Is there a destructor for Java?

Question

Is there a destructor for Java? I don't seem to be able to find any documentation on this. If there isn't, how can I achieve the same effect?

To make my question more specific, I am writing an application that deals with data and the specification say that there should be a 'reset' button that brings the application back to its original just launched state. However, all data have to be 'live' unless the application is closed or reset button is pressed.

Being usually a C/C++ programmer, I thought this would be trivial to implement. (And hence I planned to implement it last.) I structured my program such that all the 'reset-able' objects would be in the same class so that I can just destroy all 'live' objects when a reset button is pressed.

I was thinking if all I did was just to dereference the data and wait for the garbage collector to collect them, wouldn't there be a memory leak if my user repeatedly entered data and pressed the reset button? I was also thinking since Java is quite mature as a language, there should be a way to prevent this from happening or gracefully tackle this.

Answer 1

Though there have been considerable advancements in Java's GC technology, you still need to be mindful of your references. Numerous cases of seemingly trivial reference patterns that are actually rats nests under the hood come to mind.

From your post it doesn't sound like you're trying to implement a reset method for the purpose of object reuse (true?). Are your objects holding any other type of resources that need to be cleaned up (i.e., streams that must be closed, any pooled or borrowed objects that must be returned)? If the only thing you're worried about is memory dealloc then I would reconsider my object structure and attempt to verify that my objects are self contained structures that will be cleaned up at GC time.

Answer 2

There is no exactly destructor class in Java, class destroyed in java automatically by garbage collector . but you could do that using below one but it's not exact same thing :

finalize()

There was a question that spawned in-depth discussion of finalize , so that you should get more depth if required...

Answer 3

The closest equivalent to a destructor in Java is the finalize() method. The big difference to a traditional destructor is that you can't be sure when it'll be called, since that's the responsibility of the garbage collector. I'd strongly recommend carefully reading up on this before using it, since your typical RAIA patterns for file handles and so on won't work reliably with finalize().

Answer 4

If you're writing a Java Applet, you can override the Applet "destroy()" method. It is...

 * Called by the browser or applet viewer to inform
 * this applet that it is being reclaimed and that it should destroy
 * any resources that it has allocated. The stop() method
 * will always be called before destroy().

Obviously not what you want, but might be what other people are looking for.

Answer 5

Perhaps you can use a try ... finally block to finalize the object in the control flow at which you are using the object. Of course it doesn't happen automatically, but neither does destruction in C++. You often see closing of resources in the finally block.

Answer 6

Use of finalize() methods should be avoided. They are not a reliable mechanism for resource clean up and it is possible to cause problems in the garbage collector by abusing them.

If you require a deallocation call in your object, say to release resources, use an explicit method call. This convention can be seen in existing APIs (e.g. Closeable, Graphics.dispose(), Widget.dispose()) and is usually called via try/finally.

Resource r = new Resource();
try {
    //work
} finally {
    r.dispose();
}

Attempts to use a disposed object should throw a runtime exception (see IllegalStateException).

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EDIT:

I was thinking, if all I did was just to dereference the data and wait for the garbage collector to collect them, wouldn't there be a memory leak if my user repeatedly entered data and pressed the reset button?

Generally, all you need to do is dereference the objects - at least, this is the way it is supposed to work. If you are worried about garbage collection, check out Java SE 6 HotSpot[tm] Virtual Machine Garbage Collection Tuning (or the equivalent document for your JVM version).