off-heap, native heap, direct memory and native memory

1) Heap memory: memory within the JVM process that is used to hold Java Objects and is maintained by the JVMs Garbage Collector.

2) Native memory/Off-heap: is memory allocated within the processes address space that is not within the heap and thus is not freed up by the Java Garbage Collector.

3) Direct memory: is similar to native, but also implies that an underlying buffer within the hardware is being shared. For example, a buffer within the network adapter or graphics display. The goal here is to reduce the number of times the same bytes is being copied about in memory.

Finally, depending upon the OS then extra native allocations (assigning of the memory address space) can be carried out via Unsafe alloc and/or by memory mapping a file. Memory mapping a file is especially interesting as it can easily allocate more memory than the machine currently has as physical ram. Also note, that the total address space limit is restricted by the size of a pointer being used, a 32bit pointer cannot go outside of 4GB. Period.

a lot of high performant server application which run on JVM have use off-heap memory to increase performance of server such as Apache Cassandra. It used to store most of data structure on heap but in recent releases, it has been stored on off-heap memory