Garbage Collection Certification - Behaviour

• the Java garbage collector consists of three basic activities:
  1. monitors program objects to determine when they are no longer required
  2. informs selected objects that they should release any non-memory resources
  3. destroys objects and reclaims their memory resources
• the gc operates as a seperate asynchronous background thread that tracks all program objects
• an object ceases to be needed by a program when it is no longer reachable
• an object is reachable if a reference to the object exists in any variables of any executing code
• an object is subject to garbage collection when it can no longer be reached but it is not necessarily garbage collected immeadiately
• there are no guarantees as to when the gc will reclaim an object or the order in which objects are reclaimed
• there is no way to tell if and when an object will be collected, you can only tell when an object becomes eligible for garbage collection
• you can request garbage collection by calling one of

    Runtime.getRuntime().gc() // no guarantee gc will run 
    System.gc() // no guarantee gc will run   

• you can also request that the finalize() method be run for objects deemed eligible for collection but which have not yet had their finalization code run

    Runtime.runFinalization()
    System.runFinalization()

Also see

• Garbage Collection in Java
• Sun Tech Tip: Reference Objects
• Sun Tech Tip: Performance tip: Garbage Collection and setting to null

Traps

• a question that targets a specific object for garbage collection (can't be done)
• a question that presumes to force the gc to run (can only suggest it run)