注意不能用再主线程，否则会报错：

private void onBack（）{

new Thread(){

public void run(){

try{

Instrumentation instru = new Instrumentation();

instru.sendKeyDownUpSync(KeyEvent.KEYCODE_BACK);

}catch(Exception e){

Log.e("back","error!!!!------------"+e.toString());

}

}.start();

}