Find Java classes implementing an interface [duplicate]

Answer 1

Obviously, Class.isAssignableFrom() tells you whether an individual class implements the given interface. So then the problem is getting the list of classes to test.

As far as I'm aware, there's no direct way from Java to ask the class loader for "the list of classes that you could potentially load". So you'll have to do this yourself by iterating through the visible jars, calling Class.forName() to load the class, then testing it.

However, it's a little easier if you just want to know classes implementing the given interface from those that have actually been loaded:

• via the Java Instrumentation framework, you can call Instrumentation.getAllLoadedClasses()
• via reflection, you can query the ClassLoader.classes field of a given ClassLoader.

If you use the instrumentation technique, then (as explained in the link) what happens is that your "agent" class is called essentially when the JVM starts up, and passed an Instrumentation object. At that point, you probably want to "save it for later" in a static field, and then have your main application code call it later on to get the list of loaded classes.

Answer 2

If you were asking from the perspective of working this out with a running program then you need to look to the java.lang.* package. If you get a Class object, you can use the isAssignableFrom method to check if it is an interface of another Class.

There isn't a simple built in way of searching for these, tools like Eclipse build an index of this information.

If you don't have a specific list of Class objects to test you can look to the ClassLoader object, use the getPackages() method and build your own package hierarchy iterator.

Just a warning though that these methods and classes can be quite slow.

Answer 3

You could also use the Extensible Component Scanner (extcos: http://sf.net/projects/extcos) and search all classes implementing an interface like so:

Set<Class<? extends MyInterface>> classes = new HashSet<Class<? extends MyInterface>>();

ComponentScanner scanner = new ComponentScanner();
scanner.getClasses(new ComponentQuery() {
    @Override
    protected void query() {
        select().
        from("my.package1", "my.package2").
        andStore(thoseImplementing(MyInterface.class).into(classes)).
        returning(none());
    }
});

This works for classes on the file system, within jars and even for those on the JBoss virtual file system. It's further designed to work within standalone applications as well as within any web or application container.