计算一个对象所占内存大小

主要思路 就是先获取当前占用内存（usedMemory） 然后创建对象 再获取当前占用内存 两次内存差就是该对象所占内存大小   runGC（）方法提供垃圾回收在每次获取内存前可以先调用 

private static final Runtime s_runtime = Runtime.getRuntime();

 private static void runGC() throws Exception {
  // It helps to call Runtime.gc()// using several method calls:
  for (int r = 0; r < 4; ++r)
   _runGC();
 }
 private static void _runGC() throws Exception {
  long usedMem1 = usedMemory(), usedMem2 = Long.MAX_VALUE;
  for (int i = 0; (usedMem1 < usedMem2) && (i < 500); ++i) {
   s_runtime.runFinalization();
   s_runtime.gc();
   Thread.currentThread().yield();
   usedMem2 = usedMem1;
   usedMem1 = usedMemory();
   }
  }
 private static long usedMemory() {
  return s_runtime.totalMemory() - s_runtime.freeMemory();
 }