Java: Strong References, Soft References and Weak References

There are many use-cases where we want to control, when our objects are collected by GC(Garbage Collector). Package "java.lang.ref" provides reference-object classes, which support a limited degree of interaction with the garbage collector. And GC reclaim the object memory based on it's reachability.

Strong Reference

Newly created objects are strongly reachable by the thread that created it. So object will be remain in heap till thread is running or some variable hold this object reference.

//leaking array
private static List<String> leakingArrayWithStrongRef = 
                                new LinkedList<String>();

    public void StrongReferenceTest(){
        try {
            for (int i = 0; i < 500000; i++) {
                String data = LEAKING_DATA + i;
                // Add data to our leaking array...
                leakingArrayWithStrongRef.add(data);
            }
        } catch (OutOfMemoryError ofm) {
            System.out.println("OutOfMemoryError");
        } catch (Throwable e) {
            System.out.println("UnexpectedError");
        }        
    }



imageHeap Memory trend chart when unwanted objects added in Array

In this example array hold the strong reference of string. As we start adding string object in array then heap memory increase to it's max limit and throws "OutOfMemoryEerror" exception.



Soft Reference

Soft Reference are the softly reachable object. These kind of object will remain in the heap memory till GC clean them out in need of memory.


//leaking array
private static List<SoftReference<String>> leakingArrayWithSoftRef = 
                                new LinkedList<SoftReference<String>>();

    public void SoftReferenceTest(){
        try {
            for (int i = 0; i < 500000; i++) {
                SoftReference<String> data = new SoftReference<String>(LEAKING_DATA + i);
                // Add data to our leaking array...
                leakingArrayWithSoftRef.add(data);
            }
        } catch (OutOfMemoryError ofm) {
            System.out.println("OutOfMemoryError");
        } catch (Throwable e) {
            System.out.println("UnexpectedError");
        }     
    }



imageHeap Memory trend chart when unwanted objects added in Array

In this example array hold the soft reference of string object. As we start adding string object in array then heap memory increase to it's max limit and GC reclaim the soft reference object from heap. And in this code you will not get "OutOfMemoryEerror".



Weak Reference

Weak Reference are weakly reachable objects. And these kind of object will reclaim by the GC at any point of time.


//leaking array
private static List<WeakReference<String>> leakingArrayWithWeakRef = 
                                new LinkedList<WeakReference<String>>();

    public void WeakReferenceTest(){
        try {
            for (int i = 0; i < 500000; i++) {
                WeakReference<String> data = new WeakReference<String>(LEAKING_DATA + i);
                // Add data to our leaking array...
                leakingArrayWithWeakRef.add(data);
            }
        } catch (OutOfMemoryError ofm) {
            System.out.println("OutOfMemoryError");
        } catch (Throwable e) {
            System.out.println("UnexpectedError");
        }       
    }



image Heap Memory trend chart when unwanted objects added in Array



In this example array hold the weak reference of string object. As we start adding string object in array then heap memory start increasing slightly and decrease after few seconds. It's happen because GC clean these object from the array more frequently. And in this code you will not get "OutOfMemoryEerror".



PS: I have used "jconsole.exe" to generate above graphs, which comes as a part of "JDK" toolkit.

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