Understanding Java Reference Types: From Strong to Phantom References and Their Practical Applications

From JDK 1.2, Java introduced different types of references: Strong Reference, Soft Reference, Weak Reference, and Phantom Reference.

The hierarchy of references is as follows:
Strong Reference > Soft Reference > Weak Reference > Phantom Reference

In practice, different types of references offer a wide range of functionalities. For example:

• Netty framework introduced a reference counting mechanism for message management by weak references starting with version 4.x.
• The ThreadLocal implementation in the JDK stores its references as weak references in a map associated with the Thread object.
• Perhaps we can implement our own caching mechanism, where objects are reclaimed when memory is running low.

1. Strong Reference

A strong reference is the traditional reference assignment in Java. As long as the reference exists, the garbage collector will not reclaim the object.

/**-------Strong Reference--------**/
int[] t1 = new int[10240000];
System.gc();// Full GC will not reclaim the objects

2. Soft Reference

A soft reference is used to refer to objects that should only be reclaimed when the JVM is low on memory. The object is eligible for garbage collection before an OutOfMemoryError occurs.

/**-------Soft Reference-----------**/
SoftReference t2 = new SoftReference<>(new int[10240000]);
System.gc();// The object referenced by the soft reference remains intact

Next, the code is modified to continuously allocate large objects. The soft reference objects will be reclaimed only before the memory overflows (when allocation fails).

/**-------Soft Reference-----------**/
SoftReference t2 = new SoftReference<>(new int[512000000]);
SoftReference t3 = new SoftReference<>(new int[512000000]);
System.gc();

3. Weak Reference

A weak reference allows the referenced object to live only until the next garbage collection. Regardless of whether the system has enough memory, the object will be reclaimed when the garbage collection occurs.

/**-------Weak Reference-----------**/
WeakReference t3 = new WeakReference<>(new int[10240000]);
System.gc();//The weak reference object will be reclaimed after the garbage collection process.

4. Phantom Reference

A phantom reference does not affect the lifetime of the referenced object. It cannot be used to retrieve the object itself. The sole purpose of a phantom reference is to notify when the object is about to be garbage collected.