The Truth About Garbage Collection

Garbage collection (GC) is probably the most widely misunderstood feature of the Java platform. GC is typically advertised as removing all memory management responsibility from the application developer. This just isn't the case. On the other hand, some developers bend over backwards trying to please the collector, and often wind up doing much more work than is required. A solid understanding of the garbage collection model is essential to writing robust, high-performance software for the Java platform. This appendix provides an overview of the garbage collection mechanisms that will help you make intelligent choices about memory management issues. It also contains information to help you debug complex problems such as memory leaks.

A.1 Why Should You Care About Garbage Collection?

The cost of allocating and collecting memory can play a significant role in how your software performs. The overall memory requirements of your software can have a huge impact on the speed of your program when large RAM requirements force the OS to use virtual memory. This often occurs when memory is allocated, but not properly released. Although the JVM is responsible for freeing unused memory, you have to make it clear what is unused. To write successful, large-scale programs, you need to understand the basics of the GC mechanism.

A.2 The Guarantees of GC

The specification for the Java platform makes very few promises about how garbage collection actually works. Here is what the Java Virtual Machine Specification (JVMS) has to say about memory management.

The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java virtual machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor's system requirements.¹

While it can seem confusing, the fact that the garbage collection model is not rigidly defined is actually important and useful-a rigidly defined garbage collection model might be impossible to implement on all platforms. Similarly, it might preclude useful optimizations and hurt the performance of the platform in the long term.
Although there is no one place that contains a full definition of required garbage collector behavior, much of the GC model is implicitly specified through a number of sections in the Java Language Specification and JVMS. While there are no guarantees about the exact process followed, all compliant virtual machines share the basic object lifecycle described in this chapter.

A.3 The Object Lifecycle

In order to discuss garbage collection, it is first useful to examine the object lifecycle. An object typically goes through most of the following states between the time it is allocated and the time its resources are finally returned to the system for reuse.

1. Created
2. In use (strongly reachable)
3. Invisible
4. Unreachable
5. Collected
6. Finalized
7. Deallocated

A.3.1 Created

When an object is created, several things occur:²

1. Space is allocated for the object.
2. Object construction begins.
3. The superclass constructor is called.
4. Instance initializers and instance variable initializers are run.
5. The rest of constructor body is executed.

The exact costs of these operations depend on the implementation of the JVM, as well as the implementation of the class being constructed. The thing to keep in mind is that these costs exist. Once the object has been created, assuming it is assigned to some variable, it moves directly to the in use state.

A.3.2 In Use

Objects that are held by at least one strong reference are considered to be in use. In JDK 1.1.x, all references are strong references. Java 2 introduces three other kinds of references: weak, soft and phantom. (These reference types are discussed in Section A.4.1.) The example shown in Listing A-1 creates an object and assigns it to some variables.

public class CatTest {
        static Vector catList = new Vector();
        static void makeCat() {
                Object cat = new Cat();
                catList.addElement(cat);
        }
        
        public static void main(String[] arg) {
                makeCat();
                // do more stuff
        }
}

Creating and referencing an object

Figure A-1 shows the structure of the objects inside the VM just before the makeCat method returns. At that moment, two strong references point to the Cat object.

Object reference graph

When the makeCat method returns, the stack frame for that method and any temporary variables it declares are removed. This leaves the Cat object with just a single reference from the catList static variable (indirectly via the Vector).

A.3.3 Invisible

An object is in the invisible state when there are no longer any strong references that are accessible to the program, even though there might still be references. Not all objects go through this state, and it has been a source of confusion for some developers. Listing A-2 shows a code fragment that creates an invisible object.

public void run() {
    try {
        Object foo = new Object();
        foo.doSomething();
    } catch (Exception e) {
        // whatever
    }
    while (true) { // do stuff } // loop forever
}

Invisible object

In this example, the object foo falls out of scope when the try block finishes. It might seem that the foo temporary reference variable would be pulled off the stack at this point and the associated object would become unreachable. After all, once the try block finishes, there is no syntax defined that would allow the program to access the object again. However, an efficient implementation of the JVM is unlikely to zero the reference when it goes out of scope. The object referenced by foo continues to be strongly referenced, at least until the run method returns. In this case, that might not happen for a long time. Because invisible objects can't be collected, this is a possible cause of memory leaks. If you run into this situation, you might have to explicitly null your references to enable garbage collection.

A.3.4 Unreachable

An object enters an unreachable state when no more strong references to it exist. When an object is unreachable, it is a candidate for collection. Note the wording: Just because an object is a candidate for collection doesn't mean it will be immediately collected. The JVM is free to delay collection until there is an immediate need for the memory being consumed by the object. It's important to note that not just any strong reference will hold an object in memory. These must be references that chain from a garbage collection root. GC roots are a special class of variable that includes

• Temporary variables on the stack (of any thread)
• Static variables (from any class)
• Special references from JNI native code

Circular strong references don't necessarily cause memory leaks. Consider the code in Listing A-3. It creates two objects, and assigns them references to each other.

public void buidDog() {
   Dog newDog = new Dog();
   Tail newTail = new Tail();
   newDog.tail = newTail;
   newTail.dog = newDog;
}

Circular reference

Figure A-2 shows the reference graph for the objects before the buildDog method returns. Before the method returns, there are strong references from the temporary stack variables in the buildDog method pointing to both the Dog and the Tail.

Reference graph before buildDog returns

Reference graph after buildDog returns

Figure A-3 shows the graph for the objects after the buildDog method returns. At this point, the Dog and Tail both become unreachable from a root and are candidates for collection (although the VM might not actually collect these objects for an indefinite amount of time).

A.3.5 Collected

An object is in the collected state when the garbage collector has recognized an object as unreachable and readies it for final processing as a precursor to deallocation. If the object has a finalize method, then it is marked for finalization. If it does not have a finalizer then it moves straight to the finalized state. If a class defines a finalizer, then any instance of that class must have the finalizer called prior to deallocation. This means that deallocation is delayed by the inclusion of a finalizer.

A.3.6 Finalized

An object is in the finalized state if it is still unreachable after its finalize method, if any, has been run. A finalized object is awaiting deallocation. Note that the VM implementation controls when the finalizer is run. The only thing that can be said for certain is that adding a finalizer will extend the lifetime of an object. This means that adding finalizers to objects that you intend to be short-lived is a bad idea. You are almost always better off doing your own cleanup instead of relying on a finalizer. Using a finalizer can also leave behind critical resources that won't be recovered for an indeterminate amount of time. If you are considering using a finalizer to ensure that important resources are freed in a timely manner, you might want to reconsider. One case where a finalize method delayed GC was discovered by the quality assurance (QA) team working on Swing. The QA team created a stress testing application that simulated user input by using a thread to send artificial events to the GUI. Running on one version of the toolkit, the application reported an OutOfMemoryError after just a few minutes of testing. The problem was finally traced back to the fact that the thread sending the events was running at a higher priority than the finalizer thread. The program ran out of memory because about 10,000 Graphics objects were held in the finalizer queue waiting for a chance to run their finalizers. It turned out that these Graphics objects were holding onto fairly substantial native resources. The problem was fixed by assuring that whenever Swing is done with a Graphics object, dispose is called to ensure that the native resources are freed as soon as possible.
In addition to lengthening object lifetimes, finalize methods can increase object size. For example, some JVMs, such as the classic JVM implementation, add an extra hidden field to objects with finalize methods so that they can be held in a linked list finalization queue.

A.3.7 Deallocated

The deallocated state is the final step in garbage collection. If an object is still unreachable after all the above work has occurred, then it is a candidate for deallocation. Again, when and how deallocation occurs is up to the JVM.

A.4 Reference Objects

Prior to the introduction of the Java 2 platform, all references were strong references. This meant that there was no way for the developer to interact with the garbage collector, except through brute force methods such as System.gc. The java.lang.ref package was introduced as part of Java 2. Figure A-4 shows the class hierarchy for the classes in this package. This package defines reference-object classes that enable a limited degree of interaction with the garbage collector. Reference objects are used to maintain a reference to some other object in such a way that the collector can still reclaim the target object. As you might expect, the addition of these new reference objects complicates the concept of reachability as defined in the object lifecycle. Understanding this is important, even if you don't intend to make direct use of this package. Some of the core class libraries use WeakReferences internally, so you might encounter them while using memory profilers to track memory usage.

Resurrection It is possible to create new strong references to an object while executing the finalizer method. This puts the object back into an in-use state. This practice, known as resurrection, is a bad idea. The specification guarantees that a finalizer is run at most one time per object. Because the finalizer is not run a second time, resurrecting an object can lead to serious problems. For more information about resurrection, see Ken Arnold and James Gosling, The Java Programming Language, Section 2.10.2. Addison-Wesley, 1998.

Reference class hierarchy

A.4.1 Types of Reference Objects

Three types of reference objects are provided, each weaker than the last: soft, weak, and phantom. Each type corresponds to a different level of reachability:

• Soft references are for implementing memory-sensitive caches.
• Weak references are for implementing mappings that do not prevent their keys (or values) from being reclaimed.
• Phantom references are for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

Going from strongest to weakest, the different levels of reachability reflect the lifecycle of an object:

• An object is strongly reachable if some thread can reach it without traversing any reference objects.
• An object is softly reachable if it is not strongly reachable but can be reached by traversing a soft reference.
• An object is weakly reachable if it is neither strongly nor softly reachable but can be reached by traversing a weak reference. When the weak references to a weakly reachable object are cleared, the object becomes eligible for finalization.
• An object is phantom reachable if it is neither strongly, softly, nor weakly reachable, it has been finalized, and some phantom reference refers to it.
• An object is unreachable, and therefore eligible for reclamation, when it is not reachable in any of the preceding ways.

A.4.2 Example GC with WeakReference

You're likely to encounter special reference objects while using tools to look for memory leaks. Only strong references will directly interfere with garbage collection. If you find chains of objects linked by weak references, you should be able to ignore them from a GC perspective. (For additional information on the use of special reference objects, see the API documentation.) Figure A-5 shows a graph of objects in memory for a sample program. Let's say that the problem with this program is that the Dog objects are not being collected, leading to a memory leak. By using a memory profiler, you can find all the pointers to the Dog object and follow them back to their GC roots. There are two GC roots in Figure A-5, a static variable in class Kennel and a stack frame in a live thread. In this case, the WagTask thread is in an infinite loop, forcing the dog's tail to wag. The question is how to get rid of the Dog object.
There are two references pointing to the Dog object, but only one of them is interesting from a GC perspective. The WeakReference from the dogCache is not important. The interesting reference is the reference from the Tail, which chains from a stack frame in a live thread. To free the Dog, and the associated Tail, you need to terminate the thread that is wagging the Tail. Once this thread is gone, everything falls into place. When an object that is pointed to by a WeakReference is collected, the WeakReference is automatically set to null. Figure A-6 shows the result of terminating the wag thread.

Reference graph

When the thread dies, its stack is removed. Now the only strong reference to the Dog is via the Tail, and this becomes a simple circular reference that isn't reachable from a GC root. The Dog, and by extension the Tail, are no longer strongly reachable through any references. They are only weakly reachable through the dogCache. When the collector discovers this (which it does on its own schedule), it might set the weak reference to null, making the Dog and Tail totally unreachable. They then become candidates for collection and will be removed at the collector's discretion.

Results of garbage collection

A.5 References on Garbage Collection

Arnold, Ken, and James Gosling. The Java Programming Language, Second Edition, Addison-Wesley, Reading, MA, 1998.
Gosling, James, Bill Joy, and Guy Steele. The Java Language Specification, Second Edition, Addison-Wesley, Reading, MA, 2000.
Jones, Richard, and Rafael Lins. Garbage Collection: Algorithms for Automatic Dynamic Memory Management, John Wiley & Sons, New York, 1996.
Lindholm, Tim and Frank Yellin. The Java Virtual Machine Specification, Second Edition, Addison-Wesley, Reading, MA, 1999.