直接在需要的activity中重写onKeyDown方法

private long start = 0;

   @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){
            if ((System.currentTimeMillis()- startTime) > 2000){
                Toast.makeText(getApplicationContext(),"再按一次退出程序",Toast.LENGTH_LONG).show();
                startTime = System.currentTimeMillis();
            } else {
                finish();
                System.exit(0);
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }