Java Garbage collection articles

http://www.cubrid.org/blog/dev-platform/understanding-java-garbage-collection/

http://www.cubrid.org/blog/dev-platform/the-principles-of-java-application-performance-tuning/


nce article on GC
http://www.dynatrace.com/en/javabook/the-three-jvms.html

     Garbage-Collection Roots — The Source of All Object Trees

Every object tree must have one or more root objects. As long as the application can reach those roots, the whole tree is reachable. But when are those root objects considered reachable? Special objects called garbage-collection roots (GC roots; see Figure 2.2) are always reachable and so is any object that has a garbage-collection root at its own root.
There are four kinds of GC roots in Java:

1. Local variables are kept alive by the stack of a thread. This is not a real object virtual reference and thus is not visible. For all intents and purposes, local variables are GC roots.
2. Active Java threads are always considered live objects and are therefore GC roots. This is especially important for thread local variables.
3. Static variables are referenced by their classes. This fact makes them de facto GC roots. Classes themselves can be garbage-collected, which would remove all referenced static variables. This is of special importance when we use application servers, OSGi containers or class loaders in general. We will discuss the related problems in the Problem Patterns section.
4. JNI References are Java objects that the native code has created as part of a JNI call. Objects thus created are treated specially because the JVM does not know if it is being referenced by the native code or not. Such objects represent a very special form of GC root, which we will examine in more detail in the Problem Patterns section below.

Figure 2.2: GC roots are objects that are themselves referenced by the JVM and thus keep every other object from being garbage-collected.

Therefore, a simple Java application has the following GC roots:

• Local variables in the main method
• The main thread
• Static variables of the main class

Marking and Sweeping Away Garbage

To determine which objects are no longer in use, the JVM intermittently runs what is very aptly called a mark-and-sweep algorithm . As you might intuit, it's a straightforward, two-step process:

1. The algorithm traverses all object references, starting with the GC roots, and marks every object found as alive.
2. All of the heap memory that is not occupied by marked objects is reclaimed. It is simply marked as free, essentially swept free of unused objects.

Garbage collection is intended to remove the cause for classic memory leaks: unreachable-but-not-deleted objects in memory. However, this works only for memory leaks in the original sense. It's possible to have unused objects that are still reachable by an application because the developer simply forgot to dereference them. Such objects cannot be garbage-collected. Even worse, such a logical memory leak cannot be detected by any software (see Figure 2.3). Even the best analysis software can only highlight suspicious objects. We will examine memory leak analysis in the Analyzing the Performance Impact of Memory Utilization and Garbage Collection section, below.

Figure 2.3: When objects are no longer referenced directly or indirectly by a GC root, they will be removed. There are no classic memory leaks. Analysis cannot really identify memory leaks; it can only point out suspicious objects.