Is something similar to ServiceLoader in Java 1.5?

Question

How do I discover classes at runtime in the classpath which implements a defined interface?

ServiceLoader suits well (I think, I haven't used it), but I need do it in Java 1.5.

Answer 1

There's nothing built into Java 1.5 for this. I implemented it myself; it's not too complicated. However, when we upgrade to Java 6, I will have to replace calls to my implementation with calls to ServiceLoader. I could have defined a little bridge between the app and the loader, but I only use it in a few places, and the wrapper itself would be a good candidate for a ServiceLoader.

This is the core idea:

public <S> Iterable<S> load(Class<S> ifc) throws Exception {
  ClassLoader ldr = Thread.currentThread().getContextClassLoader();
  Enumeration<URL> e = ldr.getResources("META-INF/services/" + ifc.getName());
  Collection<S> services = new ArrayList<S>();
  while (e.hasMoreElements()) {
    URL url = e.nextElement();
    InputStream is = url.openStream();
    try {
      BufferedReader r = new BufferedReader(new InputStreamReader(is, "UTF-8"));
      while (true) {
        String line = r.readLine();
        if (line == null)
          break;
        int comment = line.indexOf('#');
        if (comment >= 0)
          line = line.substring(0, comment);
        String name = line.trim();
        if (name.length() == 0)
          continue;
        Class<?> clz = Class.forName(name, true, ldr);
        Class<? extends S> impl = clz.asSubclass(ifc);
        Constructor<? extends S> ctor = impl.getConstructor();
        S svc = ctor.newInstance();
        services.add(svc);
      }
    }
    finally {
      is.close();
    }
  }
  return services;
}

Better exception handling is left as an exercise for the reader. Also, the method could be parameterized to accept a ClassLoader of the caller's choosing.

Answer 2

javax.imageio.spi.ServiceRegistry is the equivalent with prior Java versions. It's available since Java 1.4.

It does not look like a general utility class, but it is. It's even a bit more powerful than ServiceLoader, as it allows some control over the order of the returned providers and direct access to the registry.

See http://docs.oracle.com/javase/7/docs/api/index.html?javax/imageio/spi/ServiceRegistry.html

Answer 3

ServiceLoader is quite basic, and has been in use (informally) within the JDK since 1.3. ServiceLoader just finally made it a first class citizen. It simply looks for a resource file named for your interface, which is basically bundled in the META-INF directory of a library jar.

That file contains the name of the class to load.

So, you'd have a file named:

META-INF/services/com.example.your.interface

and inside it is a single line: com.you.your.interfaceImpl.

In lieu of ServiceLoader, I like Netbeans Lookup. It works with 1.5 (and maybe 1.4).

Out of the box, it does the exact same thing as ServiceLoader, and it's trivial to use. But it offers a lot more flexibility.

Here's a link: http://openide.netbeans.org/lookup/

Here's a article about ServiceLoader, but it mentions Netbeans Lookup at the bottom: http://weblogs.java.net/blog/timboudreau/archive/2008/08/simple_dependen.html