之前的例子都是我们在子线程(WorkerThread)当中处理并发送消息，然后在主线程(UI线程)中获取消息并修改UI，那么可以不可以在由主线程发送消息，子线程接收呢？我们按照之前的思路写一下代码：

packagecn.lixyz.handlertest;importandroid.app.Activity;importandroid.os.Bundle;importandroid.os.Handler;importandroid.os.Message;importandroid.util.Log;importandroid.view.View;importandroid.widget.Button;public class MainActivity extendsActivity {privateButton button;privateHandler handler;private int i = 0;

@Overrideprotected voidonCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

button=(Button) findViewById(R.id.button);

button.setOnClickListener(newView.OnClickListener() {

@Overridepublic voidonClick(View v) {

Log.d("TTTT", "onClick:" +Thread.currentThread().getName());

Message msg=handler.obtainMessage();

handler.sendMessage(msg);

}

});

WorkerThread wt= newWorkerThread();

wt.start();

}class WorkerThread extendsThread {

@Overridepublic voidrun() {super.run();

handler= newHandler() {

@Overridepublic voidhandleMessage(Message msg) {super.handleMessage(msg);

Log.d("TTTT", "收到了消息对象");

}

};

}

}

}

MainActivity.java

activity_main.xml

上面的问题貌似没什么错误，但运行时却出错

抛出异常：

09-17 04:01:31.655 15854-15869/? E/AndroidRuntime﹕ FATAL EXCEPTION: Thread-120

Process: cn.lixyz.handlertest, PID: 15854

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

at android.os.Handler.(Handler.java:200)

at android.os.Handler.(Handler.java:114)

at cn.lixyz.handlertest.MainActivity$WorkerThread$1.(MainActivity.java:48)

at cn.lixyz.handlertest.MainActivity$WorkerThread.run(MainActivity.java:48)

提示我们没有调用Looper.prepare()，无法创建线程，原因是非主线程中默认是没有创建Looper对象的，需要先调用Looper.prepare()启用Looper。修改一下上面的代码。

packagecn.lixyz.handlertest;importandroid.app.Activity;importandroid.os.Bundle;importandroid.os.Handler;importandroid.os.Looper;importandroid.os.Message;importandroid.util.Log;importandroid.view.View;importandroid.widget.Button;public class MainActivity extendsActivity {privateButton button;privateHandler handler;

@Overrideprotected voidonCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

button=(Button) findViewById(R.id.button);

button.setOnClickListener(newView.OnClickListener() {

@Overridepublic voidonClick(View v) {

Message msg=handler.obtainMessage();

msg.what= 100;

handler.sendMessage(msg);

Log.d("TTTT", "sendMessage:" +Thread.currentThread().getName());

}

});

WorkerThread wt= newWorkerThread();

wt.start();

}class WorkerThread extendsThread {

@Overridepublic voidrun() {super.run();

Looper.prepare();

handler= newHandler() {

@Overridepublic voidhandleMessage(Message msg) {super.handleMessage(msg);

Log.d("TTTT", "handleMessage:" +Thread.currentThread().getName());int i =msg.what;

Log.d("TTTT", "收到了消息对象:" +i);

}

};

Looper.loop();

}

}

}

MainActivity.java

activity_main.xml

运行结果：

09-17 04:29:48.669 28397-28397/cn.lixyz.handlertest D/TTTT﹕ sendMessage:main09-17 04:29:48.669 28397-28410/cn.lixyz.handlertest D/TTTT﹕ handleMessage:Thread-141

09-17 04:29:48.669 28397-28410/cn.lixyz.handlertest D/TTTT﹕ 收到了消息对象:100

上面代码表示，消息是从MainThread发送的，WorkerThread接收到了消息。

PS：非UI线程默认是没有创建Looper对象的，所以需要主线程向子线程传递消息时，需创建Looper对象并启动循环，子线程才可以循环着从MaessageQueue中取出消息。