Simple Java “Service Provider frameworks”?

Question

I refer to "service provider framework" as discussed in Chapter 2 of Effective Java, which seems like exactly the right way to handle a problem I am having, where I need to instantiate one of several classes at runtime, based on a String to select which service, and an Configuration object (essentially an XML snippet):

But how do I get the individual service providers (e.g. a bunch of default providers + some custom providers) to register themselves?

 interface FooAlgorithm
 {
     /* methods particular to this class of algorithms */
 }

 interface FooAlgorithmProvider
 {
     public FooAlgorithm getAlgorithm(Configuration c);
 }

 class FooAlgorithmRegistry
 {
     private FooAlgorithmRegistry() {}
     static private final Map<String, FooAlgorithmProvider> directory =
        new HashMap<String, FooAlgorithmProvider>();
     static public FooAlgorithmProvider getProvider(String name)
     {
         return directory.get(serviceName);
     }
     static public boolean registerProvider(String name, 
         FooAlgorithmProvider provider)
     {
         if (directory.containsKey(name))
            return false;
         directory.put(name, provider);
         return true;
     }
 }

e.g. if I write custom classes MyFooAlgorithm and MyFooAlgorithmProvider to implement FooAlgorithm, and I distribute them in a jar, is there any way to get registerProvider to be called automatically, or will my client programs that use the algorithm have to explicitly call FooAlgorithmRegistry.registerProvider() for each class they want to use?

Answer 1

I think you need to create a META-INF/services/fully.qualified.ClassName and list things there, but I don't remember the spec (JAR File Specification or this).

The Practical API design confessions of a Java architect book chapter 8 is about SPI.

The ServiceLoader might help you to list available implementations. For example with the PersistenceProvider interface:

ServiceLoader<PersistenceProvider> loader = 
        ServiceLoader.load(PersistenceProvider.class);
Iterator<PersistenceProvider> implementations = loader.iterator();
while(implementations.hasNext()) {
    PersistenceProvider implementation = implementations.next();
    logger.info("PersistenceProvider implementation: " + implementation);
}

Answer 2

You could have the client JAR register the providers in a static initializer block within some class that you know will be called before FooAlgorithmRegistry.getProvider(), something like:

static {
    FooAlgorithmRegistry.registerProvider("test", new MyFooAlgorithmProvider());
}

But, it might be pretty hard to find a way to guarantee that this will run (static initializers are guaranteed to be run once and only once, when the class is first loaded) before the accessor method of the factory.