Do anonymous classes *always* maintain a reference to their enclosing instance?

Question

I'm working with some code where one object, "foo", is creating another object, "bar", and passing it a Callable. After this foo will return bar, and then I want foo to become unreachable (ie: available for garbage collection).

My initial thought was to just create the Callable anonymously. eg:

class Foo {
  ...

  public Bar createBar() {
    final int arg1 = ...
    final int arg2 = ...
    final int arg3 = ...
    return new Callable<Baz>() {
      @Override
      public Baz call() {
        return new Baz(arg1, arg2, arg3);
      }
    };
  }
}

It occurred to me that this might not actually work as desired, however, as an inner class typically keeps a reference to its enclosing object. I don't want a reference to the enclosing class here, because I want the enclosing object to be collected while the Callable is still reachable.

On the other hand, detecting that the enclosing instance is never actually referred to should be pretty trivial, so perhaps the Java compiler is smart enough to not include a reference in that case.

So... will an instance of an anonymous inner class hold on to a reference to its enclosing instance even if it never actually uses the enclosing instance reference?

Answer 1

Yes, instances of anonymous inner classes hold on to a reference to their enclosing instances even if these references are never actually used. This code:

public class Outer {
  public Runnable getRunnable() {
    return new Runnable() {
      public void run() {
        System.out.println("hello");
      }
    };
  }
}

When compiled with javac generates two class files, Outer.class and Outer$1.class. Disassembling the latter, the anonymous inner class, with javap -c yields:

Compiled from "Outer.java"
class Outer$1 extends java.lang.Object implements java.lang.Runnable{
final Outer this$0;

Outer$1(Outer);
  Code:
   0:   aload_0
   1:   aload_1
   2:   putfield        #1; //Field this$0:LOuter;
   5:   aload_0
   6:   invokespecial   #2; //Method java/lang/Object."<init>":()V
   9:   return

public void run();
  Code:
   0:   getstatic       #3; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #4; //String hello
   5:   invokevirtual   #5; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

}

The putfield line shows that a reference to the enclosing instance is being stored in the field this$0 (of type Outer) by the constructor even though this field is never used again.

This is unfortunate if you're attempting to create small potentially long-lived objects with anonymous inner classes as they'll hold onto the (potentially large) enclosing instance. A workaround is to use an instance of a static class (or a top-level class) instead. This is unfortunately more verbose.