Geek asks and answers

Does Java throw an exception before it really exits memory?

I want to create a large int array that almost fills all the memory available to the JVM. Take this code, for example:

    final int numBuffers = (int) ((runtime.freeMemory() - 200000L) / (BUFFER_SIZE));
    System.out.println(runtime.freeMemory());
    System.out.println(numBuffers*(BUFFER_SIZE/4)*4);
    buffers = new int[numBuffers*(BUFFER_SIZE / 4)];

When launched with a heap size of 10M, this throws an OutOfMemoryException, despite the output from printlns:

9487176
9273344

I understand that the array will have some overhead, but not 200k, of course? Why can't Java allocate memory for what it claims to have enough space? I have to set this constant, which is subtracted by something around 4M, before Java starts it (by the time printlns look more similar: 9487176 5472256)

Even more confusing if I replace the buffers with a 2D array:

buffers = new int[numBuffers][BUFFER_SIZE / 4];

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There are also many other factors that you should consider, some of which include: memory leaks either in a virtual machine (not very) or on your application (also unlikely for a simple scenario), unreliability of use Runtime.freeMemory()(GC may work immediately after a call, and available free memory may change), implementation details of each specific JVM, etc.

The fact is that, as a rule, it is not always expected that everything will be available for your application Runtime.freeMemory().

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