Garbage collection and asynchronous calls / Future objects

Question

Below is a sample code that utilizes the Future interface to make an asynchronous call. I need some clarification about the get() method.

Future<String> future = getAsyncString();
//do something ...
String msg = "";
if (validation)
    return;
else
    msg = future.get();
//do something else...
return;

The future variable is initialized in a method , so the variable will be soon cleared by the GC after the method's execution as it is no longer used. So in case that the code enters the if statement , what will be the state of the JVM? How is the JVM going to handle the wrapped result in case that noone is going to read it back? Does it affect the Thread Pool , or the thread Executor?

Answer 1

How is the JVM going to handle the wrapped result in case that noone is going to read it back?

Presumably you got the Future object from an Executor. For this executor to be able to set the result in the Future, it holds a reference to the Future. In other words, just because the method local reference to the object disappears as the call stack is popped, doesn't mean that the Future object (which is on the heap) is automatically eligible for garbage collection.

The async call is not cancelled or anything like that. The executor will perform the call, fill in the result, and presumably drop it's reference to the Future object. At this point the object becomes unreachable and eligible for garbage collection.

If you're certain that your code doesn't keep a reference to the Future object (i.e. leaking it in the // do something... part) then you can be sure that the Future object is (eventually) collected by the GC. (The executor doesn't have any subtle memory leaks here.)

[...] so the variable will be soon cleared by the GC.

To be precise, the variable will be discarded as the call stack is popped. This will eventually cause the Future object to be unreachable and eligible for garbage collection. The object will however typically not be garbage collected immediately as the method returns.