private long exitTime = 0;
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {

            if ((System.currentTimeMillis() - exitTime) > 2000) {
                ToastUtils.showMessage("再按一次退出应用程序");
                exitTime = System.currentTimeMillis();
            } else {
                Intent intent = new Intent(Intent.ACTION_MAIN);
                intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                intent.addCategory(Intent.CATEGORY_HOME);
                startActivity(intent);
                return true;
            }
            return true;

    }
    return super.onKeyDown(keyCode, event);
}