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.