handler机制是Android重要的多线程数据传输机制，所以想从源码来解析这个机制。

一般使用

在Activity中

public class MainActivity extends AppCompatActivity {

    Handler handler = new Handler(new Handler.Callback() {
        @Override
        public boolean handleMessage(Message msg) {
            switch (msg.what){
                case 1:
                    showToast((String) msg.obj);
                    break;
            }
            return false;
        }
    });

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button = findViewById(R.id.btn_start);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        Message message = Message.obtain();
                        message.what = 1;
                        message.obj = "Handler机制";
                        handler.sendMessage(message);
                    }
                }).start();
            }
        });
    }

    private void showToast(String content){
        Toast.makeText(this,content,Toast.LENGTH_SHORT).show();
    }
}

Handler机制

1 最基本的Message

Message两种创建

• 直接构造

Message message = new Message();

• 通过obtain()方法

    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    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();
    }

注释很清楚，从全局池返回一个Message实例，避免在很多情况下分配新对象

很重要的变量

Handler target;
Message next;

这两个之后会用到

2 handler.sendMessage方法

    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

这些相关的sendMessage方法最终会调用一个enqueueMessage方法

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

这个方法中将Message的target赋值为handler自己，并调用了MessageQueue.enqueueMessage方法

3 MessageQueue.enqueueMessage方法

    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;
    }

核心大概这些代码，其实就是一个单向链表

    boolean enqueueMessage(Message msg, long when) {
        //保证线程安全性
        synchronized (this) {
            
            msg.when = when;
            Message p = mMessages;
            //这个消息是第一个或这个消息无需延迟 或 这个消息延迟时间比第一个消息短
            if (p == null || when == 0 || when < p.when) {
                //把这个消息放到链表的头
                msg.next = p;
                mMessages = msg;
               
            } else {
                //循环整个链表，直到结束或这个消息延迟时间比某一个消息短
                // 1. 直到结束情况：把这个消息放到链表的末尾
                // 2.这个消息延迟时间比某一个消息短： prev -> p 改为 prev -> msg -> p
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }
        }
        return true;
    }

本质上完成对于把你传递的消息跟之前的消息排序

4. Looper的loop方法（循环消息队列）

    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;
            }

            // 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();
        }
    }

其实核心就干了一件事msg.target.dispatchMessage(msg);

    public static void loop() {
        final Looper me = myLooper();
        final MessageQueue queue = me.mQueue;
 
        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);
                }
            }
        }
    }

诡异的是这个和我们最开始Handler的实现的回调方法名并不是一样的

5.Handler的handleMessage方法

原来最后是由dispatchMessage方法调用的，其实也是因为两种方式来实现handleMessage，一种通过匿名接口回调实现，另外一种直接重写handleMessage

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

一些疑问

Handler与Looper的关联

    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());
            }
        }

        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;
    }

所以的构造都会调用这个，在这个里面调用获取了mLooper和mQueue

Looper的创建

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

    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));
    }

通过ThreadLocal的特性，保证每个线程只存在一个Looper对象。

ThreadLocal是一个关于创建线程局部变量的类。
通常情况下，我们创建的变量是可以被任何一个线程访问并修改的。而使用ThreadLocal创建的变量只能被当前线程访问，其他线程则无法访问和修改。

而这个实在ActivityThread（并没有继承线程）中被调用prepareMainLooper方法

public static void main(String[] args) {
        ...
        Looper.prepareMainLooper();
        ...

Handler 为什么可以 post Runnable

其实就是Handler自己帮你把Runnable转换成Message

    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

子线程展示Toast报错的原因

很多人都以为子线程展示Toast是因为不在主线程，其实并不是。

private static class TN extends ITransientNotification.Stub {
...
TN(String packageName, @Nullable Looper looper) {
            ...

            if (looper == null) {
                // Use Looper.myLooper() if looper is not specified.
                looper = Looper.myLooper();
                if (looper == null) {
                    throw new RuntimeException(
                            "Can't toast on a thread that has not called Looper.prepare()");
                }
            }
            mHandler = new Handler(looper, null) {
                @Override
                public void handleMessage(Message msg) {
                    switch (msg.what) {
                        case SHOW: {
                            IBinder token = (IBinder) msg.obj;
                            handleShow(token);
                            break;
                        }
                        case HIDE: {
                            handleHide();
                            // Don't do this in handleHide() because it is also invoked by
                            // handleShow()
                            mNextView = null;
                            break;
                        }
                        case CANCEL: {
                            handleHide();
                            // Don't do this in handleHide() because it is also invoked by
                            // handleShow()
                            mNextView = null;
                            try {
                                getService().cancelToast(mPackageName, TN.this);
                            } catch (RemoteException e) {
                            }
                            break;
                        }
                    }
                }
            };
        }
}

因为Toast需要初始化一个Handler，并且最后的show仍然是在Handler的回调执行的
同理，子线程展示Dialog报错也是因为这个
如果非要在子线程展示Toast,可以这样

 class LooperThread extends Thread {
        public Handler mHandler;
  
        public void run() {
           Looper.prepare();
           Toast.makeText(getApplicationContext(),"子线程展示Toast",Toast.LENGTH_SHORT).show();
           Looper.loop();
       }
  }

但是不要这么做，因为Looper会阻塞这个线程

主线程不会因为Looper.loop()里的死循环卡死

这里涉及线程，先说说说进程/线程，进程：每个app运行时前首先创建一个进程，该进程是由Zygote fork出来的，用于承载App上运行的各种Activity/Service等组件。进程对于上层应用来说是完全透明的，这也是google有意为之，让App程序都是运行在Android Runtime。大多数情况一个App就运行在一个进程中，除非在AndroidManifest.xml中配置Android:process属性，或通过native代码fork进程。
线程：线程对应用来说非常常见，比如每次new Thread().start都会创建一个新的线程。该线程与App所在进程之间资源共享，从Linux角度来说进程与线程除了是否共享资源外，并没有本质的区别，都是一个task_struct结构体，在CPU看来进程或线程无非就是一段可执行的代码，CPU采用CFS调度算法，保证每个task都尽可能公平的享有CPU时间片。
有了这么准备，再说说死循环问题：
对于线程既然是一段可执行的代码，当可执行代码执行完成后，线程生命周期便该终止了，线程退出。而对于主线程，我们是绝不希望会被运行一段时间，自己就退出，那么如何保证能一直存活呢？简单做法就是可执行代码是能一直执行下去的，死循环便能保证不会被退出，例如，binder线程也是采用死循环的方法，通过循环方式不同与Binder驱动进行读写操作，当然并非简单地死循环，无消息时会休眠。但这里可能又引发了另一个问题，既然是死循环又如何去处理其他事务呢？通过创建新线程的方式。
真正会卡死主线程的操作是在回调方法onCreate/onStart/onResume等操作时间过长，会导致掉帧，甚至发生ANR，looper.loop本身不会导致应用卡死。
转载Gityuan的知乎回答

也就是说looper本身不会，它只是分发消息。ANR只是出现在处理消息的时候（也就是我们触摸屏幕等）

总结handler机制流程图

最后整个机制可以用这个流程图表示

handler机制流程图.png