WeakReferenced object is not garbage collected after calling System.gc()

Question

I am a fresh new learner of Java. I\'m now learning the concept of WeakReference. I came across a problem which probably looks stupid but I just wanna figure out the reason.

Answer 1

Firstly, System.gc() does not ensure a garbage collection. Instead, it's just a hint that "It's a good time to run garbage collection".

Secondly, in your code when you put A a1 = wr.get(); before calling System.gc(), it creates a new strong reference to the same object referenced by a, thus even if garbage collection runs, your object will not be garbage collected.

As we have two tasks in hand

1. Ensure garbage collection
2. Don't keep any strong reference to the object you want to be garbage collected

Let's do little modification to your code

public class A {
    public static void main(String[] args) {
        A a = new A();
        WeakReference wr = new WeakReference<>(a);
        a = null;
        // A a1 = wr.get(); Removing this, this does our 2nd task
        System.out.println(a);
        // System.out.println(a1); Removing this as a1 does not exists anymore
        try {
            while (null != wr.get()) { // 1st task done, the loop ensures sending the hint until your object collected
                System.gc();
                // Thread.sleep(10000); it does not have impact
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(wr.get()); // Obviously prints null
    }

    @Override
    protected void finalize() {
        System.out.println(Thread.currentThread().getName() + ": See ya, nerds!");
    }
}