今天打算写一下Handler机制的原理及源码解析，Handler在我们的开发中用到的频率还是非常高的，同时这也是一个非常基础的知识点，但是即使是基础知识，有很多工作两三年的安卓开发依然是一知半解，搞不清楚原理，包括View、ViewGroup的事件分发及绘制流程。
在深入学习一下知识点之前，希望能够带着疑问去思考：
1.为什么在子线程实例化Handler会报错闪退，而主线程不会
2.为什么每个线程只能存在一个Looper和MessageQueue
3.多个Handler发送消息是怎么保证Looper轮询消息队列发送最新消息不错乱发给其他Handler的
4.子线程真的不能更新UI吗？
5.ThreadLocal的作用
......

 Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            Log.e("接收消息", (String) msg.obj);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Message msg = handler.obtainMessage();
        msg.obj= "hello world";
        handler.sendMessage(msg);
    }

我们在实际的开发中基本上都是这样类似的写法，但是为什么当我们在子线程实例化Handler就不行了呢

 public Handler() {
        this(null, false);
    }
 public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }
        // 获取我的轮询器
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
            // 在实例化Handler时， mLooper 为空的时候就会抛出这个异常
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

那么问题找到，Looper为空，接下来我们再看Looper里的代码

 // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
      public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

好的，问题找到，也回到了我们第4个问题ThreadLocal是什么

public class ThreadLocal<T> {
/**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        // 获取当前线程
        Thread t = Thread.currentThread();
      // 获取当前线程的私有数据变量
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }
/**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

}

 /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
//  Thread 源码里的私有变量，为 Map 键值对类型
    ThreadLocal.ThreadLocalMap threadLocals = null;

Java基础不算太差的朋友相信已经懂了，sThreadLocal.get()获取的Looper其实就保存在当前线程的私有变量threadLocals，那我们怎样给子线程加一个Looper呢，为什么在主线程可以直接使用呢？

/** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    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");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

在子线程调用Looper.prepare() 就可以给当前线程添加一个轮询器，主线程之所以不需要开发者添加，是因为在程序启动的时候，famework层就已经给我们实例化了一个，在ActivityThread的main()方法中

  public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
        SamplingProfilerIntegration.start();

        // CloseGuard defaults to true and can be quite spammy.  We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("<pre-initialized>");
        // 实例化轮询器
        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        // 开启消息轮询
        Looper.loop();

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

当你在子线程加上轮询器时，却发现并不行，因为轮询器还没有开启轮询消息

 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();
        // 进入for死循环
        for (;;) {
            Message msg = queue.next(); // might block
            // 如果消息为空就return，进入下一次轮询
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            final long end;
            try {
                msg.target.dispatchMessage(msg);
                end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (slowDispatchThresholdMs > 0) {
                final long time = end - start;
                if (time > slowDispatchThresholdMs) {
                    Slog.w(TAG, "Dispatch took " + time + "ms on "
                            + Thread.currentThread().getName() + ", h=" +
                            msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                }
            }

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

            // 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();
        }
    }

那么接下来我们从头到尾过一遍Handler机制，首先Looper在程序启动的时候系统就已经帮我们创建好了，那我们去看一下方法里面的实现。

 // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
 /**
     * Initialize the current thread as a looper, marking it as an
     * application's main looper. The main looper for your application
     * is created by the Android environment, so you should never need
     * to call this function yourself.  See also: {@link #prepare()}
     */
//   系统实例化Handler
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

  private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
       // 往当前线程的私有变量里添加 Looper
        sThreadLocal.set(new Looper(quitAllowed));
    }
// Looper 在实例化的时候也实例化了一个消息队列同时还持有了当前线程的引用
  private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

然后我们从发送消息查看源码

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
------经过几个方法的调用进入下面的方法
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
       // mQueue 在Handler 实例化的时候就从当前线程中取出消息队列并赋值了
        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) {
       // 重点在这里，把当前Handler的引用赋值给 msg 的 target
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

进入消息队列的源码

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;
        // p == null 代表前面没有消息， when 是延迟消息的时间值
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
              //  当前消息的 next  是 p 引用，形成一个单链表结构，如果是第一个消息的话，p为空
                msg.next = p;
              // 赋值消息到轮询器
                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;
          // 发送了第一个消息后   mMessages 就不为空了
                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;
    }

Looper.loop()会进入轮询，然后从Looper获取MessageQueue ，再调用MessageQueue .next()，这个方法不再解析，简单的说就是把当前的消息返回，注意理解Message的单链表结构，next是null，当msg获取的是null，进入下一次轮询，消息不为空调用msg.target.dispatchMessage(msg),target就是Handler，所以消息不会发送错乱。
最后再啰嗦一句，子线程可以更新UI，View不是只能在主线程更新UI，而是只能在创建它的线程更新UI，详情见源码的ViewRootImpl。