测试代码

      new Thread(new Runnable() {
            @Override
            public void run() {
              /*  Message message1 = Message.obtain();
                message1.obj = "Handler源码分析";
                mHandler.sendMessage(message1);
                Message message2 = Message.obtain();
                mHandler.sendMessageDelayed(message2, 1000);
                Message message3 = Message.obtain();
                mHandler.sendMessageDelayed(message3, 500);*/
                Handler handler=new Handler();
            }
        }).start();

MessageQueue 消息队列

mHandler.sendMessage(message1)源码分析

    public final boolean sendMessage(Message msg)
    {  
        //默认设置消息延迟的时间是0
        return sendMessageDelayed(msg, 0);
    }
  public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        //这里为方便测试，我们默认系统时间不考虑，即message1默认delayMillis的时间是0
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
       //默认消息队列肯定不是空的
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }
 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
      //注意这行将Hadler复制给msg.target
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

查看MessageQueue中的enqueueMessage的源码

 boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {//此时的msg.target等于Handler
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

            msg.markInUse();
            msg.when = when;//此时是message1,msg.when=0;
            Message p = mMessages;//默认肯定是空
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
              //message1进来
                msg.next = p;//msg.next是为空
                mMessages = msg;//mMessage=msg
                needWake = mBlocked;
            } 
        }
        return true;
    }

Handler的MessageQueue1.png

mHandler.sendMessageDelayed(message2, 1000);源码分析

 public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
    //这里默认是1000
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
//其他和上面的message1源码分析走的流程一样直接看最后的MessageQueue中的enqueueMessage源码

 boolean enqueueMessage(Message msg, long when) {
        synchronized (this) {
            msg.when = when;//1000
            Message p = mMessages;//mMessage==Message1,所以P等于Message1
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {//上一个message1等于0
                 msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;//prev=messsage1
                    p = p.next;//此时p等于message指向的下一个指针即为空
                    if (p == null || when < p.when) {
                        break;//跳出
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                //message2下一个指针指向P即为空
                msg.next = p;
               //message1的下一个指针指向的是message2
                prev.next = msg;
            }
            }
        return true;
    }

Handler的MessageQueue2.png

mHandler.sendMessageDelayed(message3, 500);源码分析
直接分析最后一步

 boolean enqueueMessage(Message msg, long when) {
            synchronized (this) {
            msg.markInUse();
            msg.when = when;//msg.when=500
            Message p = mMessages;//还是message1
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                 //还是不会走这儿
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                  needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;//message1
                    p = p.next;//message2
                    if (p == null || when < p.when) {//500<1000
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                //message3.next=message2
                msg.next = p; // invariant: p == prev.next
              //message1=message3
                prev.next = msg;
            }
         
        }
        return true;
    }

Handler的MessageQueue3.png

handler.sendMessage只是将消息保存到消息队列中

Loop 消息循环

我们会知道在子线程中直接new Handler会出错

image.png

为什么会报错呢？我们看下源码

public Handler() {
        this(null, false);
    }

 public Handler(Callback callback, boolean async) {
        mLooper = Looper.myLooper();//获取一个looper
        if (mLooper == null) {//looper为空的时候抛出异常，这也是为什么new Handler的时候会报错
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
       //获得消息队列
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

mLooper源码分析

  public static Looper myLooper() {
        return sThreadLocal.get();
    }
//static 也就是说一个线程中只有一个sThreadLocal 
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

ThreadLocal.get源码分析

 public T get() {
       //获取当前线程
        Thread currentThread = Thread.currentThread();
        //此时默认是空的
        Values values = values(currentThread);
        if (values != null) {
            Object[] table = values.table;
            int index = hash & values.mask;
            if (this.reference == table[index]) {
                return (T) table[index + 1];
            }
        } else {
            values = initializeValues(currentThread);
        }
       //此时默认是空的
        return (T) values.getAfterMiss(this);
    }

解决办法是

 Looper.prepare();
 Handler handler=new Handler();
Looper.loop();

Looper.loop源码分析

   public static void prepare() {
        prepare(true);
    }

private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        //这里就解决了new Handler的时候为空
        sThreadLocal.set(new Looper(quitAllowed));
    }
//new Loope主要是new一个消息队列和获得当前线程
 private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }
//设置set源码
public void set(T value) {
        Thread currentThread = Thread.currentThread();
        Values values = values(currentThread);
        if (values == null) {
            values = initializeValues(currentThread);
        }
         //key是ThreadLocal,因为它是唯一的所以值也是唯一的
        values.put(this, value);
    }

总结：其实就是创建一个 Looper ，并且保证一个 Thread 线程中，只有一个 Looper 对象
Looper.loop();源码分析

 public static void loop() {
        final Looper me = myLooper();//获得一个looper对象
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;//获得消息队列

      
        final long ident = Binder.clearCallingIdentity();

        for (;;) {//实际就是不断的获得下一个对象,直到消息队列中没有消息
            Message msg = queue.next(); //指向下一个，链式表
            if (msg == null) {
             //如果消息为空返回
                return;
            }
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
               //msg.target实际是Handler
            msg.target.dispatchMessage(msg);

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }
            msg.recycleUnchecked();
        }
    }

Handler中的分发消息

 public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
//实际最终调用的是我们自己写的handlerMessage
public void handleMessage(Message msg) {
    }

recycleUnchecked回收源码分析

void recycleUnchecked() {
       
        flags = FLAG_IN_USE;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        sendingUid = -1;
        when = 0;
        target = null;
        callback = null;
        data = null;

        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {//最大是50
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }

问题一：为什么主线程中不需要设置Looper.prepare
答：因为在我们应用启动的时候，ActivityThread 的入口函数 main() 方法中已经帮我们写了这行代码

  Looper.prepareMainLooper();
  Looper.loop();

问题二：为什么Mssage.obtain比new 一个Messae好

  public static Message obtain() {//线程池没有就new有就拿
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }