//运用list来保存们每一个activity是关键
  private List<Activity> mList = new LinkedList<Activity>();
  //为了实现每次使用该类时不创建新的对象而创建的静态对象
  private static MyApplication instance;

  //构造方法
  public MyApplication() {
  }

  //实例化一次
  public synchronized static MyApplication getInstance() {
    if (null == instance) {
      instance = new MyApplication();
    }
    return instance;
  }

  //添加要退出的Activity
  public void addActivity(Activity activity) {
    mList.add(activity);
  }

  //关闭每一个list内的activity
  public void exit() {
    try {
      for (Activity activity : mList) {
        if (activity != null)
          activity.finish();
      }
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      System.exit(0);
    }
  }

  //杀进程
  public void onLowMemory() {
    super.onLowMemory();
    System.gc();
  }

MyApplication.getInstance().addActivity(this);

  /**
   * 两次返回退出
   */
  private long mExitTime;

  public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
      if ((System.currentTimeMillis() - mExitTime) > 3000) {
        Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
        mExitTime = System.currentTimeMillis();
      } else {
        MyApplication.getInstance().exit();
      }
      return true;
    }
    return super.onKeyDown(keyCode, event);
  }