Java using up far more memory than allocated with -Xmx

Top command reflects the total amount of memory used by the Java application. This includes among other things:

• A basic memory overhead of the JVM itself
• the heap space (bounded with -Xmx)
• The permanent generation space (-XX:MaxPermSize – not standard in all JVMs)
• threads stack space (-Xss per stack) which may grow significantly depending on the number of threads
• Space used by native allocations (using ByteBufer class, or JNI)