Difference between ByteBuffer.allocateDirect() and MappedByteBuffer.load()

Direct ByteBuffers (those allocated using ByteBuffer.allocateDirect) are different to MappedByteBuffers in that they represent different sections of memory and are allocated differently. Direct ByteBuffers are a way to access a block of memory allocated outside of the JVM generally allocated with a malloc call (although most implementations will probably use an efficient slab allocator). I.e. it's just a pointer to a block of memory.

A MappedByteBuffer represents a section of memory allocated using mmap call, which is used to perform memory mapped I/O. Therefore MappedByteBuffers won't register their use of memory in the same way a Direct ByteBuffer will.

So while both are "direct" in that they represent memory outside of the JVM their purposes are different.

As an aside, in order to get the reservedMemory value you are reflectively calling to an internal method of the JVM, whose implementation is not covered by any specification, therefore there are no guarantees as to what that value returns. Direct ByteBuffers can be allocated from within JNI using NewDirectByteBuffer call from C/C++ (MappedByteBuffers likely use this) and this probably doesn't affect the reservedMemory value, which is may only changed when using the Java ByteBuffer.allocateDirect.