Explain Garbage Collection In Java, How Garbage Collectior Work , Explain Final() In Java With Program
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Garbage Collection
Since objects are dynamically allocated by using the
new
operator, you might be wondering
how such objects are destroyed and their memory released for later reallocation. In some
languages, such as C++, dynamically allocated objects must be manually released by use of
a
delete
operator. Java takes a different approach; it handles deallocation for you automatically.
The technique that accomplishes this is called garbage collection. It works like this: when no
references to an object exist, that object is assumed to be no longer needed, and the memory
occupied by the object can be reclaimed. There is no explicit need to destroy objects as in C++.
Garbage collection only occurs sporadically (if at all) during the execution of your program.
It will not occur simply because one or more objects exist that are no longer used. Furthermore,
different Java run-time implementations will take varying approaches to garbage collection,
but for the most part, you should not have to think about it while writing your programs.
The finalize( ) Method
Sometimes an object will need to perform some action when it is destroyed. For example, if
an object is holding some non-Java resource such as a file handle or character font, then you
might want to make sure these resources are freed before an object is destroyed. To handle
such situations, Java provides a mechanism called finalization. By using finalization, you can
define specific actions that will occur when an object is just about to be reclaimed by the
garbage collector.
To add a finalizer to a class, you simply define the
finalize( )
method. The Java run time
calls that method whenever it is about to recycle an object of that class. Inside the
finalize( )
method, you will specify those actions that must be performed before an object is destroyed.
The garbage collector runs periodically, checking for objects that are no longer referenced by
any running state or indirectly through other referenced objects. Right before an asset is freed,
the Java run time calls the
finalize( )
method on the object.
The
finalize( )
method has this general form:
protected void finalize( )
{
// finalization code here
}
Here, the keyword
protected
is a specifier that prevents access to
finalize( )
by code defined
outside its class. This and the other access specifiers are explained in Chapter 7.
It is important to understand that
finalize( )
is only called just prior to garbage collection.
It is not called when an object goes out-of-scope, for example. This means that you cannot
know when—or even if—
finalize( )
will be executed. Therefore, your program should provide
other means of releasing system resources, etc., used by the object. It must not rely on
finalize( )
for normal program operation.