WeakReference (T referent, ReferenceQueue q)

public WeakReference(T referent, ReferenceQueue<? super T> q)

Creates a new weak reference that refers to the given object and is registered with the given queue.

import java.lang.ref.*;

public class WeakReferenceEx1 {
    
    static WeakReference<Employee> weakRef1;
    static WeakReference<Employee> weakRef2;
    
    static ReferenceQueue<Employee> refQueue = new ReferenceQueue();
    
    static class Employee{
        String name;
        int id;

        Employee(int id, String name){
            this.id =id;
            this.name = name;
        }

        @Override
        public String toString(){
            return id +" " + name;
        }

        @Override
        protected void finalize() throws Throwable{
            try {
                System.out.println(this + " is going to be garbage collected");
            } 
            finally {
                super.finalize();
            }
        }
    }
    
    public static void main(String args[]) throws InterruptedException{
        Employee emp1 = new Employee(1, "Krishna");
        Employee emp2 = new Employee(1, "Hari");
        
        weakRef1 = new WeakReference<>(emp1, refQueue);
        weakRef2 = new WeakReference<>(emp2, refQueue);
        
        System.out.println("Calling Garbage collector");
        System.gc();
        System.out.println("Waiting for 1 second");
        Thread.sleep(1000);
        
        System.out.println("Setting emp1 and emp2 to null");
        emp1 = null;
        emp2 = null;
        System.gc();
        System.out.println("weakRef1 referent value is "+weakRef1.get());
        System.out.println("weakRef2 referent value is "+weakRef2.get());
        
        Thread.sleep(1000);
        System.out.println("weakRef1 = " + weakRef1);
        System.out.println("weakRef2 = " + weakRef2);
        
        System.out.println(refQueue.poll());
        System.out.println(refQueue.poll());
        System.out.println(refQueue.poll());
        
    }
}

Output

Calling Garbage collector
Waiting for 1 second
Setting emp1 and emp2 to null
weakRef1 referent value is null
weakRef2 referent value is null
1 Hari is going to be garbage collected
1 Krishna is going to be garbage collected
weakRef1 = java.lang.ref.WeakReference@1db9742
weakRef2 = java.lang.ref.WeakReference@106d69c
java.lang.ref.WeakReference@106d69c
java.lang.ref.WeakReference@1db9742
null

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