android 消息传递机制

1 源码分析

handler构造函数：

public Handler() {
        this(null, false);
    }
public Handler(Callback callback) {
        this(callback, false);
    }
public Handler(Looper looper) {
        this(looper, null, false);
    }
public Handler(Looper looper, Callback callback) {
        this(looper, callback, false);
    }

一般来说我们都是用Handler()来生成Handler实例的那么我们来看看这个构造函数做了些什么吧：

public Handler() {
        this(null, false);
    }
public Handler(Callback callback, boolean async) {
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
    }

mLooper = Looper.myLooper();是什么意思，我们来看看它是怎么实现的

  public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

线程本地存储区（==Thread Local Storage，简称为TLS==），每个线程都有自己的私有的本地存储区域，不同线程之间彼此不能访问对方的TLS区域，这样问题就来了，为什么不同线程之间彼此不能访问对方的TLS区域？我们来看看ThreadLocal的set方法：

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

在这个方法内部我们看到，首先通过getMap(Thread t)方法获取一个和当前线程相关的ThreadLocalMap，然后将变量的值设置到这个ThreadLocalMap对象中，当然如果获取到的ThreadLocalMap对象为空，就通过createMap方法创建。这又有了一个疑问,这个set方法不会发生并发嘛？线程隔离的秘密，就在于ThreadLocalMap这个类。ThreadLocalMap是ThreadLocal类的一个静态内部类，它实现了键值对的设置和获取（对比Map对象来理解），每个线程中都有一个独立的ThreadLocalMap副本，它所存储的值，只能被当前线程读取和修改。ThreadLocal类通过操作每一个线程特有的ThreadLocalMap副本，从而实现了变量访问在不同线程中的隔离。==因为每个线程的变量都是自己特有的，完全不会有并发错误==（当时不懂这句，自己写还是比较直观）：

static class A implements Runnable {
       int i;
       @Override
       public void run() {
           for(int j=0;j<=2;j++){
               i+=j;
           }
           System.out.println(i);
       }
   }

还有一点就是，ThreadLocalMap存储的键值对中的键是this对象指向的ThreadLocal对象，而值就是你所设置的对象了。接着我们来看看get方法：

 public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

从上面这代码块可以知道，get是从ThreadLocalMap.Entry拿出来的,也是通过set放进去的，那在哪进行set的呢：

private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

当sThreadLocal里面没有东西的时候才会执行set，否则会RuntimeException，到这里来总结下，也就是说使用handler，必须先执行Loop.prepare,问题来了：
- 仔细看看这个方法prepare的参数有什么意义？

private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }
 void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }
        。。。
    }

quitAllowed这个参数所经过的方法，明显，mQuitAllowed只有Ui线程会设为false，这也是prepare方法默认为true的原因吧

• 为什么我们直接使用Handler的时候不需要执行这个方法呢

回到最开始Handler()这个构造函数，一般来说我们都是在UI线程中直接使用这个构造函数，从前面的分析我们应该知道，要使用Handler这个消息传递，那就应该提前使用prepare方法，我们就来做个实验，在UI线程中使用这个方法，不出意料，闪退：

Caused by: java.lang.RuntimeException: 
        Only one Looper may be created per thread 
        at android.os.Looper.prepare(Looper.java:90)
        at android.os.Looper.prepare(Looper.java:85)

也就是上面sThreadLocal.get() != null这个条件，那肯定是在哪里已经执行过了，答案就是在ActivityThread的main函数里面：Looper.prepareMainLooper();ActivityThread6103行：

 public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

注意到prepare里面的参数了嘛，上面也分析了只有在mian
thread中才会为false。
实例化Handler后我们可以用很多方法来发送消息，但最终都是通过enqueueMessage方法来传递消息的：

 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

实际上是执行MessageQueue.enqueueMessage:

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

        synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

mQuitting,MessageQueue默认为false，那么哪里赋为true的呢。通过查找可以找到只有在quit(boolean safe)方法中，而且找不到赋为fasle的地方，这不就说明了当执行了MessageQueue.quit这个方法，MessageQueue也会被回收掉的，接着看后面msg.recycle()：

public void recycle() {
        if (isInUse()) {
            if (gCheckRecycle) {
                throw new IllegalStateException("This message cannot be recycled because it "
                        + "is still in use.");
            }
            return;
        }
        recycleUnchecked();
    }
void recycleUnchecked() {
        // Mark the message as in use while it remains in the recycled object pool.
        // Clear out all other details.
        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) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }

看上去有点晕啊，先来看看Message，这个类的构造函数有很多，要说的就两个：

 public static Message obtain()
 public Message()

尽量不用第二种构造方式，先来分析分析第一种：

public static Message obtain() {
        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();
    }

sPool哪里有赋值呢，通过查找，也就只有上面的recycleUnchecked方法进行赋值了，综合来看，Message对象好像是被循环利用了，根据它的next属性，就知道它是链表的数据结构，这里用到了享元模式，如果用的第二种构造方式，就得重新分配内存了，
接着分析MessageQueue.enqueueMessage,mMessages属性可以说是MessageQueue的头，最先处理的消息。

接着看这个判断语句(p == null || when == 0 || when < p.when)：p是当前的message，when表示消息处理的时间，它的值是距离开机的时间+延迟的时间，when

 public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycleUnchecked();
        }
    }

关键的代码就是msg.target.dispatchMessage(msg);target就是Handler，那么久回到Handler.dispatchMessage方法中：

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
 public void handleMessage(Message msg) {
 //空方法
    }

这些就是整个消息传递机制了

2 参考：

• http://gityuan.com/2015/12/26/handler-message-framework/
• https://blog.csdn.net/cn_hhaip/article/details/51539477
• 郭霖公众号