这是垃圾收集还是没收？

Question

Here is some code:

public class A {
  private volatile B b;

  public void methodC() {
    b.doSomething();
  }

  public void setB(B newB) {
    this.b = newB;
  }
}

'Thread 1' is executing b.doSomething() by executing methodC().

At the same time 'thread 2' set a new B object into 'b'.

My question is:

Could the object previously refereced by 'b' be garbage-collected although doSomething() method on it is still executing?

Answer 1

No, because in order to call a member function you need to have a reference to the object. Therefore the thread which is calling b.doSomething() will be holding a reference thus preventing garbage collection.

Although check Jon Harrop's answer below for a situation when b could be GC 'd.

Answer 2

Could the object previously refereced by 'b' be garbage-collected although doSomething() method on it is still executing?

Yes, the object previously refereced by 'b' might be garbage-collected even though the doSomething() method on it is still executing.

You might expect this to be reachable while one of its methods is being executed because methods are implicitly passed the this pointer and you might assume that it is always spilled to the stack. However, many optimizations can alter this and remove this from the set of global roots seen by the GC.

For example, if the body of the doSomething method ends with code that does not require this and the method is inlined then this might not be spilled from a register to the stack or its stack slot might be overwritten. In either case, the GC will no longer see this even though execution is still inside doSomething and, therefore, this might be garbage collected.