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JavaWorld.com > Java Standard Edition >

Trash talk, Part 2

The Reference Objects API allows programs to interact with the garbage collector

By Jeff Friesen, JavaWorld.com, 01/04/02

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Page 3 of 7

  • Clear the current reference object
  • Add the current reference object to the currently registered reference queue
  • Return the current reference object's referent
  • Determine if the garbage collector has placed the current reference object on a reference queue


The aforementioned operations introduce a reference queue. What are reference queues, and why are they part of the Reference Objects API? I'll answer both questions during our exploration of soft references.

Soft references



The softly reachable state manifests itself in Java through the SoftReference class. When you initialize a SoftReference object, you store a reference to a referent in that object. The object contains a soft reference to the referent, and the referent is softly reachable if there are no other references, apart from soft references, to that referent. If heap memory is running low, the garbage collector can find the oldest softly reachable objects and clear their soft references -- by calling SoftReference's inherited clear() method. Assuming there are no other references to those referents, the referents enter the resurrectable state (if they contain overridden finalize() methods) or the unreachable state (if they lack overridden finalize() methods). Assuming the referents enter the resurrectable state, the garbage collector calls their finalize() methods. If those methods do not make the referents reachable, the referents become unreachable. The garbage collector can then reclaim their memory.

To create a SoftReference object, pass a reference to a referent in one of two constructors. For example, the following code fragment uses the SoftReference(Object referent) constructor to create a SoftReference object, which encapsulates an Employee referent: 

SoftReference sr = new SoftReference (new Employee ());


Figure 2 shows the resulting object structure.

Figure 2. A SoftReference object and its Employee referent

According to Figure 2, the SoftReference object is strongly reachable through root-set variable sr. Also, the Employee object is softly reachable from the soft reference field inside SoftReference.

You often use soft references to implement image and other memory-sensitive caches. You can create an image cache by using the SoftReference and java.awt.Image classes. Image subclass objects allow images to load into memory. As you probably know, images can consume lots of memory, especially if they have large horizontal and vertical pixel dimensions and many colors. If you kept all such images in memory, the object heap would quickly fill up, and your program would grind to a halt. However, if you maintain soft references to Image subclass objects, your program can arrange for the garbage collector to notify you when it clears an Image subclass object's soft reference and moves it to the resurrectable state -- assuming no other references to Image exist. Eventually, assuming the Image subclass object lacks a finalize() method with code that resurrects the image, Image will transition to the unreachable state, and the garbage collector will reclaim its memory.

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