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java.lang.ref.PhantomReference Class in Java

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  • Last Updated : 26 Oct, 2021

When we create an object in Java, an object is strong by default. To create a Phantom Reference Object, we must explicitly specify this to the JVM. Phantom Reference Objects are created as phantom reference object is eligible for garbage collection, but it is not collected instantly. Instead, it is pushed into a ReferenceQueue so that all such enqueued references can be cleared.

Constructors of this class is shown in the table below

Constructor Parameters Constructor Description
PhantomReference (T,ReferenceQueue <T> q) : Creates a new phantom reference that refers to the given object and is registered with the given queue. It is possible to create a phantom reference with a null queue, but such a reference is completely useless: Its get method will always return null and, since it does not have a queue, it will never be enqueued.

Methods inherited from Reference Class are as follows:

Method Name Method Description
get() Returns this reference object’s referent. Because the referent of a phantom reference is always inaccessible, this method always returns null.
clear() Clears this reference object. Invoking this method will not cause this object to be enqueued. 
enque() Adds this reference object to the queue with which it is registered, if any.
isEnqueued() Tells whether this reference object has been enqueued, either by the program or by the garbage collector.

Example 1:

Java




// Java Program to illustrate PhantomReference class
// of java.lang.ref package
 
// Importing PhantomReference and ReferenceQueue classes
// from java.lanf.ref package
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
 
// Class 1
// Helper class
class HelperClass {
 
    // Method inside HelperClass
    void Display()
    {
        // Print statement whenever the function is called
        System.out.println("Display Function invoked ...");
    }
}
 
// Class 2
// Main class
public class GFG {
    // MyClass
    // Main driver method
    public static void main(String[] args)
    {
        // Creating a strong object of HelperClass
        HelperClass obj = new HelperClass();
 
        // Creating a reference queue of HelperClass type
        ReferenceQueue<HelperClass> rq
            = new ReferenceQueue<>();
 
        // Creating a phantom reference object using rq
        PhantomReference<HelperClass> pobj
            = new PhantomReference<>(obj, rq);
 
        // Display message only
        System.out.println(
            "-> Calling Display Function using strong object:");
 
        //  Calling the display method over the object of
        //  HelperClass
        obj.Display();
 
        // Display message for better readability
        System.out.println("-> Object set to null");
 
        obj = null;
 
        // Getting elements in PhantomReference object
        // using standard get() method
        obj = pobj.get();
 
        // Display status of  objects after fetching
        // PhantomReference class objects
        System.out.println(
            "-> Object status after fetching from PhantomReference now : "
            + obj);
 
        // Display message only
        System.out.println(
            "-> Calling Display Function after retrieving from weak Object");
 
        // Try block to check for exceptions
        try {
            obj.Display();
        }
 
        // Catch block to handle the exceptions
        catch (Exception E) {
 
            // Print message when an exception occurred
            System.out.println("-> Error : " + E);
        }
    }
}


Output

-> Calling Display Function using strong object:
Display Function invoked ...
-> Object set to null
-> Object status after fetching from PhantomReference now : null
-> Calling Display Function after retrieving from weak Object
-> Error : java.lang.NullPointerException

Hence, it is seen that unlike Soft and Weak References, Phantom Reference always returns null.

Example 2:

Java




// Java Program to illustrate PhantomReference class
// of java.lang.ref package
 
// Importing Phantomreference and RefereenceQueue classes
// from java.lang.ref package
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
 
// Class 1
// HelperClass
class X {
 
    // Method
    // To print simply
    void show()
    {
 
        // Display message whenever
        // show() method is called
        System.out.println("show () from X invoked..");
    }
}
 
// Class 2
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating default object of X class
        X obj = new X();
 
        // Creating new reference queue object
        ReferenceQueue<X> rq = new ReferenceQueue<X>();
 
        // Creating an object of PhantomReference class
        // of X class type with RefereneQueue object
        // reference queue
        PhantomReference<X> phantomobj
            = new PhantomReference<X>(obj, rq);
 
        // Display message
        System.out.println(
            "-> Trying to retrieve object from Phantom Reference :");
 
        // Try block to check for exceptions
        try {
 
            // this will always throw error as it has been
            // collected
            //  by the garbage collector
            phantomobj.get().show();
        }
 
        // Catch block to handle the exceptions
        catch (Exception e) {
 
            // Print and display the exception
            System.out.println(e);
        }
    }
}


Output

-> Trying to retrieve object from Phantom Reference :
java.lang.NullPointerException

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