java - Is there any way to force a JVM to use swap no matter how big the memory requirement is? - Stack Overflow

Apparently there is one way around the limits of Java heap. It is even used in a commercial product called BigMemory which basically allows you to have almost unlimited memory by transparently swapping out to OS swap and/or to disk if needed.

The idea is to use direct ByteBuffers to store your objects data. Because direct byte buffers' contents are stored in native process memory (as opposed to heap) you can rely on OS swap mechanism to swap memory out for you. I found this on this website (search for 'direct byte buffer' on the page).

Here is how you can implement it (java-pseudo-code'ish):

class NativeMemoryCache{    private Map<Object, ByteBuffer> data = new HashMap<...>();      public void put(Object key, Serializable object){      byte[] bytes = serialize(object);      //allocate native memory to store our object      ByteBuffer buf = ByteBuffer.allocateDirect(bytes.length);      buf.put(bytes);      buf.flip();      data.put(key, buf);    }      public Object get(Object key){      ByteBuffer buf = data.get(key).duplicate();      byte[] bytes = new byte[buf.remaining()];      buf.get(bytes);      return deserialize(bytes);    }      private byte[] serialize(Object obj){ ... }    private Object deserialize(byte[] bytes){ ... }  }

Hope you get the idea. You just need to implement the serialization (you can also compress your objects using zip. This will be effective if you have few big objects especially ones containing zippable data like strings).

Of course NativeMemoryCache object, data hash map and keys will be in heap, but that should not take much memory.

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