如何以编程方式获取java中进程的总内存和已用堆内存

Question

如何在java中获取进程的总内存和已用堆内存？

    strong textint mb = 1024 * 1024;
    Runtime instance = Runtime.getRuntime();
    System.out.println("***** Heap utilization statistics [MB] *****\n");
    System.out.println("Total Memory: " + instance.totalMemory() / mb);
    String str = "9280";
    Process pr = instance.exec("cmd /c jmap -heap "+str);

这是基于Windows的，我需要它用于跨平台。