Java EE (Java Enterprise Edition) General Discussion

When I generate PDF files via Apache FOP, the JVM memory consumption rises for several megabytes for each document. I tried to free as much resources as i know of:

    /**
     * Tries to free as much resources as possible to free memory.
     */
    public void cleanUp() {
        FopImageFactory.resetCache(); //empty image cache to free up memory
        MessageHandler.setScreenLogger(new ConsoleLogger(ConsoleLogger.LEVEL_INFO)); //reset logger
    }//cleanUp()

and also free (set to null) the reference to my FOP processing class. But the memory (for these FOP processing classes) still keeps increasing. Any further possibility to free FOP resources?