转载自：https://blog.csdn.net/qingchunweiliang/article/details/50448365
public class Handler {
private MessageQueue messageQueue;
public Handler() {
Looper looper=Looper.myLooper();
if (looper==null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
this.messageQueue=looper.messageQueue;
}
public void sendMessage(Message msg) {
//Looper循环中发现message后，调用message.targer就得到了当前handler，使用taget.handleMessage
//就把消息转发给了发送message时的handler的handleMessage函数
msg.target=this;
messageQueue.enqueueMessage(msg);
}
public void handleMessage(Message msg) {
}
}
public class Looper {
private static final ThreadLocal<Looper> threadLocal=new ThreadLocal<>();
/**
* 存储Message的队列，阻塞式，没有消息则一直等待
*/
final MessageQueue messageQueue;
private Looper() {
messageQueue=new MessageQueue();
}
/**为该线程创建Looper，
* 若该线程已经有Looper了则不需要再次调用prepare
*/
public static void prepare() {
if (threadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
threadLocal.set(new Looper() );
}
public static void loop() {
Looper looper=myLooper();
if (looper == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
MessageQueue messageQueue=looper.messageQueue;
for(;;){
Message message=messageQueue.next();
message.target.handleMessage(message);
}
}
/**
* 获取当先线程的Looper
* @return
*/
public static Looper myLooper() {
return threadLocal.get();
}
}
public class Message {
Handler target;
public Object obj;
public int what;
@Override
public String toString() {
return "what="+what+" obj="+obj.toString();
}
}
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
public class MessageQueue {
private BlockingQueue<Message>blockingQueue=new LinkedBlockingQueue<>();
/**
* 阻塞式，没有消息则一直等待
* @return
*/
public Message next() {
try {
return blockingQueue.take();
} catch (InterruptedException e) {
throw new RuntimeException();
}
}
/**
* 插入到消息队列尾部
* @param message
*/
void enqueueMessage(Message message) {
try {
blockingQueue.put(message);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
import java.util.HashMap;
import java.util.Map;
/**
* ThreadLocal简单实现
* @author Young
*
* @param <T>
*/
public class ThreadLocal<T> {
private Map<Thread,T>map;
public ThreadLocal() {
map=new HashMap<>();
}
public void set(T obj) {
map.put(Thread.currentThread(),obj);
}
public T get() {
return map.get(Thread.currentThread());
}
}
public class Main {
public static void main(String[] args) {
new Main().start();
}
private void start() {
//创建该线程唯一的消息队列，线程安全的阻塞队列
Looper.prepare();
onCreate();
//死循环，阻塞式，执行下面代码后主线程就会去获取消息队列里的消息，没有消息时就阻塞，有就执行。执行Looper.loop前即使消息队列里有消息，消息也不会执行，因为主线程还没有去检查消息队列。
Looper.loop();
//下面 的代码通常不会执行，除非手动让主线程消息队列退出。退出主线程消息队列后android的view布局、绘制，事件分发就不执行了，所以android APP也没必要继续执行了，所以android采用了抛出异常的方式结束APP。
System.out.println("exit........");
throw new RuntimeException("Main thread loop unexpectedly exited");
}
private void onCreate() {
//下面的操作相当于运行在android的UI线程中
final Thread thread = Thread.currentThread();
System.out.println("main thread=" + thread);
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
//若thread == Thread.currentThread()，则证明已经运行在主线程中了
System.out.println("current thread is main thread? " + (thread == Thread.currentThread()));
System.out.println(msg);
System.out.println();
}
};
// 测试1 主线程创建handler，子线程使用该handler发送消息
new Thread() {
public void run() {
try {//模拟耗时操作
Thread.sleep(1000 * 2);
} catch (InterruptedException e) {
}
Message message = new Message();
message.obj = "new Thread" + Thread.currentThread();
message.what = (int) System.currentTimeMillis();
//在子线程中发送消息
handler.sendMessage(message);
try {
Thread.sleep(1000 * 2);
} catch (InterruptedException e) {
}
message = new Message();
message.obj = "hanler...waht==1";
message.what = 1;
//在子线程中发送消息
handler.sendMessage(message);
try {
Thread.sleep(1000 * 2);
} catch (InterruptedException e) {
}
message = new Message();
message.obj = "hanler...waht==2";
message.what = 2;
//在子线程中发送消息
handler.sendMessage(message);
try {
Thread.sleep(1000 * 2);
} catch (InterruptedException e) {
}
message = new Message();
message.obj = "hanler...waht==3";
message.what = 3;
//在子线程中发送消息
handler.sendMessage(message);
}
;
}.start();
// 测试2 在thread内部创建handler，结果会抛出异常
new Thread() {
public void run() {
try {
sleep(1000 * 3);
} catch (InterruptedException e) {
}
/*
* 在线程内部使用默认构造函数创建handler会抛出异常。
* android中也可以在子线程中创建Handler，但要在初始化时传入Looper，
* Looper.getMainLooper()获取到的就是主线程的Looper，所以可以这样创建
*
* new Handler(Looper.getMainLooper()){
@Override
public void handleMessage(Message msg) {
//运行在主线程中
}
};
*/
Handler h = new Handler() {
public void handleMessage(Message msg) {
System.out.println("haneler msg...." + msg);
}
};
Message message = new Message();
message.obj = "handler in new Thread";
message.what = (int) System.currentTimeMillis();
//在子线程中发送消息
h.sendMessage(message);
}
}.start();
//上面的操作相当于运行在android的UI线程中
}
}
运行结果：
image.png
程序并没有停止，主线程消息队列仍旧阻塞等待新消息到来
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