总结4个关键概念
 
1、MessageQueue：是一种数据结构，见名知义，就是一个消息队列，存放消息的地方。每一个线程最多只可以拥有一个MessageQueue数据结构。
 
创建一个线程的时候，并不会自动创建其MessageQueue。通常使用一个Looper对象对该线程的MessageQueue进行管理。主线程创建时，会创建一
 
个默认的Looper对象，而Looper对象的创建，将自动创建一个Message Queue。其他非主线程，不会自动创建Looper，要需要的时候，通过调
 
用prepare函数来实现。
 
2、Message：消息对象，Message Queue中的存放的对象。一个Message Queue中包含多个Message。
 
Message实例对象的取得，通常使用Message类里的静态方法obtain(),该方法有多个重载版本可供选择；它的创建并不一定是直接创建一个新的实例，
 
而是先从Message Pool(消息池)中看有没有可用的Message实例，存在则直接取出返回这个实例。如果Message Pool中没有可用的Message实例，
 
则才用给定的参数创建一个Message对象。调用removeMessages()时，将Message从Message Queue中删除，同时放入到Message Pool中。除了上面这
 
种方式，也可以通过Handler对象的obtainMessage()获取一个Message实例。
 
3、Looper：
 
是MessageQueue的管理者。每一个MessageQueue都不能脱离Looper而存在，Looper对象的创建是通过prepare函数来实现的。同时每一个Looper对象
 
和一个线程关联。通过调用Looper.myLooper()可以获得当前线程的Looper对象
 
创建一个Looper对象时，会同时创建一个MessageQueue对象。除了主线程有默认的Looper，其他线程默认是没有MessageQueue对象的，所以，不能
 
接受Message。如需要接受，自己定义一个Looper对象(通过prepare函数),这样该线程就有了自己的Looper对象和MessageQueue数据结构了。
 
Looper从MessageQueue中取出Message然后，交由Handler的handleMessage进行处理。处理完成后，调用Message.recycle()将其放入Message Pool中。
4、Handler：
 
消息的处理者，handler负责将需要传递的信息封装成Message，通过调用handler对象的obtainMessage()来实现；
 
将消息传递给MessageQueue，这是通过handler对象的sendMessage()来实现的。当Looper对象看到MessageQueue中含有Message，就将其广播出去。该handler对象收到该消息后，调用相应的handler对象的handleMessage()方法
 
对其进行处理。
二、线程之间的消息如何进行传递
 
1、主线程给自己发送Message
 
public
class MainActivity
extends Activity {    
        
private Button btnTest;    
        
private TextView textView;    
        
private Handler handler;    
        @Override    
        
public
void onCreate(Bundle savedInstanceState) {    
                
super.onCreate(savedInstanceState);    
                setContentView(R.layout.main);    
                btnTest = (Button)
this.findViewById(R.id.btn_01);    
                textView = (TextView)
this.findViewById(R.id.view_01);    
                btnTest.setOnClickListener(
new View.OnClickListener() {    
                        @Override    
                        
public
void onClick(View arg0) {    
                                Looper looper = Looper.getMainLooper();
//主线程的Looper对象    
                                
//这里以主线程的Looper对象创建了handler，    
                                
//所以，这个handler发送的Message会被传递给主线程的MessageQueue。    
                                handler =
new MyHandler(looper);    
                                handler.removeMessages(0);    
                                
//构建Message对象    
                                
//第一个参数：是自己指定的message代号，方便在handler选择性地接收    
                                
//第二三个参数没有什么意义    
                                
//第四个参数需要封装的对象    
                                Message msg = handler.obtainMessage(1,1,1,
"主线程发消息了");    
                                handler.sendMessage(msg);
//发送消息    
                        }    
                });    
        }    
        
class MyHandler
extends Handler{    
                
public MyHandler(Looper looper){    
                        
super(looper);    
                }    
                
public
void handleMessage(Message msg){    
                        
super.handleMessage(msg);    
                        textView.setText(
"我是主线程的Handler，收到了消息："+(String)msg.obj);    
                }    
        }    
}    
2、其他线程给主线程发送Message
 
 
public
class MainActivity
extends Activity {    
        
private Button btnTest;    
        
private TextView textView;    
        
private Handler handler;    
        @Override    
        
public
void onCreate(Bundle savedInstanceState) {    
                
super.onCreate(savedInstanceState);    
                setContentView(R.layout.main);    
                btnTest = (Button)
this.findViewById(R.id.btn_01);    
                textView = (TextView)
this.findViewById(R.id.view_01);    
                btnTest.setOnClickListener(
new View.OnClickListener() {    
                        @Override    
                        
public
void onClick(View arg0) {    
                                
//可以看出这里启动了一个线程来操作消息的封装和发送的工作    
                                
//这样原来主线程的发送就变成了其他线程的发送，简单吧？呵呵    
                                
new MyThread().start();            
                        }    
                });    
        }    
        
class MyHandler
extends Handler{    
                
public MyHandler(Looper looper){    
                        
super(looper);    
                }    
                
public
void handleMessage(Message msg){    
                        
super.handleMessage(msg);    
                        textView.setText(
"我是主线程的Handler，收到了消息："+(String)msg.obj);    
                }    
        }    
        
//加了一个线程类    
        
class MyThread
extends Thread{    
                
public
void run(){    
                        Looper looper = Looper.getMainLooper();
//主线程的Looper对象    
                        
//这里以主线程的Looper对象创建了handler，    
                        
//所以，这个handler发送的Message会被传递给主线程的MessageQueue。    
                        handler =
new MyHandler(looper);    
                        
//构建Message对象    
                        
//第一个参数：是自己指定的message代号，方便在handler选择性地接收    
                        
//第二三个参数没有什么意义    
                        
//第四个参数需要封装的对象    
                        Message msg = handler.obtainMessage(1,1,1,
"其他线程发消息了");    
                        handler.sendMessage(msg);
//发送消息                            
                }    
        }    
}    
3、主线程给其他线程发送Message
 
 
public
class MainActivity
extends Activity {    
        
private Button btnTest;    
        
private TextView textView;    
        
private Handler handler;    
        @Override    
        
public
void onCreate(Bundle savedInstanceState) {    
                
super.onCreate(savedInstanceState);    
                setContentView(R.layout.main);    
                btnTest = (Button)
this.findViewById(R.id.btn_01);    
                textView = (TextView)
this.findViewById(R.id.view_01);    
                
//启动线程    
                
new MyThread().start();            
                btnTest.setOnClickListener(
new View.OnClickListener() {    
                        @Override    
                        
public
void onClick(View arg0) {    
                                
//这里handler的实例化在线程中    
                                
//线程启动的时候就已经实例化了    
                                Message msg = handler.obtainMessage(1,1,1,
"主线程发送的消息");    
                                handler.sendMessage(msg);    
                        }    
                });    
        }    
        
class MyHandler
extends Handler{    
                
public MyHandler(Looper looper){    
                        
super(looper);    
                }    
                
public
void handleMessage(Message msg){    
                        
super.handleMessage(msg);    
                        textView.setText(
"我是主线程的Handler，收到了消息："+(String)msg.obj);    
                }    
        }    
        
class MyThread
extends Thread{    
                
public
void run(){    
                        Looper.prepare();
//创建该线程的Looper对象，用于接收消息    
                        
//注意了：这里的handler是定义在主线程中的哦，呵呵，    
                        
//前面看到直接使用了handler对象，是不是在找，在什么地方实例化的呢？    
                        
//现在看到了吧？？？呵呵，开始的时候实例化不了，因为该线程的Looper对象    
                        
//还不存在呢。现在可以实例化了    
                        
//这里Looper.myLooper()获得的就是该线程的Looper对象了    
                        handler =
new ThreadHandler(Looper.myLooper());    
                        
//这个方法，有疑惑吗？    
                        
//其实就是一个循环，循环从MessageQueue中取消息。    
                        
//不经常去看看，你怎么知道你有新消息呢？？？    
                        Looper.loop();    
                }    
                
//定义线程类中的消息处理类    
                
class ThreadHandler
extends Handler{    
                        
public ThreadHandler(Looper looper){    
                                
super(looper);    
                        }    
                        
public
void handleMessage(Message msg){    
                                
//这里对该线程中的MessageQueue中的Message进行处理    
                                
//这里我们再返回给主线程一个消息    
                                handler =
new MyHandler(Looper.getMainLooper());    
                                Message msg2 = handler.obtainMessage(1,1,1,
"子线程收到:"+(String)msg.obj);    
                                handler.sendMessage(msg2);    
                        }    
                }    
        }    
}
 
4、其他线程给自己发送Message 
 
 
public
class MainActivity
extends Activity {    
        
private Button btnTest;    
        
private TextView textView;    
        
private Handler handler;    
        @Override    
        
public
void onCreate(Bundle savedInstanceState) {    
                
super.onCreate(savedInstanceState);    
                setContentView(R.layout.main);    
                btnTest = (Button)
this.findViewById(R.id.btn_01);    
                textView = (TextView)
this.findViewById(R.id.view_01);    
                btnTest.setOnClickListener(
new View.OnClickListener() {    
                        @Override    
                        
public
void onClick(View arg0) {    
                                
//启动线程    
                                
new MyThread().start();            
                        }    
                });    
        }    
        
class MyHandler
extends Handler{    
                
public MyHandler(Looper looper){    
                        
super(looper);    
                }    
                
public
void handleMessage(Message msg){    
                        
super.handleMessage(msg);    
                        textView.setText((String)msg.obj);    
                }    
        }            
        
class MyThread
extends Thread{    
                
public
void run(){    
                        Looper.prepare();
//创建该线程的Looper对象    
                        
//这里Looper.myLooper()获得的就是该线程的Looper对象了    
                        handler =
new ThreadHandler(Looper.myLooper());    
                        Message msg = handler.obtainMessage(1,1,1,
"我自己");    
                        handler.sendMessage(msg);    
                        Looper.loop();    
                }    
                
//定义线程类中的消息处理类    
                
class ThreadHandler
extends Handler{    
                        
public ThreadHandler(Looper looper){    
                                
super(looper);    
                        }    
                        
public
void handleMessage(Message msg){    
                                
//这里对该线程中的MessageQueue中的Message进行处理    
                                
//这里我们再返回给主线程一个消息    
                                
//加入判断看看是不是该线程自己发的信息    
                                
if(msg.what == 1 && msg.obj.equals(
"我自己")){    
                                        handler =
new MyHandler(Looper.getMainLooper());    
                                        Message msg2 = handler.obtainMessage(1,1,1,
"禀告主线程:我收到了自己发给自己的Message");    
                                        handler.sendMessage(msg2);                                    
                                }    
                        }    
                }    
        }    
}    
附注：
 
上面四个例子的布局文件是同一个文件main.xml
 
<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

<?xml version=
"1.0" encoding=
"utf-8"?>    
<LinearLayout xmlns:android=
"http://schemas.android.com/apk/res/android"    
        android:rientation="vertical"    
        android:layout_width="fill_parent"    
        android:layout_height="fill_parent"    
        >    
<TextView    android:id="@+id/view_01"    
        android:layout_width="fill_parent"    
        android:layout_height="wrap_content"    
        android:text="@string/hello"    
        />    
<Button android:id="@+id/btn_01"    
        android:layout_width="fill_parent"    
        android:layout_height="wrap_content"    
        android:text="测试消息" />    
</LinearLayout>