Garbage Collection

Introduction

• All GC involves in making sure the heap doesnt run out of space.
• Purpose of the GC is to find and delete objects that cannot be reached.
• An object is eligible to be garbage collected when no live thread can access it.

Ways in which Objects can be GC'ed

• Nulling a reference - Explicitly assigning a reference to null will make it eligible for GC.
• Reassigning a reference - Reassigning the reference to another object will cause older object to be eligible for GC.
  • Local variables in a method will be eligible for GC when the method returns.
• Isolating a reference - Even if objects have valid references, but still there is no way to reach them, they will be GC'ed.

Invoking GC

• Just a request, no guarantee that GC will happen.
• Runtime.getRuntime().gc() or System.gc() as a convenience method.

Finalize Method

• Any code that needs to be run before the object is GC'ed can be included in the finalize() method.
• The finalize() method is defined in Object. (protected void finalize() throws Throwable)
• For any object, finalize() will called ONLY ONCE by the GC. (An object can be gc'ed only once!)
• Sometimes calling finalize() can cause the object not to be GC'ed if a reference to the current object is assigned somewhere else.
• Any exception thrown but not caught by a finalizer that is called by the GC will be ignored.
• The JVM remembers that finalize() already ran and will not run it again.
• Enum clases cannot have finalize() methods.
• Finalizers are not chained explicitly like constructors are. So a subclass finalizer needs to invoke super.finalize() explicitly.
• Runtime.getRuntime().runFinalization() / System.runFinalization() can request for any pending finalizers be run for objects eligible for GC.