Give me an example

When trying to recreate the java.lang.OutOfMemoryError: Requested array size exceeds VM limit error, let’s look at the following code:


for (int i = 3; i >= 0; i--) {
	try {
		int[] arr = new int[Integer.MAX_VALUE-i];
		System.out.format("Successfully initialized an array with %,d elements.\n", Integer.MAX_VALUE-i);
	} catch (Throwable t) {
		t.printStackTrace();
	}
}

The example iterates four times and initializes an array of long primitives on each turn. The size of the array this program is trying to initialize grows by one with every iteration and finally reaches Integer.MAX_VALUE. Now, when launching the code snippet on 64-bit Mac OS X with Hotspot 7, you should get the output similar to the following:

java.lang.OutOfMemoryError: Java heap space
	at eu.plumbr.demo.ArraySize.main(ArraySize.java:8)
java.lang.OutOfMemoryError: Java heap space
	at eu.plumbr.demo.ArraySize.main(ArraySize.java:8)
java.lang.OutOfMemoryError: Requested array size exceeds VM limit
	at eu.plumbr.demo.ArraySize.main(ArraySize.java:8)
java.lang.OutOfMemoryError: Requested array size exceeds VM limit
	at eu.plumbr.demo.ArraySize.main(ArraySize.java:8)

Note that before facing java.lang.OutOfMemoryError: Requested array size exceeds VM limit on the last two attempts, the allocations failed with a lot more familiar java.lang.OutOfMemoryError: Java heap space message. It happens because the 2^31-1 int primitives you are trying to make room for require 8G of memory which is less than the defaults used by the JVM.

This example also demonstrates why the error is so rare – in order to see the VM limit on array size being hit, you need to allocate an array with the size right in between the platform limit and Integer.MAX_INT. When our example is run on 64bit Mac OS X with Hotspot 7, there are only two such array lengths: Integer.MAX_INT-1 and Integer.MAX_INT.