Wednesday, March 9, 2011

OutOfMemory != allocation failure

IBM says --
Q: I am getting an OutOfMemoryError. Does this mean that the Java heap is exhausted?
A: Not necessarily. Sometimes the Java heap has free space but an OutOfMemoryError can occur. The error could occur because of
* out of memory but not due to a call to new()
* Excessive memory allocation in other parts of the application, unrelated to the JVM, if the JVM is just a part of the process, rather than the entire process (JVM through JNI, for instance).

Q: How can I confirm if the OutOfMemoryError was caused by the Java heap becoming exhausted?
A: Run with the -verbosegc option. VerboseGC will show messages such as "Insufficient heap space to satisfy allocation request"

Q: When I see an OutOfMemoryError, does that mean that the Java program will exit?
A: No. My experiments below show you can continue even after new() failed.
    static ArrayList<byte[]> list = new ArrayList<byte[]>();
    public static void main(String[] kkkj) throws InterruptedException {
        byte[] array = null;
        for (;;) {
            Sizeof.printHeapSegments4JDK6();
            out.println(Sizeof.total_ie_Committed() + "\t>  " + Sizeof.used()
                    + " <-- bytes in use. Trying to new up....");
            try {
                array = new byte[20 * 1000 * 1000];
            } catch (OutOfMemoryError e) {
                StringBuilder stat = Sizeof.printHeapSegments4JDK6();
                System.err.println(stat);
                e.printStackTrace();
            }
            list.add(array);
        }
    }

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