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Sunday, September 2, 2007

JavaOne 2007 - Performance Tips 2 - Finish the finalizers!

by Eduardo Rodrigues

Continuing from my last post about some lessons learned at JavaOne'07 on Java performance since JDK 1.5, there's something we usually do not pay much attention to but which can get us some trouble: object finalizers.

Every time we override the protected void finalize() throws Throwable method, we are implicitly creating a postmortem hook to be called by the Garbage Collector after it finds that the object is unreachable and before it actually reclaims the object's memory space. In general, we override finalize() with the best of the intentions which is to ensure that all necessary disposal of system resources and any other cleanup will be performed before the object is permanently discarded. So why is that an issue?

Well, we all should know that finalize() is an empty method declared in java.lang.Object class, therefore, inherited by any existing Java class. So, when it's overridden, the JVM can't assume the default trivial finalization for the object anymore which means that "fast allocation" won't happen here. In fact, "finalizable" objects have much slower allocation simply because the VM must keep track of all finalize() hooks. Besides, those objects also give much more work to the GC. It takes at least 2 GC cycles (which are also slower) to reclaim a "finalizable" object. The first is the usual one when the GC identifies the object as garbage. The difference is that now it has to enqueue the object on finalization queue. Only during a next cycle GC will dequeue and call the object's finalize() method and, if we're lucky, discard the object and reclaim its space, or else, it may take another cycle just to finally get rid of that object.

If we look closer, we'll notice that putting more pressure on the GC and slowing down both initialization and finalization processes are not the only problems here. Let's take a quick look at the J2SE 5.0 API Javadoc for the Object.finalize() method:

"(...) After the finalize method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded. The finalize method is never invoked more than once by a Java virtual machine for any given object. Any exception thrown by the finalize method causes the finalization of this object to be halted (...)"

It is quite clear to me that there's a potential temporary (or even permanent) "memory leak" matter hidden in that piece of Javadoc. Since the JVM is obligated to execute the finalize() method before discarding any object overriding it, in fact, due to the additional GC cycles described above, not only that specific object will be retained longer in the heap but also any other objects that are still reachable from it. In the other hand, even after executing finalize(), the VM will not reclaim an object's space if, by any means, it may still be accessed by any object or class, in any living thread, even if they're also ready to be finalized. Like it isn't enough, if any exception is thrown uncaught during finalize() execution, the finalization of the object is halted and there's a good chance that, in this case, this object will be retained forever as garbage.

At last, the fact that the finalize() method should never be invoked more that once for any given object certainly implies the use of synchronization which is one more performance threatening element.

So, next time you consider writing a finalizer in a class, please, take a second look at it. And if you really have to do that, be really careful with the code you write and try to follow these tips:
  • Use finalizers only as a last resort!

  • Even if you do not explicitly override the finalize() method, library classes you extend may have done it. Look at the example bellow:

    class MyFrame extends JFrame {
    private byte[] buffer = new byte[16*1024*1024];
    (...)
    }

    In JDK 1.5 and earlier, the 16MB buffer will survive, at least, 2 GC cycles before any MyFrame instance is discarded. That's because JFrame library class does declare a finalizer. So, try to split objects in cases like this:

    class MyFrame {
    private JFrame frame;
    private byte[] buffer = new byte[16*1024*1024];
    (...)
    }

  • Even if you're considering to use a finalizer to dispose expensive and scarce resources, keep in mind that, being scarce, it's very likely that they will be exhausted before memory (assuming that memory is usually plentiful). So, in these cases, prefer to pool scarce resources instead.
To be continued...

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