How do I "close" a ClassLoader?

Question

I have a case where I need to create a lot of class loaders in my application to temporarily make some code visible while user supplied scripts are running. I'm using an URLClassLoader for this and it works pretty well.

When the script terminates, I want to "unload" or "close" the class loader to free the resources.

Is it enough to set the reference to the class loader to null? I'm especially wondering if I'll eventually run out of file handles because the extra classes are in JAR files.

PS: Must work with Java 5 and up. Yeah, I know...

Answer 1

A little late, but hopefully this'll be helpful for those who come to this Question later (like me).

With Java 7, a close() method has been added to URLClassLoader, which is exactly what OP was asking for.

EDIT (thanks to @Hot Licks): OK, so it isn't exactly what the OP has asked for. It doesn't free up all the resources, or make the resources and the loader collectible. It simply prevents the loading of more resources using the class loader. It does, however, close the jar file that was loaded with the URLClassLoader.

Answer 2

If you can't use Java7 and it's close() method, use reflection to close all open JAR archives of a classloader, like so:

public void close() {
try {
   Class clazz = java.net.URLClassLoader.class;
   java.lang.reflect.Field ucp = clazz.getDeclaredField("ucp");
   ucp.setAccessible(true);
   Object sun_misc_URLClassPath = ucp.get(this);
   java.lang.reflect.Field loaders = 
      sun_misc_URLClassPath.getClass().getDeclaredField("loaders");
   loaders.setAccessible(true);
   Object java_util_Collection = loaders.get(sun_misc_URLClassPath);
   for (Object sun_misc_URLClassPath_JarLoader :
        ((java.util.Collection) java_util_Collection).toArray()) {
      try {
         java.lang.reflect.Field loader = 
            sun_misc_URLClassPath_JarLoader.getClass().getDeclaredField("jar");
         loader.setAccessible(true);
         Object java_util_jar_JarFile = 
            loader.get(sun_misc_URLClassPath_JarLoader);
         ((java.util.jar.JarFile) java_util_jar_JarFile).close();
      } catch (Throwable t) {
         // if we got this far, this is probably not a JAR loader so skip it
      }
   }
} catch (Throwable t) {
   // probably not a SUN VM
}
return;
}

Answer 3

When all classes that the class loader loaded no longer have any references, and all references to the class loader itself have been erased, the class loader and the classes it loaded will be garbage collected as a group.

Note that this is dependent on having the JVM attribute set that causes unreferenced classes to be unloaded. It's set by default in most environments, but may not be in some embedded cases.

[Note that it's a non-trivial matter to remove references to a class. Any other class that references it by name will of course prevent removal. So the class must be loaded using ClassLoader.findClass or something similar.]

Answer 4

If you do not longer have classes (and object) loaded from that classloader, and if you do not keep any reference to that classloader, it will automatically handled by the garbage collector.

Answer 5

There are no close() methods in URL class loader or any of its parent classes, so you're out of luck.

Shouldn't GC handle this?