Life cycle of a new object without reference

Question

My question is related to:

• Java: What is the purpose of creating an object in the heap with no reference
• Java - Can objects which are executing methods be garbage-collected?

What actually happens when we have something like this in our code:

 (new SomeClass()).longMethod();

Is there still some sort of unnamed (strong ?) reference pointing to the newly created object on Heap put on Stack?

If nothing is on Stack, then how does Garbage Collector know to preserve the object for the duration of the method?

Is it possibly the same as

{ 
  // very local scope
  SomeClass throwAwayRef = new SomeClass();
  throwAwayRef.longMethod();
}

Answer 1

You can look at the bytecode for insight:

   0: new           #16                 // class SomeClass
   3: dup
   4: invokespecial #18                 // Method SomeClass."<init>":()V
   7: invokevirtual #19                 // Method SomeClass.longMethod:()V

• new actually allocates the object, a reference to which is pushed on the stack.
• dup duplicates the top-of-stack; now the top two stack items are references to the newly created object.
• invokespecial here calls the constructor of SomeClass, popping the stack; now the stack consists of just a single reference to our SomeClass instance. The instance is not GCed because a reference to it exists on the stack.
• invokevirtual here calls longMethod. Again, the instance is not GCed because a reference to it still exists on the stack (and is popped after the method completes, after which it is eligible for GC).

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(new SomeClass()).longMethod();

is not the same as

{ 
  // very local scope
  SomeClass throwAwayRef = new SomeClass();
  throwAwayRef.longMethod();
}

at the bytecode level, since the latter involves an astore and an aload. However, the two are certainly functionally equivalent. The SomeClass instance still becomes eligible for GC after longMethod completes (the stacks for the two snippets will look identical when invokevirtual is executed).

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Reference:

• Java bytecode instruction listings