How does finalize() work in java?

问题:

So, I recently discovered the finalize method in Java (not sure why I missed it before, but there it is). This seems like it could be the answer to a lot of the issues I'm working with, but I wanted to get a bit more information first.

Online, I found this diagram illustrating the process of garbage collection and finalize:

A couple of questions:

1. This takes place in a separate thread, correct?
2. What happens if I instantiate a new object during finalize? Is that allowed?
3. What happens if I call on a static method from finalize?
4. What happens if I establish a new reference to the object from within finalize?

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I suppose I should explain why I'm interested. I work with LWJGL a lot, and it seems that if I could use finalize to cause Java objects to automatically clean up OpenGL resources, then I could do some really nice things in terms of an API.

回答1:

I don't think there are any guarantees about what thread will be used. New objects may be instantiated and static methods may be called. Establishing a new reference to your object will prevent it from being garbage collected, but the finalize method will not be called again--you don't want to do this.

Cleaning up resources is precisely what the finalize method is for, so you should be good there. A couple of warnings, though:

The method is not guaranteed to be called. If you have tied up resources that will not automatically be freed when your program stops do not depend on finalize.

When the method is called is not guaranteed. With memory tight, this will be sooner. With lots of free memory, it will be later if at all. This may suit you fine: with lots of memory you may not be all that concerned about freeing up the resources. (Though hanging on to them may interfere with other software running at the same time, in which case you would be concerned.)

My ususal solution is to have some kind of dispose method that does the clean up. I call it explicitly at some point if I can, and as soon as I can. Then I add a finalize method that just calls the dispose method. (Note that the dispose method must behave well when when called more than once! Indeed, with this kind of programming I might call dispose several times outside finalize, not being sure if previous calls were made successfully and yet wanting it to be called effectively as soon as possible.) Now, ideally, my resources are freed as soon as I no longer need them. However, if I lose track of the object with the resources, the finalize method will bail me out when memory runs short and I need the help.

回答2:

finalize() is called by the Java Garbage Collector when it detects that no references to that particular object exists. finalize() is inherited by all Java objects through the Object class.

As far as I am aware you would have no difficulty making static method calls from a finalize() method I and you could establish a new reference to it from finalize() - however I would say this is poor programming practice.

You shouldn't rely on finalize() for clearing up, and it is better to clear up as you go. I prefer to use try, catch, finally for clearing up, rather than using finalize(). In particular, by using finalize() you will cause the JVM to hold onto all other objects that your finalizable object references, just in case it makes calls to them. This means your holding onto memory you might not need to use. More importantly, this also means you can cause the JVM to never end up disposing of objects, because they have to hold onto them incase another objects finalize method needs it e.g. a race condition.

Also, consider that it is entirely possible that GC won't be called. Therefore you can't actually guarantee that finalize() will ever be called.

Clear up resources as and when you are finished with them, and do not rely on finalize() to do it is my advice.

回答3:

First of all, remember there is no guarantee that finalization will even be run at all for all your objects. You can use it to free memory allocated in native code associated with an object, but for pure Java code most use cases are only to perform a "backup" mechanism of cleaning up resources. This means in most cases you should free resources manually and finalizers could act only a sort of helper to clean up if you forget to do it the standard way. However, you can't use them as the only or the main mechanism of cleanup. Even more generally, you shouldn't write any code whose correctness depends on finalizers being run.

Ad 1. As far as I know, there are no guarantees about what thread calls finalize(), though in practice this will probably be one of the GC threads.

Ad 2. Instantiating new objects is allowed. However, there are a number of pitfalls with handling object references in finalizers. In particular, if you store a hard reference to the object being finalized in some live object, you can prevent your about-to-be-garbage-collected object from being cleaned up. This kind of object resurrection may lead to exhausting your resources if it gets out of control. Also, watch out for exceptions in finalize() - they may halt the finalization, but there's no automatic way for your program to learn about them. You need to wrap the code in try-catch blocks and propagate the information yourself. Also, long execution time of finalizers may cause the queue of objects to build up and consume lots of memory. Some other noteworthy problems and limitations are described in this JavaWorld article.

Ad 3. There shouldn't be any issues with calling static methods from finalizers.

Ad 4. As mentioned in point 2, it is possible to prevent an object from being garbage collected (to resurrect it) by placing a reference to it in another live object during finalization. However, this is tricky behavior and probably not good practice.

To sum up, you can't rely on finalizers for cleaning up your resources. You need to handle that manually and finalizers in your case may at best be used as a backup mechanism to cover up after sloppy coding to some degreee. This means, unfortunately, your idea of making the API nicer by using finalizers to clean up OpenGL resources probably won't work.