it was only possible to define ids of filters to apply using @JsonFilter on classes. With 2.3.0 you can apply this annotation (as well as @JsonView) on properties as well, to use different filters for different instances of same class:

public class POJO {
@JsonFilter("filterA") public Value value1;
@JsonFilter("filterB")public Value value2;

// similarly with JsonView (was added in 2.2)
@JsonView(AlternateView.class) public AnotherValue property;

But this is not all! Applicability of JSON Filters is also expanded, so that in addition to regular POJOs and their properties, it will now apply to:

  1. Any getters (they will get filtered just like regular properties)
  2. Java Maps

and in future we may also add ability to filter out List or array elements; this will be possible since the filtering interfaces were extended allow alternate calls that specify value index, instead of property name.

3. Uniqueness checks

Although JSON specification does not mandate enforcing uniqueness of Object property names (and use of databinder on serialization should prevent generation of duplicates), there are situations where one would want to be extra careful, and make parser check uniqueness.

With 2.3.0, there are two new features you can turn on to cause an exception to be thrown if duplicate property names are encountered:

  1. DeserializationFeature.FAIL_ON_READING_DUP_TREE_KEY: when reading JSON Trees (JsonNode), and encountering a duplicate name, throw a JsonMappingException
  2. JsonParser.Feature.STRICT_DUPLICATE_DETECTION and JsonGenerator.Feature.STRICT_DUPLICATE_DETECTION: when reading or writing JSON using Streaming API (either directly, or for data-binding or building/serializing Tree instances), duplicates will be reported by a JsonParsingException

The main difference (beyond applicability; first feature only affects cases of building a JsonNode out of JSON input) is that duplicate detection at Streaming API level does incur some overhead (up to 30-40% more time spent); whereas duplicate detection at Tree Model level has little if any overhead. Difference is due to additional storage and checking requirements, as Streaming API does not need to keep track of set of property names encountered unless checking is required, whereas Tree Model will have to keep track of properties anyway. As a consequence, tree level checks are basically close to free to add.

4, Object Id handling

There are two improvements to Object Identity handling.

First, by enabling, SerializationFeature.USE_EQUALITY_FOR_OBJECT_ID you can loosen checks so that all values that are deemed equal (by calling Object.equals()) are considered "same object"; basically this allows canonicalization of objects. It mostly makes sense when using ORM libraries, or other data sources that may not use exact object instances; or if you want to reconstruct shared references from sources that do not support it.

Second, when using YAML data format module, all Object Id references are now written using YAML native constructs called anchors, and references handled as native references. Same is also true for Type Ids; YAML module will now use tags for this purpose.
This change should not onlt make result more compact and "YAML-looking", but should also improve interoperability with native YAML tools. Latter should be most useful for a common use case for YAML, that of configuration files, where you can more easily share common configuration blocks with anchors and references.

5. Contextual Attributes for custom (de)serialization

One thing that has been missing so far from both SerializaterProvider and DeserializationContext objects (both of which extends DatabindContext base class) has been ability to assign and access temporary values during serialization/deserialization. In absence of something like this, custom JsonSerializer and JsonDeserializer implementations have had to use ThreadLocal to retain such values, adding more complexity.

But not any more: starting with 2.3.0, there is concept of "databind attributes" (similar to, say, Servlet attributes), managed and accessed using two simple methods -- getAttribute(Object key) and setAttribute(Object key, Object value). These can be used for keeping track of per-call (serialization or deserialization) state, pass data between (de)serializers and so on. All values are cleared when context is created (for a readValue() or writeValue() call); and similarly values are cleared at the end, so that no explicit clean up is required.

This may not sound like a huge feature in itself, but it actually opens up interesting possibilities for future: specifically, it may make sense to add new "standard attributes" that are set by databind module itself, when specific feature is enabled. For example, perhaps it would make sense to keep track of POJO that is being currently serialized/deserialized, to be accessible to actual (de)serializers.

6. Null handling for serialization

Another relative small, but often requested feature is ability to control how Java nulls are serialized, using more granular control than global rules like "all nulls to be serialized as empty Strings" (which is possible to do already). This is supported by adding a new property for @JsonSerialize annotation:

public class POJO {
public Value value;

in which case instance of MyNullSerializer would be used to write a JSON value for property "value", if POJO property has value null.

7. Other misc improvements

JAX-RS module has a new mechanisms for fully customizing ObjectReader and ObjectWriter instances, above and beyond what module itself can do. You can find more details on Issue#33.

XML module has been improved as well; it is finally possible to properly serialize root-level array and java.util.Collection values. ObjectMapper.convertValue() now works properly.

CSV module now supports filtering using JSON Views and JSON Filters; this was not working correctly with earlier versions.

Monday, October 28, 2013

On Jackson: Serializing Lists, Maps with properties

1. Problem

One of questions occasionally brought up is why Jackson ignores any properties that a java.util.List or java.util.Map may have. For example, for:

public class StringList extends ArrayList<String> {
public int id;

the serialized version will just be a JSON array, without property "id". This is also true for Java Maps: only contents are serialized, not properties. This is because serializers (and deserializers) have special handling; and in case of Lists, there isn't even any place to add properties in.

2. Solution

But is there any way to add support for additional properties, without resorting to custom serializers and deserializers?

Yes! With Jackson 2.1 and above there is a new annotation, @JsonFormat, and specifically its "shape" property.

Like so:

public class StringListexten ds ArrayList<String> {
public int id;

public Iterator<String> getValues() { return iterator(); }
public void setValues(Collection<String> v) { addAll(v); }

What does this do? It will basically instruct Jackson to forget about special handling for Collections and Maps, and instead consider them POJOs. This means that introspection is used to find logical properties to (de)serialize.

One caveat: since there is no special handling for contents, you HAVE to add getter/setter to actually include contents.

3. Output

For type like above, you would get something like:

{ "id":123, "values":["abc","xyz"]}

Similarly for Maps, you would get properties at one JSON Object level, and values (if included) as a JSON Object-valued property:

{ "id" : 345,
"value" : {
"key2":"value2" }

Or, for extra credit, you can even try using "any setter" and "any getter", to create a Map with flat set of values -- this works, since Map is just like any POJO from Jackson's point of view now.

Tuesday, September 10, 2013

Java Classmate 0.9 released -- getting ready for 1.0

UPDATE: (October 2013) -- Version 1.0 is now out!


Getting close to Java Classmate 1.0: version 0.9 now out!

Java Classmate is a highly specialized library that can be used to properly and completely resolve type information about Java generic types used in field and method signatures. This is information that is not, oddly enough, easily accessible via JDK -- if you try figure out, for example, what is actual return type for method '' below:

  public class MyStringKeyMap<V> extends Map<String,V> { }
  public class Processor<T> {
    public T foo();
  public class IntegerProcessor extends Processor<MyStringKeyMap<Integer>> { }

you are in for quite a ride. As a programmer, you should be able to figure out that it is equivalent to Map<String,Integer>. But dynamically, from your code, how would you figure it out? I won't bother you with all the complications: for more background feel free to read "Why 'java.lang.reflect.Type' does not cut it".
And my suggested solution is Java Classmate, as per "Use Java Classmate for resolving generic signatures".

Last change to test

Java Classmate library has been used by quite a few frameworks, such as Hibernate and JBoss, and considered for inclusion by others (Netty uses code adopted from JCM, to avoid adding external dependencies; Jackson uses "predecessor", and this is code from which JVM was originally built). There hasn't been much activity for past 12 months; no new bug reports, and things appear to Just Work, which is great.

So at this point version 0.9 is released, and I am hoping to get some last piece of feedback before releasing the "official" 1.0 version. Please let me know of any issues; the best way is via project's Github issue tracker.

Tuesday, August 13, 2013

On Jackson 2.2

Here's another thing I need to write about, from my "todo bloklog" (blog backlog): overview of Jackson 2.2 release.
As usual, official 2.2 release notes are worth checking out for more detailed listing.

1. Overview

Jackson 2.2.0 was released in April, 2013 -- that is, four months ago -- and the latest patch version currently available is 2.2.2. It has proven a nice, stable release, and is currently used by frameworks such as DropWizard (my current favorite Java service platform). This is also the current stable version, as the development for 2.3 is not yet complete.

As opposed to earlier releases (2.0 major, 2.1 minor) which overflowed with new functionality, focus with 2.2 was to really stabilize functionality and close as many open bugs as possible; especially ones related to new 2.x functionality.
Related to this, we wanted to improve parity (coverage of features to different parts; that is, that both serialization and deserialization support things; that Map/Node/POJO handling would be as similar as possible).

2. Enhancements to serializer, deserializer processing

One problem area, with respect to writing custom handlers for structured non-POJO types (esp. container types: arrays, Collections, Maps), was that BeanSerializerModifier and BeanDeserializerModifier handlers could only be used for POJO types.

But custom handling is needed for container types too; and especially so when adding support for third-party libraries like Trove, Guava and HPPC. 2.2 extended these interfaces to allow post-processing serializers/deserializers for all types (also including scalar types).

Ability to post-process (de)serializers of all types should reduce the need for writing custom (de)serializers from scratch: it is possible -- for example -- to take the default (de)serializer, and use post-processor to create (de)serializer that delegates to the standard version for certain cases, or for certain part of processing.

3. Converters

The biggest new feature was adding annotation-based support for adding things called "Converters". It can be seen as sort of further extension for the idea that one should be able to refine handling with small(er) component, instead of having to write custom handlers from scratch.

The basic idea is simple: to serialize custom types (that Jackson would not know how to handle correctly) one can write converters that know how to take a custom type and convert it into an intermediate object that Jackson already knows how to serialize. This intermediate form could be simple java.util.Map or JsonNode (tree model), or even just another more traditional POJO.

And for deserialization, do the reverse: let Jackson deserialize JSON into this intermediate type; and call converter to get to the custom type.

Typically you will write one or two converters (one if you just need converter for either serialization or deserialization; two if both); and then either annotate the type that needs converter(s); or property of that type that needs converter(s):

public class Point {
private int x, y;
public MyPoint(int x, int y) {
this.x = x;
this.y = y;
public int x() { return x; }
public int y() { return y; }

class SerializationConverter extends StdConverter<ConvertingBean, int[]> {
public int[] convert(MyPoint p) {
return new int[] { p.x(), p.y() };
// similarly for DeserializationConverter: StdConverter is convenient base class

This feature was inspired by similar concept in JAXB, and should have been included a long time ago (actually, 2.1 already added internal support for converters; 2.2 just added annotation and connected the dots).
One thing worth noting regarding above is that use of StdConverter is strongly recommended; although you may directly implement Converter there is usually little need. Also note that although example associated converters directly with the type, you can also add them to property definition; this can be useful when dealing with third-party types (although you can also use mix-in annotations for those cases).

4. Android

One "unusual" area for improvements was work to try to make Jackson run better on Android platform. Android has its set of quirks; and although Jackson was already working well from functionality perspective, there were obvious performance problems. This was especially true for data-binding, where initial startup overhead has been problematic.

One simple improvement was elimination of file VERSION.txt. While it seemed harmless enough thing for J2SE, Android's package loader has surprising overhead when loading resources from within a jar -- at least on some versions, contents of jar are retained in memory basically DOUBLING amount of memory needed. 2.2 replaced text-file based version discovery with simple class generation (as part of build, that is, static source generation).

Version 2.2 also contained significant amount of internal refactorings, to try to reduce startup overhead, by both simplifying set up of (de)serializers, and to try to improve lazy-loading aspects.
One challenge, however, is that we still do not have a good set of benchmarks to actually verify effects of these changes. So while the intent was to improve startup performance, we do not have solid numbers to report, yet.

On plus side, there is some on-going work to do more performance measurements; and I hope to write more about these efforts once related work is made public (it is not yet; I am not driving these efforts, but have helped).

5. JAX-RS: additional caching

Another area of performance improvements was that of JAX-RS provider. Earlier versions did reuse internal `ObjectMapper`, but had to do more per-call annotation processing. 2.2 added simple caching of results of annotation introspection, and should help reduce overhead.

One other important change was structural: before 2.2, there were multiple separate github projects (three; one for JSON, another for Smile, third for XML). With 2.2 we now have a single Github project, jackson-jaxrs-providers, with multiple Maven sub-projects that share code via a base package. This should simplify development, and reduce likelihood of getting cut'n paste errors.

6. AfterBurner becomes Production Ready

One more big milestone concerned Afterburner module (what is it? Check out this earlier entry). With a little help from my friends (special thanks to Steven Schlansker for his significant contributions!), all known issues were addressed and new checks added, such that we can now consider Afterburner production ready.

Given that use of Afterburner can give you 30-50% boost in throughput, when using data-binding, it might be good time to check it out.

Thursday, August 08, 2013

Brief History of Jackson the JSON processor

(Disclaimer: this article talks about Jackson JSON processor -- not other Jacksons, like American cities or presidents -- those others can be found from Wikipedia)

0. Background

It occurred to me that although it is almost six years since I released the first public version of Jackson, I have not actually written much about events surrounding Jackson development -- I have written about its features, usage, and other important things. But not that much about how it came about.

Since still remember fairly well how things worked out, and have secondary archives (like this blog, Maven/SVN/Github repositories) available for fact-checking the timeline, it seems like high time to write a short(ish) historical document on the most popular OSS project I have authored.

1. Beginning: first there was Streaming

Sometime in early 2007, I was working at, and had successfully used XML as the underying data format for couple of web services. This was partly due to having written Woodstox, a high-performance Java XML parser. I was actually relatively content with the way things worked with XML, and had learnt to appreciate benefits of open, standard, text-based data format (including developer-debuggability, interoperability and -- when done properly -- even simplicity).
But I had also been bitten a few times by XML data-binding solutions like JAXB; and was frustrated both by complexities of some tools, and by direction that XML-centric developers were taking, focusing unnecessarily in the format (XML) itself, instead of how to solve actual development problems.

So when I happened to read about JSON data format, I immediately saw potential benefits: the main one being that since it was a Data Format -- and not a (Textual) Markup Format (like XML) -- it should be much easier to convert between JSON and (Java) objects. And if that was simpler, perhaps tools could actually do more; offer more intuitive and powerful functionality, instead of fighting with complex monsters like XML Schema or (heaven forbid) lead devs to XSLT.
Other features of JSON that were claimed as benefits, like slightly more compact size (marginally so), or better readabilty (subjective) I didn't really consider particularly impresive.
Beyond appreciating good fit of JSON for web service use case, I figured that writing a simple streaming tokenizer and generator should be easy: after all, I had spent lots of time writing low-level components necessary for tokenizing content (I started writing Woodstox in late 2003, around time Stax API was finalized)

Turns out I was right: I got a streaming parser working and in about two weeks (and generator in less than a week). In a month I had things working well enough that the library could be used for something. And then it was ready to be released ("release early, release often"); and rest is history, as they say.

Another reason for writing Jackson, which I have occasionally mentioned, was what I saw as a sorry state of JSON tools -- my personal pet peeve was use of org.json's reference implementation. While it was fine as a proof-of-concept, I consider(ed) it a toy library, too simplistic, underpowered thing for "real" work. Other alternatives just seemed to short-change one aspect or another: I was especially surprised to find total lack of modularity (streaming vs higher levels) and scant support for true data-binding -- solutions tended to either assume unusual conventions or require lots of seemingly unnecessary code to be written. If I am to write code, I'd rather do it via efficient streaming interface; or if not, get a powerful and convenient data-binding. Not a half-assed XML-influenced tree model, which was en vogue (and sadly, often still is).

And the last thing regarding ancient history: the name. I actually do not remember story behind it -- obviously it is a play on JSON. And I vaguely recall toying with the idea of calling library "Jason", but deciding that might sound too creepy (I knew a few Jasons, and didn't want confusion). Compared to Woodstox -- where I actually remember that my friend Kirk P gave the idea (related to Snoopy's friend, bird named Woodstock!) -- I actually don't really know who to give credit to the idea, or inspiration to it.

2. With a FAST Streaming library...

Having written (and quickly published in August 2007) streaming-only version of Jackson, I spent some time optimizing and measuring things, as well as writing some code to see how convenient library is to use. But my initial thinking was to wrap things up relatively soon, and "let Someone Else write the Important Pieces". And by "important pieces" I mostly meant a data-binding layer; something like what JAXB and XMLBeans are to XML Streaming components (SAX/Stax).

The main reasons for my hesitation were two-fold: I thought that

  1. writing a data-binding library will be lots of work, even if JSON lends itself much more easily to doing that; and
  2. to do binding efficiently, I would have to use code-generation; Reflection API was "known" to be unbearably slow

Turns out that I was 50% right: data-binding has consumed vast majority of time I have spent with Jackson. But I was largely wrong with respect to Reflection. But more on that in a bit.

In short term (during summer and autumn of 2008) I did write "simple" data-binding, to bind Java Lists and Maps to/from token streams; and I also wrote a simple Tree Model, latter of which has been rewritten since then.

3. ... but No One Built It, So I did

Jackson the library did get relatively high level of publicity from early on. This was mostly due to my earlier work on Woodstox, and its adoption by all major second-generation Java SOAP stacks (CXF nee XFire; Axis 2). Given my reputation for producing fast parsers, generators, there was interest in using what I had written for JSON. But early adopters used things as is; and no one did (to my knowledge) try to build higher-level abstractions that I eagerly wanted to be written.

But that alone might not have been enough to push me to try my luck writing data-binding. What was needed was a development that made me irritated enough to dive in deep... and sure enough, something did emerge.

So what was the trigger? It was the idea of using XML APIs to process JSON (that is, use adapters to expose JSON content as if it was XML). While most developers who wrote such tools consider this to be a stop-gap solution to ease transition, many developers did not seem to know this.
I thought (and still think) that this is an OBVIOUSLY bad idea; and initially did not spend much time refuting merits of the idea -- why bother, as anyone should see the problem? I assumed that any sane Java developer would obviously see that "Format Impedance" -- difference between JSON's Object (or Frame) structure and XML Hierarchic model -- is a major obstacle, and would render use of JSON even MORE CUMBERSOME than using XML.

And yet I saw people suggesting use of tools like Jettison (JSON via Stax API), even integrating this into otherwise good frameworks (JAX-RS like Jersey). Madness!

Given that developers appeared intent ruining the good thing, I figured I need to show the Better Way; just talking about that would not be enough.
So, late in 2008, around time I moved on from Amazon, I started working on a first-class Java/JSON data-binding solution. This can be thought of as "real" start of Jackson as we know it today; bit over one year after the first release.

4. Start data-binding by writing Serialization side

The first Jackson version to contain real data-binding was 0.9.5, released December of 2008. Realizing that this was going to be a big undertaking, I first focused on simpler problem of serializing POJOs as JSON (that is, taking values of Java objects, writing equivalent JSON output).
Also, to make it likely that I actually complete the task, I decided to simply use Reflection "at first"; performance should really matter only once thing actually works. Besides, this way I would have some idea as to magnitude of the overhead: having written a fair bit of manual JSON handling code, it would be easy to compare performance of hand-written, and fully automated data-binder.

I think serializer took about a month to work to some degree, and a week or two to weed out bugs. The biggest surprise to me was that Reflection overhead actually was NOT all that big -- it seemed to add maybe 30-40% time; some of which might be due to other overhead beside Reflection access (Reflection is just used for dynamically calling get-methods or accessing field values). This was such a non-issue for the longest time, that it took multiple years for me to go back to the idea of generating accessor code (for curious, Afterburner Module is the extension that finally does this).

My decision to start with Serialization (without considering the other direction, deserialization) was good one for the project, I believe, but it did have one longer-term downside: much of the code between two parts was disjoint. Partly this was due to my then view that there are many use cases where only one side is needed -- for example, Java service only every writing JSON output, but not necessarily reading (simple query parameters and URL path go a long way). But big part was that I did not want to slow down writing of serialization by having to also consider challenges in deserialization.
And finally, I had some bad memories from JAXB, where requirements to have both getters AND setters was occasionally a pain-in-the-buttocks, for write-only use cases. I did not want to repeat mistakes of others.

Perhaps the biggest practical result of almost complete isolation between serialization and deserialization side was that sometimes annotations needed to be added in multiple places; like indicating both setter and getter what the JSON property name should be. Over time I realized that this was not a good things; but the problem itself was only resolved in Jackson 1.9, much later.

5. And wrap it up with Deserialization

After serialization (and resulting 0.9.5) release, I continued work with deserialization, and perhaps surprisingly finished it slightly faster than serialization. Or perhaps it is not that surprising; even without working on deserialization concepts earlier, I had nonetheless tackled many of issues I would need to solve, including that of using Reflection efficiently and conveniently; and that of resolving generic types (which is a hideously tricky problem in Java, as readers of my blog should know by now).

Result of this was 0.9.6 release in January 2009.

6. And then on to Writing Documentation

After managing to get the first fully functional version of data-binding available, I realized that the next blocker would be lack of documentation. So far I had blogged occasionally about Jackson usage; but for the most part I had relied on resourcefulness of the early adopters, those hard-working hardy pioneers of development. But if Jackson was to become the King of JSON on Java platform, I would need to do more for it users.

Looking blog at my blog archive I can see that some of the most important and most read articles on the site are from January of 2009. Beyond the obvious introductions to various operating modes (like "Method 2, Data Binding"), I am especially proud of "There are Three Ways to Process Json!" -- an article that I think is still relevant. And something I wish every Java JSON developer would read, even if they didn't necessarily agree with all of it. I am surprised how many developers blindly assume that one particular view -- often the Tree Model -- is the only mode in existence.

7. Trailblazing: finally getting to add Advanced Features

Up until version 1.0 (released May 2009), I don't consider my work to be particularly new or innovative: I was using good ideas from past implementations and my experience in building better parsers, generators, tree models and data binders. I felt Jackson was ahead of competition in both XML and JSON space; but perhaps the only truly advanced thing was that of generic type resolution, and even there, I had more to learn yet (eventually I wrote Java ClassMate, which I consider the first Java library to actually get generic type resolution right -- more so than Jackson itself).

This lack of truly new, advanced (from my point of view) features was mostly since there was so much to do, all the foundational code, implementing all basic and intermediate things that were (or should have been) expected from a Java data-binding library. I did have ideas, but in many cases had postponed those until I felt I had time to spare on "nice-to-have" things, or features that were more speculative and might not even work; either functionally, or with respect to developers finding them useful.

So at this point, I figured I would have the luxury of aiming higher; not just making a bit Better Mousetrap, but something that is... Something Else altogether. And with following 1.x versions, I started implementing things that I consider somewhat advanced, pushing the envelope a bit. I could talk or write for hours on various features; what follows is just a sampling. For slightly longer take, read my earlier "7 Killer Features of Jackson".

7.1 Support for JAXB annotations

With Jackson 1.1, I also started considering interoperability. And although I thought that compatibility with XML is a Bad Idea, when done at API level, I thought that certain aspects could be useful: specifically, ability to use (a subset of) JAXB annotations for customizing data-binding.

Since I did not think that JAXB annotations could suffice alone to cover all configuration needs, I had to figure a way for JAXB and Jackson annotations to co-exist. The result is concept of "Annotation Introspector", and it is something I am actually proud of: even if supporting JAXB annotations has been lots of work, and caused various frustrations (mostly as JAXB is XML-specific, and some concepts do not translate well), I think the mechanism used for isolating annotation access from rest of the code has worked very well. It is one area that I managed to design right the first time.

It is also worth mentioning that beyond ability to use alternative "annotation sets", Jackson's annotation handling logic has always been relatively advanced: for example, whereas standard JDK annotation handling does not support overriding (that is; annotations are not "inherited" from overridden methods), Jackson supports inheritance of Class, Method and even Constructor annotations. This has proven like a good decision, even if implementing it for 1.0 was lots of work.

7.2 Mix-in annotations

One of challenges with Java Annotations is the fact that one has to be able to modify classes that are annotated. Beyond requiring actual access to sources, this can also add unnecessary and unwanted dependencies from value classes to annotations; and in case of Jackson, these dependencies are in wrong direction, from design perspective.

But what if one could just loosely associate annotations, instead of having to forcible add them in classes? This was the thought exercise I had; and led to what I think was the first implementation in Java of "mix-in annotations". I am happy that 4 years since introduction (they were added in Jackson 1.2), mix-in annotations are one of most loved Jackson features; and something that I still consider innovative.

7.3 Polymorphic type support

One feature that I was hoping to avoid having to implement (kind of similar, in that sense, to data-binding itself) was support for one of core Object Serialization concepts (but not necessarily data-binding concept; data is not polymorphic, classes are): that of type metadata.
What I mean here is that given a single static (declared) type, one will still be able to deserialize instances of multiple types. The challenge is that when serializing things there is no problem -- type is available from instance being serialized -- but to deserialize properly, additional information is needed.

There are multiple problems in trying to support this with JSON: starting with obvious problem of JSON not having separation of data and metadata (with XML, for example, it is easy to "hide" metadata as attributes). But beyond this question, there are various alternatives for type identifiers (logical name or physical Java class?), as well as alternative inclusion mechanisms (additional property? What name? Or, use wrapper Array or Object).

I spent lots of time trying to figure out a system that would satisfy all the constraints I put; keep things easy to use, simple, and yet powerful and configurable enough.
It took multiple months to figure it all out; but in the end I was satisfied with my design. Polymorphic type handling was included in Jackson 1.5; less than one year after release of 1.0. And still most Java JSON libraries have no support at all for polymorphic types: or at most support fixed use of Java class name -- I know how much work it can be, but at least one could learn from existing implementations (which is more than I had)

7.4 No more monkey code -- Mr Bean can implement your classes

Of all the advanced features Jackson offers, this is my own personal favorite: and something I had actually hoped to tackle even before 1.0 release.

For full description, go ahead and read "Mr Bean aka Abstract Type Materialization"; but the basic idea is, once again, simple: why is it that even if you can define interface of your data type as a simple interface, you still need to write monkey to code around it? Other languages have solutions there; and some later Java Frameworks like Lombok have presented some alternatives. But I am still not aware of a general-purpose Java library for doing what Mr Bean does (NOTE: you CAN actually use Mr Bean outside of Jackson too!).

Mr Bean was included in Jackson 1.6 -- which was a release FULL of good, innovative new stuff. The reason it took such a long time for me to build was hesitation -- it is the first time I used Java bytecode generation. But after starting to write code I learnt that it was surprisingly easy to do; and I just wished I had started earlier.
Part of simplicity was due to the fact that literally the only thing to generate were accessors (setters and/or getters): everything else is handled by Jackson, by introspecting resulting class, without having to even know there is anything special about dynamically generated implementation class.

7.5 Binary JSON (Smile format)

Another important milestone with Jackson 1.6 was introduction of a (then-) new binary data format called Smile.

Smile was borne out of my frustration with all the hype surrounding Google's protobuf format: there was tons of hyperbole caused by the fact that Google was opening up the data format they were using internally. Protobuf itself is a simple and very reasonable binary data format, suitable for encoding datagrams used for RPC. I call it "best of 80s datagram technology"; not as an insult, but as a nod to maturity of the idea -- it is automating things that back in 80s (and perhaps earlier) were hand-coded whenever data communication was needed. Nothing wrong in there.

But my frustration had more to do with creeping aspects of pre-mature optimization; and the myopic view that binary formats were the only way to achieve acceptable performance for high-volume communication. I maintain that this is not true for general case.

At the same time, there are valid benefits from proper use of efficient binary encodings. And one approach that seemed attractive to me was that of using alternative physical encoding for representing existing logical data model. This idea is hardly new; and it had been demonstrated with XML, with BNUX, Fast Infoset and other approaches (all that predate later sad effort known as EXI). But so far this had not been tried with JSON -- sure, there is BSON, but it is not 1-to-1 mappable to JSON (despite what its name suggest), it is just another odd (and very verbose) binary format.
So I thought that I should be able to come up with a decent binary serialization format for JSON.

Timing for this effort was rather good, as I had joined Ning earlier that year, and had actual use case for Smile. At Ning Smile was dynamically used for some high-volume systems, such as log aggregation (think of systems like Kafka, Splunk). Smile turns out to work particularly well when coupled with ultra-fast compression like LZF (implemented at and for Ning as well!).

And beyond Ning, I had the fortune of working with creative genius(es) behind ElasticSearch; this was a match made in heaven, as they were just looking for an efficient binary format to complement their use of JSON as external data format.

And what about the name? I think I need to credit mr. Sunny Gleason on this; we brainstormed the idea, and it came about directly when we considered what "magic cookie" (first 4 bytes used to identify format) to use -- using a smiley seemed like a crazy enough idea to work. So Smile encoded data literally "Starts With a Smile!" (check it out!)

7.6 Modularity via Jackson Modules

One more major area of innovation with Jackson 1.x series was that of introduction of "Module" concept in Jackson 1.7. From design/architectural perspective, it is the most important change during Jackson development.

The background to modules was my realization that I neither can nor want to be the person trying to provide Jackson support for all useful Java libraries; for datatypes like Joda, or Collection types of Guava. But neither should users be left on their own, to have to write handlers for things that do not (and often, can not) work out of the box.

But if not me or users, who would do it? The answer of "someone else" does not sound great, until you actually think about it a bit. While I think that the ideal case is that the library maintainers (of Joda, Guava, etc) would do it, I think that the most likely case is that "someone with an itch" -- developer who happens to need JSON serialization of, say, Joda datetime types -- is the person who can add this support. The challenge, then, is that of co-operation: how could this work be turned to something reusable, modular... something that could essentially be released as a "mini-library" of its own?

This is where the simple interface known as Module comes in: it is simply just a way to package necessary implementations of Jackson handlers (serializers, deserializers, other components they rely on for interfacing with Jackson), and to register them with Jackson, without Jackson having any a priori knowledge of the extension in question. You can think them of Jackson equivalent of plug-ins.

8. Jackson 2.x

Although there were more 1.x releases after 1.6, all introducing important and interesting new features, focus during those releases started to move towards bigger challenges regarding development. It was also challenging to try to keep things backwards-compatible, as some earlier API design (and occasionally implementation) decisions proved to be sub-optimal. With this in mind, I started thinking about possibility of making bigger change, making a major, somewhat backwards-incompatible change.

The idea of 2.0 started maturing at around time of releasing Jackson 1.8; and so version 1.9 was designed with upcoming "bigger change" in mind. It turns out that future-proofing is hard, and I don't know how much all the planning helped. But I am glad that I thought through multiple possible scenarios regarding potential ways versioning could be handled.

The most important decision -- and one I think I did get right -- was to change the Java and Maven packages Jackson 2.x uses: it should be (and is!) possible to have both Jackson 1.x and Jackson 2.x implementations in classpath, without conflicts. I have to thank my friend Brian McCallister for this insight -- he convinced me that this is the only sane way to go. And he is right. The alternative of just using the same package name is akin to playing Russian Roulette: things MIGHT work, or might not work. But you are actually playing with code of other people; and they can't really be sure whether it will work for them without trying... and often find out too late if it doesn't.

So although it is more work all around for cases where things would have worked; it is definitely much, much less work and pain for cases where you would have had problems with backwards compatibility. In fact, amount of work is quite constant; and most changes are mechanical.

Jackson 2.0 took its time to complete; and was released February 2012.

9. Jackson goes XML, CSV, YAML... and more

One of biggest changes with Jackson 2.x has been the huge increase in number of Modules. Many of these handle specific datatype libraries, which is the original use case. Some modules implement new functionality; Mr Bean, for example, which was introduced in 1.6 was re-packaged as a Module in later releases.

But one of those Crazy Ideas ("what if...") that I had somewhere during 1.x development was to consider possibility of supporting data formats other than JSON.
It started with the obvious question of how to support Smile format; but that was relatively trivial (although it did need some changes to underlying system, to reduce deep coupling with physical JSON content). Adding Smile support lead me to realize that the only JSON-specific handling occurs at streaming API level: everything above this level only deals with Token Streams. So what if... we simply implemented alternative backends that can produce/consume token streams? Wouldn't this allow data-binding to be used on data formats like YAML, BSON and perhaps even XML?

Turns out it can, indeed -- and at this point, Jackson supports half a dozen data formats beyond JSON (see here); and more will be added over time.

10. What Next?

As of writing this entry I am working on Jackson 2.3; and list of possible things to work on is as long as ever. Once upon a time (around finalizing 1.0) I was under false impression that maybe I will be able to wrap up work in a release or two, and move on. But given how many feature-laden versions I have released since then, I no longer thing that Jackson will be "complete" any time soon.

I hope to write more about Jackson future ... in (near I hope) future. I hope above gave you more perspective on "where's Jackson been?"; and perhaps can hint at where it is going as well.

Saturday, August 03, 2013

Jackson 2.1 was released... quite a while ago :)

Ok, so I have not been an active blogger for a while. Like, since about a year ago. I am hoping to catch up a bit, so let's start with intermediate Jackson releases that have gone out the door since I last wrote about Jackson.

1. Jackson 2.1

Version 2.1 was released almost a year ago, October 2012. After big bang of 2.0 release -- what with all the crazy new features like Object Id handling (for fully cyclic object graphs), 2.1 was expected to be more minor release in every way.

But, that was not to be... instead, 2.1 packed an impressive set of improvements of its own.
Focus was on general usability: improved ergonomics, bit of performance improvements (for data-binding) and the usual array of bug fixes that required bigger changes in internals (and occasionally additional API) than what can be done in a patch release.

For more complete handling of what exactly was added, you can check out my Jackson 2.1 Overview presentation I gave at Wordnik (thanks Tony and folks!). Note that links to this and other presentations can be found from Jackson Docs github repo.
For full list of changes, check 2.1 Release Notes.

But here's a Reader's Digest version.

2. Shape-shifting

@JsonFormat annotation was added in Jackson 2.0, but was not used by many datatypes. With 2.1, there are interesting (and back then, experimental; but it is much more stable now!) new features to let you change the "shape" (JSON Structure) of some of common Java datatypes:

  • Serialize Enums as JSON Objects instead of Strings: useful for serialization, but can not deserialize back (how would that work? Enums are singletons)
  • Collections (Sets, Lists) as JSON Objects (instead of arrays): useful for custom Collections that add extra properties -- can also deserialize, with proper use of @JsonCreator annotations (or custom deserializer)
  • POJOs as Arrays! Instead of having name/value pairs, you will get JSON arrays where position indicates which property is being used (make sure to use @JsonPropertyOrder annotation to define ordering!)

Of these, the last option is probably the most interesting. It can make JSON as compact as CSV; and in fact can compete with binary formats in many cases, especially if values are mostly Strings.
A simple example would be:

  public class Point {
    public int x, y;

which, when serialized, could look like:

  [ 1, 2]

instead of earlier

  { "x":1, "y":2 }

and obviously works for reading as well (that is, you can read such tabular data back).

3. Chunked (partial) Binary Data reads, writes

When dealing with really large data, granularity of JsonParser and JsonGenerator works well, except for case of long JSON Strings; for example, ones that contain Base64-encoded data. Since these values may be potentially very large, and since they are quite often just stored on disk (or read from disk to send) -- and there is no benefit from keeping the whole value in memory at all -- it makes sense to offer some way to allow streaming for values, not just between values.

To this end, JsonParser and JsonGenerator now do have methods that allow one to read and write large binary data chunks without retaining more than a limited amount of data in memory (one buffer-full, like 8 or 16kB) at any given point. Access is provided via and, with methods:

JsonGenerator.writeBinary(InputStream, int expectedLength)

Note that while direction of arguments may look odd, it actually makes sense when you try using it: you will provide handler for content (which implements OutputStream), and source for content to write (InputStream).

4. Format auto-detection support for data-binding

Another innovative new feature is ability to use already existing data format auto-detection, without having to use Streaming API. Earlier versions included support for JsonParser auto-detecting type of input, for data formats that support this (some binary formats do not; I consider this a flaw in such formats; of text formats, CSV does not): at least JSON, XML, Smile and YAML support auto-detection.

You enable support through ObjectReader for example like so:

  ObjectMapper mapper = new ObjectMapper();
  XmlMapper xmlMapper = new XmlMapper(); // XML is special: must start with its own mapper
  ObjectReader reader = mapper
    .reader(POJO.class) // for reading instances of POJO
    .withFormatDetection(new JsonFactory(), xmlMapper.getFactory(), new SmileFactory();

and then you can use resulting reader normally:

  User user = mapper.readValue(new File("input.raw"), User.class);

and input that is in XML, JSON or Smile format will be property decoded, and bound to resulting class. I personally use this to support transparent usage of Smile (binary JSON) format as a pluggable optimization over JSON.

5. Much improved XML module

Although XML module has existed since earlier 1.x versions, 2.0 provided first solid version. But it did not include support for one commonly used JAXB feature: ability to use so-called "unwrapped" Lists. 2.1 fixes this and fully supports both wrapped and unwrapped Lists.

But beyond this feature, testing was significantly extended, and a few specific bugs were fixed. As a result version 2.1 is the first version that I can fully recommend as replacement for JAXB processing in production environments.

6. Delegating serializer, deserializer

Final new feature is support for so-called delegating serializers and deserializers. The basic idea is simple: instead of having to build fully custom handlers you only need to implement converters that can convert your custom types into something that Jackson can automatically handle (supports out of the box).

Details of this are included in 2.1 presentation; most commonly you will just extend com.fasterxml.jackson.databind.deser.std.StdDelegatingDeserializer and com.fasterxml.jackson.databind.ser.std.StdDelegatingSerializer.

Saturday, August 18, 2012

Replacing standard JDK serialization using Jackson (JSON/Smile),

1. Background

The default Java serialization provided by JDK is a two-edged sword: on one hand, it is a simple, convenient way to "freeze and thaw" Objects you have, handling about any kind of Java object graphs. It is possibly the most powerful serialization mechanism on Java platform, bar none.

But on the other hand, its shortcomings are well-document (and I hope, well-known) at this point. Problems include:

  • Poor space-efficiency (especially for small data), due to inclusion of all class metadata: that is, size of output can be huge, larger than about any alternative, including XML
  • Poor performance (especially for small data), partly due to size inefficiency
  • Brittleness: smallest changes to class definitions may break compatibility, preventing deserialization. This makes it a poor choice for both data exchange between (Java) systems as well as long-term storage

Still, the convenience factor has led to many systems using JDK serialization to be the default serialization method to use.

Is there anything we could do to address downsides listed above? Plenty, actually. Although there is no way to do much more for the default implementation (JDK serialization implementation is in fact ridiculously well optimized for what it tries to achieve -- it's just that the goal is very ambitious), one can customize what gets used by making objects implement interface. If so, JDK will happily use alternate implementation under the hood.

Now: although writing custom serializers may be fun sometimes -- and for specific case, you can actually write very efficient solution as well, given enough time -- it would be nice if you could use an existing component to address listed short-comings.

And that's what we'll do! Here's one possible way to improve on all problems listed above:

  1. Use an efficient Jackson serializer (to produce either JSON, or perhaps more interestingly, Smile binary data)
  2. Wrap it in nice, to make it transparent to code using JDK serialization (albeit not transparent for maintainers of the class -- but we will try minimizing amount of intrusive code)

2. Challenges with

First things first: while conceptually simple, there are couple of rather odd design decisions that make use of bit tricky:

  1. Instead of passing instances of,, instead and are used; and they do NOT extend stream versions (even though they define mostly same methods!). This means additional wrapping is needed
  2. Externalizable.readExternal() requires updating of the object itself, not that of constructing new instances: most serialization frameworks do not support such operation
  3. How to access external serialization library, as no context is passed to either of methods?

These are not fundamental problems for Jackson: first one requires use of adapter classes (see below), second that we need to use "updating reader" approach that Jackson was supported for a while (yay!). And to solve the third part, we have at least two choices: use of ThreadLocal for passing an ObjectMapper; or, use of a static helper class (approach shown below)

So here are the helper classes we need:

final static class ExternalizableInput extends InputStream
  private final ObjectInput in;

  public ExternalizableInput(ObjectInput in) { = in;

  public int available() throws IOException {
    return in.available();

  public void close() throws IOException {

  public boolean  markSupported() {
    return false;

  public int read() throws IOException {

  public int read(byte[] buffer) throws IOException {

  public int read(byte[] buffer, int offset, int len) throws IOException {
    return, offset, len);

  public long skip(long n) throws IOException {
   return in.skip(n);

final static class ExternalizableOutput extends OutputStream { private final ObjectOutput out; public ExternalizableOutput(ObjectOutput out) { this.out = out; } @Override public void flush() throws IOException { out.flush(); } @Override public void close() throws IOException { out.close(); } @Override public void write(int ch) throws IOException { out.write(ch); } @Override public void write(byte[] data) throws IOException { out.write(data); } @Override public void write(byte[] data, int offset, int len) throws IOException { out.write(data, offset, len); } }

/* Use of helper class here is unfortunate, but necessary; alternative would
* be to use ThreadLocal, and set instance before calling serialization.
* Benefit of that approach would be dynamic configuration; however, this
* approach is easier to demonstrate.
class MapperHolder { private final ObjectMapper mapper = new ObjectMapper(); private final static MapperHolder instance = new MapperHolder(); public static ObjectMapper mapper() { return instance.mapper; } }

and given these classes, we can implement Jackson-for-default-serialization solution.

3. Let's Do a Serialization!

So with that, here's a class that is serializable using Jackson JSON serializer:

  static class MyPojo implements Externalizable
        public int id;
        public String name;
        public int[] values;

        public MyPojo() { } // for deserialization
        public MyPojo(int id, String name, int[] values)
   = id;
   = name;
            this.values = values;

        public void readExternal(ObjectInput in) throws IOException {
            MapperHolder.mapper().readerForUpdating(this).readValue(new ExternalizableInput(in));
} public void writeExternal(ObjectOutput oo) throws IOException { MapperHolder.mapper().writeValue(new ExternalizableOutput(oo), this); }

to use that class, use JDK serialization normally:

  // serialize as bytes (to demonstrate):
MyPojo input = new MyPojo(13, "Foobar", new int[] { 1, 2, 3 } ); ByteArrayOutputStream bytes = new ByteArrayOutputStream(); ObjectOutputStream obs = new ObjectOutputStream(bytes); obs.writeObject(input); obs.close(); byte[] ser = bytes.toByteArray();

// and to get it back:
ObjectInputStream ins = new ObjectInputStream(new ByteArrayInputStream(ser)); MyPojo output = (MyPojo) ins.readObject();

And that's it.

4. So what's the benefit?

At this point, you may be wondering if and how this would actually help you. Since JDK serialization is using binary format; and since (allegedly!) textual formats are generally more verbose than binary formats, how could this possibly help with size of performance?

Turns out that if you test out code above and compare it with the case where class does NOT implement Externalizable, sizes are:

  • Default JDK serialization: 186 bytes
  • Serialization as embedded JSON: 130 bytes

Whoa! Quite unexpected result? JSON-based alternative 30% SMALLER than JDK serialization!

Actually, not really. The problem with JDK serialization is not the way data is stored, but rather the fact that in addition to (compact) data, much of Class definition metadata is included. This metadata is needed to guard against Class incompatibilities (which it can do pretty well), but it comes with a cost. And that cost is particularly high for small data.

Similarly, performance typically follows data size: while I don't have publishable results (I may do that for a future post), I expect embedded-JSON to also perform significantly better for single-object serialization use cases.

5. Further ideas: Smile!

But perhaps you think we should be able to do better, size-wise (and perhaps performance) than using JSON?

Absolutely. Since the results are not exactly readable (to use Externalizable, bit of binary data will be used to indicate class name, and little bit of stream metadata), we probably do not greatly care what the actual underlying format is.
With this, an obvious choice would be to use Smile data format, binary counterpart to JSON, a format that Jackson supports 100% with Smile Module.

The only change that is needed is to replace the first line from "MapperHolder" to read:

private final ObjectMapper mapper = new ObjectMapper(new SmileFactory());

and we will see even reduced size, as well as faster reading and writing -- Smile is typically 30-40% smaller in size, and 30-50% faster to process than JSON.

6. Even More compact? Consider Jackson 2.1, "POJO as array!"

But wait! In very near future, we may be able to do EVEN BETTER! Jackson 2.1 (see the Sneak Peek) will introduce one interesting feature that will further reduce size of JSON/Smile Object serialization. By using following annotation:


you can further reduce the size: this occurs as the property names are excluded from serialization (think of output similar to CSV, just using JSON Arrays).

For our toy use case, size is reduced further from 130 bytes to 109; further reduction of almost 20%. But wait! It gets better -- same will be true for Smile as well, since while it can reduce space in general, it still has to retain some amount of name information normally; but with POJO-as-Arrays it will use same exclusion!

7. But how about actual real-life results?

At this point I am actually planning on doing something based on code I showed above. But planning is in early stages so I do not yet have results from "real data"; meaning objects of more realistic sizes. But I hope to get that soon: the use case is that of storing entities (data for which is read from DB) in memcache. Existing system is getting CPU-bound both from basic serialization/deserialization activity, but especially from higher number of GCs. I fully expect the new approach to help with this; and most importantly, be quite easy to deploy: this because I do not have to change any of code that actually serializes/deserializes Beans -- I just have to modify Beans themselves a bit.

Forcing escaping of HTML characters (less-than, ampersand) in JSON using Jackson

1. The problem

Jackson handles escaping of JSON String values in minimal way using escaping where absolutely necessary: it escapes two characters by default -- double quotes and backslash -- as well as non-visible control characters. But it does not escape other characters, since this is not required for producing valid JSON documents.

There are systems, however, that may run into problems with some characters that are valid in JSON documents. There are also use cases where you might prefer to add more escaping. For example, if you are to enclose a JSON fragment in XML attribute (or Javascript code), you might want to use apostrophe (') as quote character in XML, and force escaping of all apostrophes in JSON content; this allows you to simple embed encoded JSON value without other transformations.

Another specific use case is that of escaping "HTML funny characters", like less-than, greater-than, ampersand and apostrophe characters (double-quote are escaped by default).

Let's see how you can do that with Jackson.

2. Not as easy to change as you might think

Your first thought may be that of "I'll just do it myself". The problem is two-fold:

  1. When using API via data-binding, or regular Streaming generator, you must pass unescaped String, and it will get escaped using Jackson's escaping mechanism -- you can not pre-process it (*)
  2. If you decide to post-process content after JSON gets written, you need to be careful with replacements, and this will have negative impact on performance (i.e. it is likely to double time serialization takes)

(*) actually, there is method 'JsonGenerator.writeRaw(...)' which you can use to force exact details, but its use is cumbersome and you can easily break things if you are not careful. Plus it is only applicable via Streaming API

3. Jackson (1.8) has you covered

Luckily, there is no need for you to write custom post-processing code to change details of content escaping.

Version 1.8 of Jackson added a feature to let users customize details of escaping of characters in JSON String values.
This is done by defining a CharacterEscapes object to be used by JsonGenerator; it is registered on JsonFactory. If you use data-binding, you can set this by using ObjectMapper.getJsonFactory() first, then define CharacterEscapes to use.

Functionality is handled at low-level, during writing of JSON String values; and CharacterEscapes abstract class is designed in a way to minimize performance overhead.
While there is some performance overhead (little bit of additional processing is required), it should not have significant impact unless significant portion of content requires escaping.
As usual, if you care a lot about performance, you may want to measure impact of the change with test data.

4. The Code

Here is a way to force escaping of HTML "funny characters", using functionality Jackson 1.8 (and above) have.

import org.codehaus.jackson.SerializableString;

// First, definition of what to escape public class HTMLCharacterEscapes extends CharacterEscapes { private final int[] asciiEscapes; public HTMLCharacterEscapes() {
// start with set of characters known to require escaping (double-quote, backslash etc) int[] esc = CharacterEscapes.standardAsciiEscapesForJSON();
// and force escaping of a few others: esc['<'] = CharacterEscapes.ESCAPE_STANDARD; esc['>'] = CharacterEscapes.ESCAPE_STANDARD; esc['&'] = CharacterEscapes.ESCAPE_STANDARD; esc['\''] = CharacterEscapes.ESCAPE_STANDARD; asciiEscapes = esc; }
// this method gets called for character codes 0 - 127 @Override public int[] getEscapeCodesForAscii() { return asciiEscapes; }
// and this for others; we don't need anything special here @Override public SerializableString getEscapeSequence(int ch) { // no further escaping (beyond ASCII chars) needed: return null; } }

// and then an example of how to apply it
public ObjectMapper getEscapingMapper() {
ObjectMapper mapper = new ObjectMapper();
mapper.getJsonFactory().setCharacterEscapes(new HTMLCharacterEscapes());
return mapper;

// so we could do:
public byte[] serializeWithEscapes(Object ob) throws IOException
return getEscapingMapper().writeValueAsBytes(ob);

And that's it.

Thursday, May 24, 2012

Doing actual non-blocking, incremental HTTP access with async-http-client

Async-http-client library, originally developed at Ning (by Jean-Francois, Tom, Brian and maybe others and since then by quite a few others) has been around for a while now.
Its main selling point is the claim for better scalability compared to alternatives like Jakarta HTTP Client (this is not the only selling points: its API also seems more intuitive).

But although library itself is capable of working well in non-blocking mode, most examples (and probably most users) use it in plain old blocking mode; or at most use Future to simply defer handling of respoonses, but without handling content incrementally when it becomes available.

While this lack of documentation is bit unfortunate just in itself, the bigger problem is that most usage as done by sample code requires reading the whole response in memory.
This may not be a big deal for small responses, but in cases where response size is in megabytes, this often becomes problematic.

1. Blocking, fully in-memory usage

The usual (and potentially problematic) usage pattern is something like:

  AsyncHttpClient asyncHttpClient = new AsyncHttpClient();
  Future<Response> f = asyncHttpClient.prepareGet(" ").execute();
  Response r = f.get();
byte[] contents = r.getResponseBodyAsBytes();

which gets the whole response as a byte array; no surprises there.

2. Use InputStream to avoid buffering the whole entity?

The first obvious work around attempt is to have a look at Response object, and notice that there is method "getResponseBodyAsStream()". This would seemingly allow one to read response, piece by piece, and process it incrementally, by (for example) writing it to a file.

Unfortunately, this method is just a facade, implemented like so:

 public InputStream getResponseBodyAsStream() {
return new ByteArrayInputStream(getResponseBodyAsBytes());

which actually is no more efficient than accessing the whole content as a byte array. :-/

(why is it implemented that way? Mostly because underlying non-blocking I/O library, like Netty or Grizzly, provides content using "push" style interface, which makes it very hard to support "pull" style abstractions like -- so it is not really AHC's fault, but rather a consequence of NIO/async style of I/O processing)

3. Go fully async

So what can we do to actually process large response payloads (or large PUT/POST request payloads, for that matter)?

To do that, it is necessary to use following callback abstractions:

  1. To handle response payloads (for HTTP GETs), we need to implement AsyncCompletionHandler interface.
  2. To handle PUT/POST request payloads, we need to implement BodyGenerator (which is used for creating a Body instance, abstraction for feeding content)

Let's have a look at what is needed for the first case.

(note: there are existing default implementations for some of the pieces -- but here I will show how to do it from ground up)

4. A simple download-a-file example

Let's start with a simple case of downloading a large file into a file, without keeping more than a small chunk in memory at any given time. This can be done as follows:

public class SimpleFileHandler implements AsyncHandler<File>
 private File file;
 private final FileOutputStream out;
 private boolean failed = false;

 public SimpleFileHandler(File f) throws IOException {
  file = f;
  out = new FileOutputStream(f);

 public com.ning.http.client.AsyncHandler.STATE onBodyPartReceived(HttpResponseBodyPart part)
   throws IOException
  if (!failed) {

 public File onCompleted() throws IOException {
  if (failed) {
   return null;
  return file;

 public com.ning.http.client.AsyncHandler.STATE onHeadersReceived(HttpResponseHeaders h) {
  // nothing to check here as of yet

 public com.ning.http.client.AsyncHandler.STATE onStatusReceived(HttpResponseStatus status) {
  failed = (status.getStatusCode() != 200);
  return failed ?  STATE.ABORT : STATE.CONTINUE;

 public void onThrowable(Throwable t) {
  failed = true;

Voila. Code is not very brief (event-based code seldom is), and it could use some more handling for error cases.
But it should at least show the general processing flow -- nothing very complicated there, beyond basic state machine style operation.

5. Booooring. Anything more complicated?

Downloading a large file is something useful, but while not a contriver example, it is rather plain. So let's consider the case where we not only want to download a piece of content, but also want uncompress it, in one fell swoop. This serves as an example of additional processing we may want to do, in incremental/streaming fashion -- as an alternative to having to store an intermediate copy in a file, then uncompress to another file.

But before showing the code, however, it is necessary to explain why this is bit tricky.

First, remember that we can't really use InputStream-based processing here: all content we get is "pushed" to use (without our code ever blocking with input); whereas InputStream would want to push content itself, possibly blocking the thread.

Second: most decompressors present either InputStream-based abstraction, or uncompress-the-whole-thing interface: neither works for us, since we are getting incremental chunks; so to use either, we would first have to buffer the whole content. Which is what we are trying to avoid.

As luck would have it, however, Ning Compress package (version 0.9.4, specifically) just happens to have a push-style uncompressor interface (aptly named as "com.ning.compress.Uncompressor"); and two implementations:

  1. com.ning.compress.lzf.LZFUncompressor
  2. com.ning.compress.gzip.GZIPUncompressor (uses JDK native zlib under the hood)

So why is that fortunate? Because interface they expose is push style:

 public abstract class Uncompressor
  public abstract void feedCompressedData(byte[] comp, int offset, int len) throws IOException;
  public abstract void complete() throws IOException;

and is thereby usable to our needs here. Especially when we use additional class called "UncompressorOutputStream", which makes an OutputStream out of Uncompressor and target stream (which is needed for efficient access to content AHC exposes via HttpResponseBodyPart)

6. Show me the code

Here goes:

public class UncompressingFileHandler implements AsyncHandler<File>,
   DataHandler // for Uncompressor
 private File file;
 private final OutputStream out;
 private boolean failed = false;
 private final UncompressorOutputStream uncompressingStream;

 public UncompressingFileHandler(File f) throws IOException {
  file = f;
  out = new FileOutputStream(f);

 public com.ning.http.client.AsyncHandler.STATE onBodyPartReceived(HttpResponseBodyPart part)
   throws IOException
  if (!failed) {
   // if compressed, pass through uncompressing stream
   if (uncompressingStream != null) {
   } else { // otherwise write directly

 public File onCompleted() throws IOException {
  if (uncompressingStream != null) {
  if (failed) {
   return null;
  return file;

 public com.ning.http.client.AsyncHandler.STATE onHeadersReceived(HttpResponseHeaders h) {
  // must verify that we are getting compressed stuff here:
  String compression = h.getHeaders().getFirstValue("Content-Encoding");
  if (compression != null) {
   if ("lzf".equals(compression)) {
    uncompressingStream = new UncompressorOutputStream(new LZFUncompressor(this));
   } else if ("gzip".equals(compression)) {
    uncompressingStream = new UncompressorOutputStream(new GZIPUncompressor(this));
  // nothing to check here as of yet

 public com.ning.http.client.AsyncHandler.STATE onStatusReceived(HttpResponseStatus status) {
  failed = (status.getStatusCode() != 200);
  return failed ?  STATE.ABORT : STATE.CONTINUE;

 public void onThrowable(Throwable t) {
  failed = true;

 // DataHandler implementation for Uncompressor; called with uncompressed content:
 public void handleData(byte[] buffer, int offset, int len) throws IOException {
  out.write(buffer, offset, len);

Handling gets bit more complicated here, since we have to handle both case where content is compressed; and case where it is not (since server is ultimately responsible for applying compression or not).

And to make call, you also need to indicate capability to accept compressed data. For example, we could define a helper method like:

public File download(String url) throws Exception
 AsyncHttpClient ahc = new AsyncHttpClient();
 Request req = ahc.prepareGet(url)
  .addHeader("Accept-Encoding", "lzf,gzip")
 ListenableFuture<File> futurama = ahc.executeRequest(req,
new UncompressingFileHandler(new File("download.txt"))); try { // wait for 30 seconds to complete return futurama.get(30, TimeUnit.MILLISECONDS); } catch (TimeoutException e) { throw new IOException("Failed to download due to timeout"); } }

which would use handler defined above.

7. Easy enough?

I hope above shows that while doing incremental, "streaming" processing is bit more work, it is not super difficult to do.

Not even when you have bit of pipelining to do, like uncompressing (or compressing) data on the fly.

Thursday, May 03, 2012

Jackson Data-binding: Did I mention it can do YAML as well?

Note: as useful earlier articles, consider reading "Jackson 2.0: CSV-compatible as well" and "Jackson 2.0: now with XML, too!"

1. Inspiration

Before jumping into the actual beef -- the new module -- I want to mention my inspiration for this extension: the Greatest New Thing to hit Java World Since JAX-RS called DropWizard.

For those who have not yet tried it out and are unaware of its Kung-Fu Panda like Awesomeness, please go and check it out. You won't be disappointed.

DropWizard is a sort of mini-framework that combines great Java libraries (I may be biased, as it does use Jackson), starting with trusty JAX-RS/Jetty8 combination, building with Jackson for JSON, jDBI for DB/JDBC/SQL, Java Validation API (impl from Hibernate project) for data validation, and logback for logging; adding bit of Jersey-client for client-building and optional FreeMarker plug-in for UI, all bundled up in a nice, modular and easily understandable packet.
Most importantly, it "Just Works" and comes with intuitive configuration and bootstrapping system. It also builds easily into a single deployable jar file that contains all the code you need, with just a bit of Maven setup; all of which is well documented. Oh, and the documentation is very accessible, accurate and up-to-date. All in all, a very rare combination of things -- and something that would give RoR and other "easier than Java" frameworks good run for their money, if hipsters ever decided to check out the best that Java has to offer.

The most relevant part here is the configuration system. Configuration can use either basic JSON or full YAML. And as I mentioned earlier, I am beginning to appreciate YAML for configuring things.

1.1. The Specific inspirational nugget: YAML converter

The way DropWizard uses YAML is to parse it using SnakeYAML library, then convert resulting document into JSON tree and then using Jackson for data binding. This is useful since it allows one to use full power of Jackson configuration including annotations and polymorphic type handling.

But this got me thinking -- given that the whole converter implementation about dozen lines or so (to work to degree needed for configs), wouldn't it make sense to add "full support" for YAML into Jackson family of plug-ins?

I thought it would.

2. And Then There Was One More Backend for Jackson

Turns out that implementation was, indeed, quite easy. I was able to improve certain things -- for example, module can use lower level API to keep performance bit better; and output side also works, not just reader -- but in a way, there isn't all that much to do since all module has to do is to convert YAML events into JSON events, and maybe help with some conversions.

Some of more advanced things include:

  • Format auto-detection works, thanks to "---" document prefix (that generator also produces by default)
  • Although YAML itself exposes all scalars as text (unless type hints are enabled, which adds more noise in content), module uses heuristics to make parser implementation bit more natural; so although data-binding can also coerce types, this should usually not be needed
  • Configuration includes settings to change output style, to allow use of more aesthetically pleasing output (for those who prefer "wiki look", for example)

At this point, functionality has been tested with a broad if shallow set of unit tests; but because data-binding used is 100% same as with JSON, testing is actually sufficient to use module for some work.

3. Usage? So boring I tell you

Oh. And you might be interested in knowing how to use the module. This is the boring part, since.... there isn't really much to it.

You just use "YAMLFactory" wherever you would normally use "JsonFactory"; and then under the hood you get "YAMLParser" and "YAMLGenerator" instances, instead of JSON equivalents. And then you either use parser/generator directly, or, more commonly, construct an "ObjectMapper" with "YAMLFactory" like so (code snippet itself is from test "")

  ObjectMapper mapper = new ObjectMapper(new YAMLFactory());
User user = mapper.readValue("firstName: Billy\n"
+"lastName: Baggins\n"
+"gender: MALE\n"
+"userImage: AQIDBAY=",

and to get the functionality itself, Maven dependency is:


4. That's all Folks -- until you give us some Feedback!

That's it for now. I hope some of you will try out this new backend, and help us further make Jackson 2.0 the "Universal Java Data Processor"

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