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

As of JDK 18, no. JDK 18 omits enclosing instance fields from inner classes that don’t use it.

However, prior to JDK 18, yes, instances of anonymous inner classes hold on to a reference to their enclosing instances even if these references are never actually used. For example, 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.