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java.lang.ref.SoftReference Class in Java

  • Last Updated : 06 Aug, 2021

When we create an object in Java, an object isn’t soft by default. To create a Soft Reference Object, we must explicitly specify this to the JVM. In Soft reference, even if the object is free for garbage collection then also it’s not garbage collected until JVM is in need of memory badly. The objects get cleared from the memory when JVM runs out of memory.

Why Soft Reference Objects are used?

When a soft reference object is created, it is marked for garbage collection. However, it is not garbage collected until and unless there is a deficiency of memory in the JVM.

java.lang.ref.SoftReference Class in Java

Constructors in the SoftReference Class :



Constructor parameters

Constructor Description

SoftReference ( T referent)Creates a new soft reference that refers to the given object.
SoftReference ( T referent, ReferenceQueue <T> q)Creates a new soft reference that refers to the given object and is registered with the given queue.

get() Method:

Java




// Java program to show the demonstration
// of get() method of SoftReference Class
 
import java.lang.ref.SoftReference;
class GFG {
    public static void main (String[] args) {
       
        // creating a strong object of MyClass
          MyClass obj = new MyClass ();
       
          // creating a weak reference of type MyClass
          SoftReference<MyClass> sobj = new SoftReference<>(obj);
       
          System.out.println ("-> Calling Display Function using strong object:");
          obj.Display ();   
       
          System.out.println ("-> Object set to null");
       
          obj = null;
       
        // Calling the get() method
          obj = sobj.get();
          System.out.println ("-> Calling Display Function after retrieving from soft Object");
          obj.Display ();
    }
}
class MyClass {
      void Display ()
    {
        System.out.println ("Display Function invoked ...");
    }
}
Output
-> Calling Display Function using strong object:
Display Function invoked ...
-> Object set to null
-> Calling Display Function after retrieving from soft Object
Display Function invoked ...

Example Showing enqueue() and isEnqueued() method of SoftReference Class:

Java




// Java program demonstrating all the methods
// of SoftRefernce Class
 
import java.lang.ref.SoftReference;
class GFG {
    public static void main (String [] args) {
      
        // Creating object of Class X
        X obj = new X ();
       
        // Creating a soft reference of type X
          SoftReference <X> softobj = new SoftReference <X> (obj);
       
          System.out.println ("-> Retrieving object from Soft Reference using get ()");
          softobj.get().show();
       
          System.out.println ("-> Is it possible to queue object using enqueue ()");
          System.out.println (softobj.enqueue ());
       
          System.out.println ("-> Checking if reference is queued using isEnqueued ()");
          System.out.println (softobj.isEnqueued ());
    }
}
class X {
    void show()
    {
        System.out.println ("show () from X invoked..");
    }
}
Output
-> Retrieving object from Soft Reference using get ()
show () from X invoked..
-> Is it possible to queue object using enqueue ()
false
-> Checking if reference is queued using isEnqueued ()
false
Output
-> Retreiving object from Soft Reference using get ()
show () from X invoked..
-> Is it possible to queue object using enqueue ()
false
-> Checking if reference is queued using isEnqueued ()
false

Methods inherited from Reference Class: 

Method Name

Method Description

clear ()Clears this reference object.
enqueue ()Adds this reference object to the queue with which it is registered, if any.
get ()Returns this reference object’s referent.
isEqueued ()Tells whether this reference object has been enqueued, either by the program or by the garbage collector.

 

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