Converting Forms in Restlet to POJOs with Jackson and Guava

Punchline first. This article presents code that you can use to convert HTML forms into POJOs, mapping property names in the natural way, with support for compound objects and arrays/lists. A long-winded set-up for that punchline follows.

Many of my Restlet resources are designed to support both JSON and HTML representations with resource interfaces that look like this:

public interface PersonResource {
    @Get Person getPerson();
}

where Person is a POJO that might look like this:

public class Person {
    public String getName() { ...}
    public List<Address> getAddresses() { ... }
    ... other bean stuff: setters, fields ...
}

and the implementation of the resource looks like this:

public class PersonServerResource 
        extends ServerResource implements PersonResource {
    @Override public Person getPerson() {
        return ... construct Person from domain data ...
    }
}

I register custom ConverterHelpers with the Restlet Engine. One of them knows how to use Jackson to convert Person into JSON, one knows how to obtain a Freemarker HTML template associated with the Person type and render it with the Person as its data model.

It works like magic. When I use a browser to point to the resource, it returns the HTML generated with Freemarker. When I ask for it with, say, jQuery, requesting JSON, I get ... JSON. When I get a Restlet ClientResource proxy for PersonResource at that URI, I get a Person object back when I call getPerson() on the client side; the value returned by getPerson() on the server is serialized to JSON, sent over the wire, and deserialized back into a Person object.

The only problem I had was for PUT and POST method handlers, because there was no automatic way to convert form data to my domain types. I had to create an additional method to handle forms as parameters:

public interface PeopleResource {
    @Post("json:json") Person addPerson(Person person);
    @Post("form:html") Person addPersonForm(Form person);
}

public class PeopleServerResource 
        extends ServerResource implements PeopleResource {


    @Override public Person addPerson(Person person) {
        ...
    }
    @Override Person addPersonForm(Form personForm) {
        Person person = new Person();
        // Extract values from personForm and set
        // them to person.
        return addPerson(person);
    }
}

I suffered this for a long time, and then I began to think that the application/www-form-urlencoded media type was, at least for simple types, not far from JSON. Could I translate form data to JSON and then use Jackson to deserialize that? Of course I could. In fact, it wasn't too hard to define some conventions whereby sequences (lists/arrays) and nested structures could be modeled. An encoded form for Person as produced  by an HTML page might look like this, in part:

name=Tim&address.1.street=Main&address.1.city=Anytown...
     ...&address.2.street=Elm&address.2.city=Smallton...

and the corresponding JSON would be:

{
    "name":"Tim",
    "address":[{
        "street":"Main",
        "city":"Anytown",
        ...
    },{
        "street":"Elm",
        "city":"Smallton",
        ...
    },
    ...
    ]
}

I wrote FormDeserializer to do this. It takes a Jackson ObjectMapper to do the heavy lifting. Here's the Javadoc for the deserialization method:

public  T deserialize(Form source, Class targetType)

Converts a Form to a Java object of the target type, using each name=value pair to set the corresponding property on the target object.

Compound names, using period (.) as the delimiter, are treated as pseudo-dereferences (a la JavaScript or Groovy) to set properties of sub-objects, e.g., a.b=c for bean targets is treated like a call to target.getA().setB(c).

Numeric components of compound names are treated as indices into a sequence named by the preceding components, e.g., a.1=c is treated as target.getA()[1] = c (or target.getA().set(1, c), if the "a" property of the target is a list rather than an array). Unless an element with index 0 is set, the indices are origin 1.

Sequences are also created by names with multiple values, e.g., a=x&a=y is equivalent to a.1=x&a.2=y, with the value of the "a" property in the target being a sequence of two values.
When a name appears both indexed and non-indexed, the last assignment wins: a=x&a.1=a1&a.2=a2 will set the "a" property to a sequence of values [a1, a2], but a.1=a1&a.2=a2&a=xwill set the "a" property to x.

If the top level consists only of integer indices, the subobjects will be interpreted as elements of a sequence (and T must be a List or List subtype instead).

The values are deserialized using the Jackson ObjectMapper that was used to construct this FormDeserializer. Any type that can be deserialized from a string can be used. While it is possible for Jackson to deserialize object graphs that have internal references, it is not possible for the form values to refer outside of themselves.

Runtime exceptions from Jackson conversion are propagated without exception translation. This could be considered a bug.

Parameters:

source - the Restlet Form object to be deserialized

targetType - the type of the target object into which the form is to be deserialized.

Returns:

the deserialized object of the target type

The converter class that uses this machinery is FormConverter. It uses a marker annotation, FormDeserializable, that signals when a class is suitable for deserialization from a form.

The upshot is that I can now write:

public interface PeopleResource {
    @Post Person addPerson(Person person);
}
// And no need for additional method in implementation!


Big reduction in the amount of boilerplate I have to write, and the code is easier to read.

Here are the links to the code in one place:

• FormDeserializer.java
• FormConverter.java
• FormDeserializable.java

I have @Inject tags on some constructors, but you can ignore them unless you want to use them for dependency injection.

The implementation makes heavy use of Guava. If you don't or can't use Guava, then you'll have to roll your own machinery. (Good luck!)

Restlet Guice extension considered ... unnecessary

I wrote some code back in 2008 to help inject my Restlet resources using Guice and then blogged about it. In 2009, the Restlet team added it to their "incubator" as a potential Restlet extension. (Calling it a Guice extension is a bit of a misnomer, since part of it is independent of Guice and applies to any JSR-330-compliant DI framework.) They've talked about promoting it out of the incubator as early as release 2.2. Update: It's now part of Restlet 2.2 and I've updated code references to point to the 2.2 branch.

More recently, I argued (and Restlet's Jérôme Louvel seemed to accept the argument) that Restlet needs a uniform way to create a non-standard Finder for all the Restlet classes that have methods accepting a Class<? extends ServerResource> in place of a Restlet. This would make the use of the Guice extension invisible when wiring up the routing structure: Currently you have to convert the resource type to a Restlet explicity,

    router.attach("/path/to/resource", finderFactory.finder(MyResource.class));

and the extra support would let you write:

    router.attach("/path/to/resource", MyResource.class);

It's all very gratifying, except I realized suddenly this week that for most people who just want to inject their ServerResource subclasses it's completely unnecessary. It's much simpler to define doInit() to inject the resource's members, and then you can just use the second, simpler form above. I wrote an abstract base class extending ServerResource that does just that:

SelfInjectingServerResource.java

Notice that SelfInjectingServerResource uses only the standard @Inject annotation and nothing specific to Guice. In order to make it actually work with Guice, you need to tell Guice that you want self-injecting server resources. The following class is a Guice Module that accomplishes this by requesting static injection of the SelfInjectingServerResource class and providing an implementation of its nested MembersInjector interface to perform the injection.

SelfInjectingServerResourceModule.java

Install this module when creating the top-level injector, make sure that the server resources you want injected inherit from SelfInjectingServerResource, and it just works. Here's a JUnit test class that shows a trivial example of the idea in practice:

SelfInjectingServerResourceModuleTest.java

We'd like this technique to co-exist with other techniques that manage resource injection differently (e.g., the current Guice extension), without worrying about whether it's OK to extend SelfInjectingServerResource, so this code uses an atomic boolean to prevent multiple injections. (AtomicBoolean is probably overkill, but it can't hurt to be safe.)

[Deleted obsolete discussion of how to use prior to adoption as part of Restlet.]

I said that the old Guice extension was unnecessary for most users, but there are a few things the old code does that the new code doesn't do:

1. It does constructor injection, letting you have final fields with injected values.
2. It lets you create a Finder for a resource interface, decoupling the target of the routing from the resource implementation.
3. It lets you use qualifiers when looking up resource types, further decoupling the target of an attachment from the implementation.
4. It doesn't require inheritance from a common abstract base class.
5. You don't have to remember to call super.doInit() when you override doInit

As it happens, I'm taking advantage of #2 in my current work. (Note that there's no way to avoid the explicit finder call in this case, even with added Restlet support, because the Restlet signatures expect a ServerResource subclass.) So I'm stuck with my mistake for the foreseeable future. But no one else need be, now.

Update (2013 July 9)

I got rid of my dependency on #2 above, and I am now using the new approach exclusively, and I strongly recommend others do the same.

Further update (2014 Sep 1)

The Restlet Guice extension package docs discuss how to use each of three approaches, so you aren't forced to use the approach that I prefer.

Further distributed CacheLoader developments

Updated 2012-Mar-13 and 2012-Mar-20; see notes at end.

In a discussion on the Guava mailing list after my previous post, Charles Fry and Louis Wasserman had some great ideas that spurred me to rewrite the distributed CacheLoader functionality from scratch.

The first part is static factory methods to turn an AsyncFunction into a CacheLoader:

CacheLoaders.java

This is independent of any mention of Hazelcast; it works with any AsyncFunction. The Iterable-ness of the keys passed to loadAll is preserved all the way through, so asynchronous processing can start before the keys have been completely iterated. This could be handy if, for example, the keys themselves are being generated asynchronously.

The other piece is a class with methods to create an AsyncFunction out of a Hazelcast-based Executor:

HazelcastAsyncFunction.java

Note that the public API of this class does not use any Hazelcast types.

The whole implementation is much cleaner and takes full advantage of existing Guava machinery. Here's what it looks like in use in my code:

    AsyncFunction asyncLookup =
        HazelcastAsyncFunction
            .from(syncLookup());
            .onExecutor(hazelcastExecutor())
            .withTaskKeyFunction(mapAccountToServer());

    LoadingCache cache = CacheBuilder.newBuilder().build(
        fromAsyncFunction(asyncLookup, 30L, TimeUnit.SECONDS));
    ...
    // Later on, this call causes the account lookups to be
    // magically distributed across the cluster:
    Map accountNameToInfo = cache.get(accountNames);

Update 2012-Mar-13: Louis Wasserman pointed out (in comments to this post) a race in the implementation of CacheLoaders. I've updated the code to remove the race. It doesn't use as many cool Guava tricks, but it's quite a bit simpler now.

Update 2012-Mar-20: Unpacked the example to make it less frightening.

An invokeAll for Hazelcast ExecutorServices and a distributed CacheLoader

[A more recent posting describes a much cleaner implementation of the functionality described in this article.]

The ExecutorService returned by Hazelcast[Instance].getExecutorService() does not support invokeAll, but I needed this functionality for work I'm doing, so I rolled my own restricted version:

ExecUtil.java

ExecUtil has several variants of invokeAll. The generic variants take these values:

• an ExecutorService, which is must have been returned by a Hazelcast getExecutorService call;
• an Iterable of DistributedTasks<T>, where T is the common result type; and
• a FutureCallback<T>, which is a Guava interface for specifying what to do with each returned result (and what to do, if anything, with exceptions thrown during task execution).

The non-generic variants use the Void result type. Generic and non-generic have two variants each, one that waits indefinitely and one that waits a given amount of time for all the results to finish.

The use of Iterable<DistributedTask<T>> allows lazy provision of the tasks. (Supporting this feature prevented a simpler implementation with just a CountDownLatch initialized to the number of tasks, because we don't know the number of tasks in advance.)

The main awkwardness is that both the Callable<T> used to create the DistributedTask and T itself must be Serializable, but this is, of course, a requirement imposed by DistributedTask.

Update (2012-Mar-11): I added a static method to ExecUtil that wraps an ExecutorService, implementing invokeAll in terms of ExecUtil.invokeAll if the argument was created by Hazelcast. Also added a ConcurrentFuction that wraps an existing Function for concurrent or distributed application.

A concurrent Guava CacheLoader

I then realized that this machinery could be used to provide a nice implementation of Guava's CacheLoader, used to build LoadingCache instances with CacheBuilder. LoadingCache has a getAll(Iterable<? extends K> keys) method that returns a Map<K, V> from keys to cached values. It calls CacheLoader.loadAll(Iterable<? extends K> keys) -- if it's implemented -- to load the values in order to cache them. If loadAll isn't implemented, it just loads the keys sequentially.

I wrote a CacheLoader implementation that can use ExecUtil.invokeAll to load the values using DistributedTasks. While I was at it, I made it so that if you don't have a Hazelcast-based ExecutorService, you can use a normal ExecutorService to load the values concurrently (in the same JVM).

The result is ConcurrentCacheLoader:

ConcurrentCacheLoader.java

It has a nested Builder class that allows you to specify:

• the ExecutorService used (including shortcuts for default HazelcastInstance ExecutorServices),
• a time limit for loadAll,
• a function to map a cache key to a key object for the DistributedTask, and
• the actual function that maps cache keys to values.

There's a minimal set of tests here:

ConcurrentCacheLoaderTest.java

Here's how I'm using it:

Sample usage

That's a ConcurrentCacheLoader.Builder used to build an argument to a CacheBuilder method.

The upshot is that my call to getAll runs on all the nodes in my Hazelcast cluster.

Addendum:

The motivation for all this was the desire to make dozens of calls to JClouds' BlobStore.getBlob() in the space of a single HTTP request. The standard for-loop approach was taking too long, and I suddenly realized I could be asking the Hazelcast cluster to do the work.

Deploying Restlet components in Elastic Beanstalk

I spent some time finding a way to deploy a Restlet component in ElasticBeanstalk without giving up the option of deploying it as a standard Java executable for local development and testing. People have expressed interest in seeing the approach, but I don't have the time to create a full-fledged framework, so I've extracted the following template instead (links to code embedded in the prose). Just copy to your own environment, changing package and class names as you see fit, add your Guice Modules, define your Restlet applications and resources, and you should be able to run the resulting component both standalone and (by bundling everything in a WAR) via Elastic Beanstalk. I make no claims that this code is correct, that it is safe to use, or even that it will compile. You have to read it, compile it, and judge for yourself.

The code depends on Restlet, Guice, Rocoto, Guava, and (minimally) SLF4J. I've included commented-out code that uses the Restlet-Guice extension, which is in incubator status in the Restlet codebase.

The Main class is a GuiceServletContextListener and it has a main method. When you run com.example.server.Main from the command line, the GuiceServletContextListener methods are ignored. When you deploy a WAR that contains the associated web.xml file, the main routine is never called. You can inject the current DeploymentMode into your classes if you want to change behavior depending whether you're running standalone or in Elastic Beanstalk (i.e., as a servlet).

The MainComponent class is where your Restlet component logic goes. Its lifecycle is managed by the MainService class, which is where you can manage other services with lifecycles. (If you use JClouds BlobStores, for example, you can ensure that a singleton BlobStoreContext is closed when the MainService is stopped. Another example: a LifecycleService for the Hazelcast data grid) There is some trickiness to handling shutdown in various cases; make sure you nest try-finally clauses so that every one of your services gets a chance to shut down even if the previous shutdown attempt throws an exception or error.

The MainServletModule and MainServlet classes deal with the details of embedding the component in a servlet (which is necessary in order to use Elastic Beanstalk).

AwsCredentialsModule is an example of how you can inject AWS credentials from either of two sources: system properties defined on the command line (or via Ant invocation) and system properties defined by an Elastic Beanstalk configuration.

The flow of control when running standalone is that the main routine creates an instance of Main with the STANDALONE deployment mode, creates the injector, gets the singleton MainService, and starts it, which causes (in a different thread), the MainComponent to be started.

The flow of control when running as a servlet is that a default instance of Main is created (due to the web.xml directive) and gets the SERVLET deployment mode. Its contextInitialized call creates the injector, which injects the MainService and starts it, causing (in a different thread), the MainComponent to be started. 

Files:

• web.xml - web application descriptor
• Main - has main() routine for standalone; is context listener for servlet mode
• MainComponent - the Restlet component we want to be able to deploy in both modes
• MainService - manages lifecycle of MainComponent and (optionally) other services
• MainServletModule - included in module list in Main; configures Restlet-Servlet bridge
• MainServlet - injects MainComponent and returns it as the component it wraps
• DeploymentMode - enum with two values: STANDALONE and SERVLET
• AwsCredentialsModule - example of passing AWS credentials for binding with @Named

Overriding common bindings with test bindings in Jukito

This post is about how to get common bindings for Jukito tests without having to write too much boilerplate. (Obviously not a general interest post!)

A recent conversation on the Jukito newsgroup discusses various proposals for reducing the amount of boilerplate involved in setting up common bindings for tests that have a common superclass. I've noticed that the underlying assumption to these proposals is that bindings in modules are inheritable in the same way that methods are, but that's not generally the case. You can override methods by defining a new version in a subclass, but to override the bindings in a module, you need to use Modules.override, which doesn't work via inheritance.

I've written a template to illustrate how to override with test bindings; it uses Jukito 1.1 as is:

http://pastebin.com/BLQnMEqx

The key is to declare a static method in a common test superclass that returns a single module, which is easy to do with Modules.combine. Then in the concrete test class, use

  Modules.override(commonBindings).with(testBindings)

This strikes what I think is the right balance between compactness and clarity: You have one place in the concrete test class that says what the basic bindings are (commonBindings) and what test bindings are being used to override for the purposes of this test (testBindings). There's no hunting around through the inheritance hierarchy for modules with the right characteristics.

Because the Modules.combine/override approach is so simple, the case for adding special Jukito support (based on inheritance alone, annotations alone, or on a combination of inheritance and annotations) for auto-installing commonly-used modules becomes much weaker. Jukito is already pretty magical; it doesn't need to add mystery to its magic.

Java Standards Annoyances

The Java Standards Annoyances session at OSCON 2011 looks like fun. I wish I could be there.

If I were there, after first expressing congratulations mixed with condolences to Ben Evans and the London Java Community on the election to the EC, I'd probably say that the most important part of building standards is knowing when and what not to standardize. The PMO measures success quantitatively, in terms of numbers of JSR stage transitions, but the inevitable result has been that a lot of the JSRs that made it to final release are junk that should never have been accepted in the first place. The EC actually did one useful thing during my time as a member: It rejected the PFD of a (well-intentioned) JSR that was not ready for prime time.

So talk of "streamlining" the process makes me nervous. I wouldn't want to make it easier to introduce garbage, and I don't trust the EC to be a responsible gatekeeper, nor do I trust the JCP as a whole to provide high-quality feedback during public review; there's too much noise, and it's too easy for a spec lead to ignore substantial criticism.

There are good JSRs. Naturally I think the JSRs on whose EGs I have served are good ones, the most recent being JSR 334 ("Coin"). But I think the enthusiasm and energy of the community should be directed more at building great libraries and frameworks than at getting them accepted as standards.

Resigned from EC

Today I resigned from the SE/EE Executive Committee of the Java Community Process. I lasted about a year before giving up hope that the ECs would ever do anything meaningful.

The last straw for me was Oracle's failure to address the ambiguous licensing terms in JSRs 336 and 337 (the Java SE7/8 umbrella JSRs) before the EC had to vote on them. At first I abstained, but I was so dismayed by Oracle's silence that I changed my vote to No, joining the Apache Software Foundation and Google.

Several of the other EC members expressed their own disappointment while voting Yes. I'm reasonably certain that the bulk of the Yes votes were due to contractual obligations rather than strongly-held principles. It's not that I'm shocked, shocked that votes can be bought, but it finally made it clear to me that my vote was worthless.

Add to that Oracle's expressed intent to proceed with the SE7/8 JSRs whatever the outcome of the vote, and one can only conclude that the SE/EE EC is never going to be more than a rubber stamp for Oracle. (The belligerent tone with which this message was delivered does not come across in the public minutes, but it was loud and clear over my phone connection.)

That's not to say that I'm against the technical aspects of the JSRs. In fact, I'm a member of the Expert Group for JSR 334 ("Project Coin") and I'm participating in discussions about some of the library-related work on JSR 335 ("Project Lambda"). I think these are both good projects that will ultimately be beneficial to the Java language and libraries.

But here's a funny thing: To my own surprise, I'm coming to believe something heretical, that it actually is not all that crucial for Java to move forward, at least not to the constituency I felt that I represented on the EC, the tens of thousands of Java developers who don't work for a big company with an Oracle contract.

The big boys want big apparent forward motion because it means more stuff to sell, more contracts and control. As a result, we are whipped to a frenzy with messages (both subliminal and explicit) that Java is falling behind, losing mind-share, being lapped by C#, anything to sell the idea that more is desperately needed, when in fact most folks could make do with a lot less.

So while I think things like Project Coin and Project Lambda are worth working on, the Java ecosystem is already so amazingly rich that the absence of these features (and of all the other good things planned for SE7 and SE8) in practice doesn't get in the way of real progress for developers like me, who just want to put together maintainable, type-safe programs, taking advantage of field-tested readily-available libraries and frameworks.

It's nice to be back in the real world, writing Java code. Here's a partial list of what I'm using, besides Java:

Ant
Freemarker
Guava
Guice
Hazelcast
iText
Ivy
Jackson
JavaScript
JClouds
Joda Time
jQuery
JUnit
Mockito
Restlet
Rhino
SnakeYAML
YUI Compressor

Vote for Bob Lee and prove Doug Lea wrong!

In explaining why he is not seeking another term on the JCP SE/EE Executive Committee, Doug Lea encourages us to prove him wrong about the growing irrelevance of the Java Community Process.

The best hope for proving Doug wrong lies with folks who have experience working on JSRs that weren't simply rubber stamps — and Bob Lee is the best of those folks. Bob is running for one of two open seats on the SE/EE EC against Azul, Eclipse, Google, and three others, and (if you're a member of the JCP) I urge you to vote for him.

Bob is running as an individual, which means that his votes will not be tied to the commercial and legal interests of a large corporation, but will instead reflect his personal expertise and judgment about the best interests of the Java community.

There's no denying that the JCP has been moribund for too long, and that Oracle's ham-handed moves have made the prospect of a true revitalization far less likely than anyone expected a year or so ago. But it's worth a shot, and I don't know anyone better equipped to help make that shot than Bob Lee. (Except Doug Lea, of course, but that's water under the bridge.)

Doug Lea in New York Times article

Doug Lea, my colleague and co-author, is quoted in a New York Times article about the Oracle-Google patent infringement lawsuit.

Moderating Chinese comments

I've been rejecting comments in Chinese whenever I can't be sure that the comment isn't spam, i.e., always. Google Translate renders them as unobjectionable but content-free, but so far none of them seem to have come from an actual person who read my post.

If you can read my blog posts and you'd like to comment in Chinese, please include an English translation.

Google Collections is dead, long live Guava!

Kevin Bourrillion wants to get the word out: Google Collections is dead, long live Guava! Guava is a proper superset of Google Collections, so the passing of the latter is cause to rejoice, not mourn.

I've only scratched the surface of Guava in my own work on Seat Yourself, but I can tell already that it makes large parts of our "common" code unnecessary. Being able to replace your own code with a well-tested open source library is a wonderful thing, like getting someone else to mow the lawn for free.

In what feels to me like a dark time, as big companies privately negotiate the fate of Java and the JCP, Google's ongoing and generous investment in and championing of open Java-related technologies (Guava, GWT, GAE/J, Android, to name a few) is a welcome bright spot. Without their work ... well, I'd still be a Java developer, but I'd probably be looking for a way out.

That I'm still enthusiastic about coding in Java, in spite of the murky politics, is largely due to my friends at Google, which should tell you something.

The Minstrel Boy

The Minstrel Boy, a piece I wrote nearly 25 years ago during graduate work in Computer Science at Cornell (with a minor in Music Composition), received its first public performance yesterday at the Unitarian-Universalist Congregation at Shelter Rock. I had shown the score to the music director there, Stephen Michael Smith, who has a track record of fostering new works and composers, and to my surprise and pleasure, he very quickly arranged for a group of singers—Leslie Craigie, Farah Chandu, Leslie Pirchinello, Chee Shun Tan, and Brace Negron—to perform it as part of the regular Sunday service. (Pianists Evan Solomon and Akira Eguchi assisted during rehearsals.)

The piece is a setting for SSATB (with solo sections) of Thomas Moore's poem of the same name. It's normally sung to the tune of "The Moreen", and I used that melody heavily in my setting. The composition teacher under whose supervision I wrote it, Steven Stucky, was not, I think, entirely on board with my wholesale appropriation of these existing materials—he might reasonably have felt that I should be producing my own music instead of rehashing others', at least on his watch—but I persisted anyway.

This was the second world premiere of my work in as many months: In April, TracyLynn Conner performed a song I wrote for her, as part of a American Cancer Society benefit concert.

Both of these events were big thrills for me. If you don't count my music for theater, arrangements, orchestrations, or works for personal occasions, the last time I had a world premiere was in ... 1984.

Concurrently initialized singletons in Guice

William Pietri asked whether it would be possible to take advantage of concurrency and Guice's knowledge of dependencies to provide interdependent (and expensive to create) singleton services in parallel.
For example, say that service A depends on services B and C and that B and C don't know about each other (and so could be constructed concurrently). Obviously, you could explicitly construct B and C in separate tasks running in a thread pool, then construct A once B and C exist.

class A {
    A(B b, C c) { /* ... use b and c ... */ }
}

class B {
    B() { /* ... takes a long time ... */ }
}

class C {
    C() { /* ... takes a long time ... */ }
}

class Services {
    A a; B b; C c;
    void init() throws InterruptedException {
        final CountDownLatch ready = new CountDownLatch(2);
        Executor pool = Executors.newCachedThreadPool();
        pool.execute(new Runnable() {
            public void run() { b = new B(); ready.countDown(); }
        })
        pool.execute(new Runnable() {
            public void run() { c = new C(); ready.countDown(); }
        })
        ready.await();
        a = new A(b, c);
        // Now publish a, b, and c safely -- not shown.
    }
}

But it's a pain to have to orchestrate this yourself. You have to know the dependencies intimately and write a lot of tricky code. If the dependencies change, the code has to be completely rethought. Wouldn't it be nice if Guice could do this for you automatically without giving up on the opportunities for concurrency? Well, it turns out that it can.
The idea is to use a variant of singleton scope that runs the providers of all bindings for that scope in separate threads. Naturally some of these providers block in their own thread while their dependencies finish in other threads, but those services that can be constructed in parallel will be.
Here's what the Guice version looks like:

@ConcurrentSingleton
class A {
    @Inject A(B b, C c) { /* ... use b and c ... */ }
}

@ConcurrentSingleton
class B {
    @Inject B() { /* ... takes a long time ... */ }
}

@ConcurrentSingleton
class C {
    @Inject C() { /* ... takes a long time ... */ }
}

No special initialization code needed, just inject A, B, and C wherever they are needed. (Note that if you have circular dependencies, this probably won't work. So don't have circular dependencies.)
ConcurrentSingleton is just a scope annotation. The actual scope implementation is ConcurrentSingletonScope. I have a fleshed-out version of the A, B, C example, ConcurrentSingletonExample, that is slightly different from the code above; it uses Providers for B and C so that A can do some work before getting B and C in the constructor.
You aren't forced to do all the expensive work in the constructor. You could also inject a start method with injected parameters to provide needed dependencies.
In the same thread that provoked the ConcurrentSingleton response, Jesse Wilson suggested the use of the experimental lifecycle facility in Guava. This could prove useful in the current context if, for example, you have singleton services that take a while to stop and could benefit from being stopped in parallel.

Update 2009-11-30

I added a call to pool.shutdown() in the scope implementation to make sure the pool threads don't prevent the JVM from exiting. It might not be necessary, but I don't think it hurts.

Update 2010-7-3

Added explicit notice that the sources are in the public domain.

Update 2018-10-16

Added a GitHub repository with the concurrent singleton code and other utilities, as the old links to code weren't working any more. Now using MIT open source license.

The Emperor's New Clothes

I posted Ray Mason's video of The Emperor's New Clothes, a Theatre Three Children's Theatre production of a show that Jeff Sanzel and I wrote together, on my Facebook profile and my YouTube channel, broken up into scene length segments.

See the links section in the right-hand column -- you'll want to look at the YouTube version unless you're my Facebook friend, and even then you might prefer to watch the YouTube playlist, something for which there is no analogue in Facebook ... is there?

I haven't bothered to post to MySpace this time around.

Joshua Rosenblum's site

Josh Rosenblum has a website worth checking out:

http://rosenblummusic.com

The site was designed and implemented by Josh's 13-year-old son, Julian.

And don't be afraid to go ahead and buy those albums! :-)

Dependency injection in Restlet 2.0 with Guice

I'm very excited about Restlet 2.0, which is in testing now and scheduled for release at the end of 2009. Among other things, the newly refactored Resource API encourages more readable code through the use of a small number of annotations. Here's a very simple server-side resource:

public class DefaultResource extends ServerResource {
    @Get public String represent() {
        return "Default resource, try /hello/resource or /hello/handler";
    }
}

ServerResource has a no-arg constructor, so you don't need to pass Context, Request, and Response objects to super().

Nice as this is, I still want to inject my resources using Guice, so I've updated the Restlet-Guice classes from my previous post to support both the new ServerResources and the old Handlers. It incorporates the enhancement mentioned in the updates: You can override methods to use different providers for Application, Context, Request, and Response, although for most purposes the default providers should suffice.

Update 2009-Dec-11

I've been working on a Guice extension for Restlet as an incubator project that uses the ideas in this posting. My goal is to make this extension independent of Guice at the interface level, so that people can provide implementations for different DI frameworks, of which Guice is only the first.

Musical numbers from my shows

After years of vague promises to cast members to provide a record of the shows that Jeff Sanzel and I have written together for Theatre Three children's theatre productions, I finally got up my courage to edit together the tapes I'd made into more or less coherent individual musical numbers. I've been posting them to YouTube, Facebook, and MySpace as I finish them.

The tapes were made by setting up a camera (or two) before each performance, pointing it towards the stage, turning it on, and leaving it there unattended. In some cases I also recorded a separate audio track with a decent stereo mic. There are several problems with this approach:

• The camera is stationary, so any zooming or panning has to be applied after the fact. This makes for blurry closeups.


• The autofocus is often confused by stage lighting changes.


• Each performance is slightly different, so synching lip movements can be tricky. I got it wrong in several instances.


• The mic in the camera is not as good as the separate stereo mic, and the sound quality varies depending on where the camera is placed. So only The Fairy Princess has good sound.

But I hope the end results are entertaining, if only for the performers who took part in the productions.

Eventually I'd like to create a video or sequence of videos covering an entire show, for watching in a YouTube playlist, for example. I made studio recordings of Hansel and Gretel and The Fairy Princess (and I hope to do so for The Elves and the Shoemaker), so it is possible that I could use that material for the soundtrack.

New CD: "Colloquy"

I am once more proud to announce the release of a CD that I produced: Colloquy, an album of music by Gary William Friedman. And once more, I am several weeks late in my announcement. Maybe I planned it that way. Yeah, that's it.

I wrote the liner notes for this album, and I won't repeat them here, so go get a copy and read them. Or don't read them, just listen to the music -- I don't mind.

Update:

There's also a video about the making of one of the pieces on the album.

New CD: "Before Love Has Gone"

I'm proud to announce the release, one month ago, of a new album from Stevie Holland, Before Love Has Gone, that I co-produced with Todd Barkan and Gary William Friedman. (The actual release date was June 24, but I kept forgetting to blog about it and decided to wait out the full month.)

Go get it and listen to it. The whole album is great, but if you're really stingy and can only spring for one tune, I recommend "Lazy Afternoon". It's phenomenal.

Update:

There's also an EPK for the album.