Understanding the finalize Method in Java
finalize method in the Object class is often a point of discussion regarding whether it should be used or not. Below are some important pointers on the finalize method:
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1. When Is It Called?
- Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
- A subclass overrides the
finalizemethod to dispose of system resources or perform other cleanup.
2. Contract of finalize
- The general contract of
finalizeis that it is invoked if and when the Java™ virtual machine determines that there is no longer any means by which this object can be accessed by any thread that has not yet died, except as a result of an action taken by the finalization of some other object or class which is ready to be finalized.
3. Purpose and Behavior
- The
finalizemethod may take any action, including making this object available again to other threads. - The usual purpose of
finalize, however, is to perform cleanup actions before the object is irrevocably discarded. - For example, the
finalizemethod for an object representing an input/output connection might perform explicit I/O transactions to break the connection before the object is permanently discarded.
4. Default Implementation
- The
finalizemethod of classObjectperforms no special action; it simply returns normally. - Subclasses of
Objectmay override this definition.
protected void finalize() throws Throwable { }
5. Thread Behavior
- The Java programming language does not guarantee which thread will invoke the
finalizemethod for any given object. - It is guaranteed, however, that the thread that invokes
finalizewill not be holding any user-visible synchronization locks. - If an uncaught exception is thrown by the
finalizemethod, the exception is ignored and finalization of that object terminates.
6. Finalization Cycle
- After the
finalizemethod has been invoked for an object, no further action is taken until the JVM has again determined that there is no longer any means by which this object can be accessed. - This includes possible actions by other objects or classes that are ready to be finalized, at which point the object may be discarded.
7. One-Time Invocation
- The
finalizemethod is never invoked more than once by the JVM for any given object.
8. Exception Handling
- Any exception thrown by the
finalizemethod causes the finalization of this object to be halted but is otherwise ignored.
9. General Recommendation
- In general, it’s best not to rely on
finalize()for cleaning up. - An object may not be eligible for garbage collection during the lifetime of the application, which can cause resources not to be closed and may lead to resource exhaustion.
10. Best Practices When Overriding finalize()
- If overriding
finalize(), it is good programming practice to use a try-catch-finally block and always callsuper.finalize(). - This ensures that resources used by the object’s calling class are properly released.
protected void finalize() throws Throwable {
try {
close(); // close open files
} finally {
super.finalize();
}
}