(3) 在非主執行緒中直接new Handler() 會報如下的錯誤:

E/AndroidRuntime( 6173): Uncaught handler: thread Thread-8 exiting due to uncaught exception 

E/AndroidRuntime( 6173): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare() 

原因：是非主執行緒中預設沒有建立Looper物件，需要先呼叫Looper.prepare()啟用Looper。

    注意：寫在Looper.loop()之後的程式碼不會被執行，這個函式內部應該是一個迴圈，當呼叫mHandler.getLooper().quit()後，loop才會中止，其後的程式碼才能得以執行。

    把下面例子中的mHandler宣告成類成員，在主執行緒通過mHandler傳送訊息即可。        

    Android官方文件中Looper的介紹： Class used to run a message loop for a thread. Threads by default do not have a message loop associated with them; to create one, call prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped.

class LooperThread extends Thread {
public Handler mHandler;
public void run(){
Looper.prepare();
mHandler = new Handler(){
public void handleMessage(Message msg){
// process incoming messages here
}
};
Looper.loop();
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello" />

    <Button
        android:id="@+id/handlerThreadBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="startHandlerThread" />

</LinearLayout>

package com.tayue;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Handler.Callback;
import android.os.HandlerThread;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
/**
 * @author xionglei
 */
public class TestHandlerActivity extends Activity implements OnClickListener{
    
    public Button handlerThreadBTN;
    MyHandlerThread handlerThread;
    Handler handler;
    
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //列印UI執行緒的名稱
        System.out.println("onCreate  CurrentThread = " + Thread.currentThread().getName());
        setContentView(R.layout.main);
        
        handlerThreadBTN = (Button) findViewById(R.id.handlerThreadBtn);
        handlerThreadBTN.setOnClickListener(this);
        
        handlerThread = new MyHandlerThread("myHanler");
        handlerThread.start();
        //注意： 這裡必須用到handler的這個構造器，因為需要把callback傳進去，從而使自己的HandlerThread的handlerMessage來替換掉Handler原生的handlerThread
        handler = new Handler(handlerThread.getLooper(), handlerThread);       
    }

    @Override
    public void onClick(View v) {
        //點選按鈕後來開啟執行緒
        handler.sendEmptyMessage(1);
    }    
    
    private class MyHandlerThread extends HandlerThread implements Callback {

        public MyHandlerThread(String name) {
            super(name);
        }

        @Override
        public boolean handleMessage(Message msg) {
            //列印執行緒的名稱
            System.out.println(" handleMessage CurrentThread = " + Thread.currentThread().getName());
            return true;
        }            
    }         
}

點選按鈕，列印的日誌如下（這裡點選了3次） 

07-06 09:32:48.776: I/System.out(780): onCreate  CurrentThread = main 

07-06 09:32:55.076: I/System.out(780):  handleMessage CurrentThread = myHanler 

07-06 09:32:58.669: I/System.out(780):  handleMessage CurrentThread = myHanler 

07-06 09:33:03.476: I/System.out(780):  handleMessage CurrentThread = myHanler

private boolean iscancel = false; //使用者手動取消登入的標誌位

    handlerThread = new HandlerThread("myHandlerThread");
                    handlerThread.start();
                    handler = new MyHandler(handlerThread.getLooper());
                // 將要執行的執行緒物件新增到執行緒佇列中
                        handler.post(new Runnable() {
                            @Override
                            public void run() {
                                Message message = handler.obtainMessage();
                                UserBean user = Bbs.getInstance().Login(username, password);//耗時任務
                                Bundle b = new Bundle();
                                b.putSerializable("user", user);
                                message.setData(b);
                                message.sendToTarget(); //或使用 handler.sendMessage(message);
                            }
                        });
    class MyHandler extends Handler {

            public MyHandler(Looper looper) {
                super(looper);
            }

            @Override
            public void handleMessage(Message msg) {
                if(iscancel == false){
                    // 操作UI執行緒的程式碼
                    Bundle b = msg.getData();
                    UserBean user = (UserBean)b.get("user");
                                     ......
               }
           }
    }