//運(yùn)用list來保存?zhèn)兠恳粋€(gè)activity是關(guān)鍵
  private List<Activity> mList = new LinkedList<Activity>();
  //為了實(shí)現(xiàn)每次使用該類時(shí)不創(chuàng)建新的對象而創(chuàng)建的靜態(tài)對象
  private static MyApplication instance;

  //構(gòu)造方法
  public MyApplication() {
  }

  //實(shí)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);
  }

  //關(guān)閉每一個(gè)list內(nèi)的activity
  public void exit() {
    try {
      for (Activity activity : mList) {
        if (activity != null)
          activity.finish();
      }
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      System.exit(0);
    }
  }

  //殺進(jìn)程
  public void onLowMemory() {
    super.onLowMemory();
    System.gc();
  }

MyApplication.getInstance().addActivity(this);

  /**
   * 兩次返回退出
   */
  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);
  }