How can clear memory with out restarting the engine.
I doubt there is such a thing. With any java based server once you have a program running it will occupy memory by creating objects. If the program is not leaky then the memory will be reclaimed by java garbage collector once the objects are no longer in use. If that does not happen then you have to restart the program or in fact debug and correct the leaky logic. Other than that take a look at this article.