这里主要是先了解整个消息传递的过程，知道这样做的好处和必要性。而不是直接介绍里面的几个关键类，然后介绍这个机制，这样容易头晕。而且网络上已经有很多这样的文章了，那些作者所站的高度对于我这种初学者来说有点高，我理解起来是比较稀里糊涂的，所以这里从一个问题出发，一步一步跟踪代码，这里只是搞清楚 handler 是怎么跨线程收发消息的，具体实现细节还是参考网上的那些大神的 Blog 比较权威。
PS. 本来是想分章节书写，谁知道这一套军体拳打下来收不住了，所以下面基本是以一种很流畅的过程解释而不是很跳跃，细心看应该会对理解 Handler 机制有所收获。
Q1: 假如有一个耗时的数据处理，而且数据处理的结果是对 UI 更新影响的，而 Android 中 UI 更新不是线程安全的，所以规定只能在主线程中更新。
下面我们有两种选择：
主线程版本：public class MainActivity extends AppCompatActivity {private static final String TAG = "MainActivity";private Button btnTest;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.layout_test);init();}private void init() {btnTest = (Button) findViewById(R.id.btn_test);btnTest.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {// 假装数据处理int i = 0;for (i = 0; i < 10; i++) {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}// 假装更新 UILog.d(TAG, "Handle it！" + i);}});}
}复制代码直接在主线程中处理数据，接着直接根据处理结果更新 UI。我想弊端大家都看到了，小则 UI 卡顿，大则造成 ANR。
子线程版本：public class MainActivity extends AppCompatActivity {private static final String TAG = "MainActivity";private Button btnTest;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.layout_test);init();}private void init() {btnTest = (Button) findViewById(R.id.btn_test);btnTest.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {new Thread(new Runnable() {@Overridepublic void run() {// 假装数据处理int i;for (i = 0; i < 10; i++) {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}// 返回处理结果handler.sendEmptyMessage(i);}}).start();}});}Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {// 假装更新 UILog.d(TAG, "Handle MSG = " + msg.what);}};
}复制代码这是一种典型的处理方式，开一个子线程处理数据，通过 Android 中提供的 Handler 机制进行跨线程通讯，把处理结果返回给主线程，进而更新 UI。这里我们就是探讨 Handler 是如何把数据发送过去的。
到这里，我们了解到的就是一个 Handler 的黑盒机制，子线程发送，主线程接收。接下来，我们不介绍什么 ThreadLocal、Looper 和 MessageQueue。而是直接从上面的代码引出它们的存在，从原理了解它们存在的必要性，然后在谈它们内部存在的细节。
一切罪恶源于 handler.sendEmptyMessage();，最终找到以下函数 sendMessageAtTime(Message msg, long uptimeMillis)：
Handler.class
/*** Enqueue a message into the message queue after all pending messages* before the absolute time (in milliseconds) <var>uptimeMillis</var>.* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>* Time spent in deep sleep will add an additional delay to execution.* You will receive it in {@link #handleMessage}, in the thread attached* to this handler.* * @param uptimeMillis The absolute time at which the message should be* delivered, using the* {@link android.os.SystemClock#uptimeMillis} time-base.* * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the* looper processing the message queue is exiting. Note that a* result of true does not mean the message will be processed -- if* the looper is quit before the delivery time of the message* occurs then the message will be dropped.*/
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {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);
}复制代码MessageQueue 出来了，我们避免不了了。里面主要是 Message next() 和 enqueueMessage(Message msg, long when) 方法值得研究，但是现在还不是时候。
从 MessageQueue queue = mQueue; 中可以看出我们的 handler 对象里面包含一个 mQueue 对象。至于里面存的什么怎么初始化的现在也不用太关心。大概有个概念就是这是个消息队列，存的是消息就行，具体实现细节后面会慢慢水落石出。
后面的代码就是说如果 queue 为空则打印 log 返回 false；否则执行 enqueueMessage(queue, msg, uptimeMillis); 入队。那就好理解了，handler 发送信息其实是直接把信息封装进一个消息队列。
Handler.class
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;if (mAsynchronous) {msg.setAsynchronous(true);}return queue.enqueueMessage(msg, uptimeMillis);
}复制代码这里涉及 Message，先说下这个类的三个成员变量：
/*package*/ Handler target;/*package*/ Runnable callback;/*package*/ Message next;复制代码所以 msg.target = this; 把当前 handler 传给了 msg。
中间的 if 代码先忽略，先走主线：执行了 MessageQueue 的 enqueueMessage(msg, uptimeMillis);方法。接着看源码
MessageQueue.class
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.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;
}复制代码代码有点长，不影响主线的小细节就不介绍了，那些也很容易看懂的，但是原理还是值得分析。if (mQuitting)...，直接看看源码初始化赋值的函数是在 void quit(boolean safe) 函数里面，这里猜测可能是退出消息轮训，消息轮训的退出方式也是值得深究，不过这里不影响主线就不看了。 msg.markInUse(); msg.when = when; 标记消息在用而且继续填充 msg，下面就是看注释了。我们前面介绍的 Message 成员变量 next 就起作用了，把 msg 链在一起了。所以这里的核心就是把 msg 以一种链表形式插进去。似乎这一波分析结束了，在这里划张图总结下：
推荐自己根据所观察到的变量赋值进行绘制图画，这样印象更加深刻。
OK，消息是存进去了，而且也是在 handler 所在的线程中。那么到底怎么取出信息呢？也就是前面小例子
Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {// 假装更新 UILog.d(TAG, "Handle MSG = " + msg.what);}
};复制代码handleMessage() 什么时候调用？这里基本断了线索。但是如果你之前哪怕看过类似的一篇文章应该都知道其实在 Android 启动时 main 函数就做了一些操作。这些操作是必要的，这也就是为什么我们不能直接在子线程中 new Handler();。
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 libcoreEventLogger.setReporter(new EventLoggingReporter());// Make sure TrustedCertificateStore looks in the right place for CA certificatesfinal File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());TrustedCertificateStore.setDefaultUserDirectory(configDir);Process.setArgV0("<pre-initialized>");Looper.prepareMainLooper(); // -------1ActivityThread thread = new ActivityThread();thread.attach(false);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler(); // -------2}if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);Looper.loop(); // -------3throw new RuntimeException("Main thread loop unexpectedly exited");
}复制代码可以看出这里在获取 sMainThreadHandler 之前进行了 Looper.prepareMainLooper(); 操作，之后进行了 Looper.loop(); 操作。
下面开始分析：
Loopr.class/** 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));
}/*** 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()}*/
public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}
}/*** Return the Looper object associated with the current thread. Returns* null if the calling thread is not associated with a Looper.*/
public static @Nullable Looper myLooper() {return sThreadLocal.get();
}复制代码前两个方法是在自己创建 Looper 的时候用，第三个是主线程自己用的。由于这里消息传递以主线程为线索。prepare(false);说明了这是主线程，在 sThreadLocal.set(new Looper(quitAllowed)); 中的 quitAllowed 为 false 则说明主线程的 MessageQueue 轮训不能 quit。这句代码里还有 ThreadLocal 的 set() 方法。先不深究实现，容易晕，这里需要知道的就是把一个 Looper 对象“放进”了 ThreadLocal，换句话说，通过 ThreadLocal 可以获取不同的 Looper。
最后的 sThreadLocal.get(); 展示了 get 方法。说明到这时 Looper 已经存在啦。
现在看看 Looper 类的成员变量吧！
Looper.class
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.classfinal MessageQueue mQueue;
final Thread mThread;复制代码在这里先介绍一下 ThreadLocal 的上帝视角吧。直接源码，可以猜测这是通过一个 ThreadLocalMap 的内部类对线程进行一种 map。传进来的泛型 T 正是我们的 looper。所以 ThreadLocal 可以根据当前线程查找该线程的 Looper，具体怎么查找推荐看源码，这里就不介绍了。
/*** 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() {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();
}* Sets the current thread's copy of this thread-local variable* to the specified value. Most subclasses will have no need to* override this method, relying solely on the {@link #initialValue}* method to set the values of thread-locals.** @param value the value to be stored in the current thread's copy of* this thread-local.*/
public void set(T value) {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);
}复制代码分析到这里，handler 和 looper 都有了，但是消息还是没有取出来？
这是看第三句 Looper.loop();。
Looper.class
/*** Run the message queue in this thread. Be sure to call* {@link #quit()} to end the loop.*/
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 blockif (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 loggerfinal Printer logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}final long traceTag = me.mTraceTag;if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}try {msg.target.dispatchMessage(msg);} finally {if (traceTag != 0) {Trace.traceEnd(traceTag);}}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();}
}复制代码一开始也是获取 Looper，但是那么多 Looper 怎么知道这是哪个 Looper 呢？这先放着待会马上解释。把 loop() 函数主要功能搞懂再说。
接下来就是获取 Looper 中的 MessageQueue了，等等，这里提出一个疑问，前面说了 Handler 中也存在 MessageQueue，那这之间存在什么关系吗？（最后你会发现其实是同一个）
先往下看，一个死循环，也就是轮训消息喽，中间有一句 msg.target.dispatchMessage(msg); 而前面介绍 msg.target 是 handler 型参数。所以和 handler 联系上了。
Handler.class
/*** Handle system messages here.*/
public void dispatchMessage(Message msg) {if (msg.callback != null) {handleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}handleMessage(msg);}
}复制代码逻辑很简单，总之就是调动了我们重写的 handleMessage() 方法。
Step 1：Looper.prepare();
在 Looper 中有一个静态变量 sThreadLocal，把创建的 looper “存在” 里面，创建 looper 的同时创建 MessageQueue，并且和当前线程挂钩。
Step 2：new Handler();
通过上帝 ThreadLocal，并根据当前线程，可获取 looper，进而获取 MessageQueue，Callback之类的。
```java
Handler.class
/**
- Use the {@link Looper} for the current thread with the specified callback interface
- and set whether the handler should be asynchronous.
*- Handlers are synchronous by default unless this constructor is used to make
- one that is strictly asynchronous.
*- Asynchronous messages represent interrupts or events that do not require global ordering
- with respect to synchronous messages. Asynchronous messages are not subject to
- the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*- @param callback The callback interface in which to handle messages, or null.
- @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
- each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*- @hide
*/
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());}复制代码
mLooper = Looper.myLooper();
if (mLooper == null) {throw new RuntimeException("Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; // 前面的两个 MessageQueue 联系起来了，疑问已解答。
mCallback = callback;
mAsynchronous = async;复制代码}`` 这个函数可以说明在 new Handler() 之前该线程必需有 looper，所以要在这之前调用Looper.prepare();`。
Step 3：Looper.loop();
进行消息循环。
基本到这里整个过程应该是清楚了，这里我画下我的理解。
那么我们现在来看一下 handler 是怎么准确发送信息和处理信息的。注意在 handler 发送信息之前，1、2、3 步已经完成。所以该获取的线程已经获取，直接往该线程所在的 MessageQueue 里面塞信息就行了，反正该信息会在该 handler 所在线程的 looper 中循环，最终会通过消息的 target 参数调用 dispatchMessage()，而在 dispatchMessage() 中会调用我们重写的 handleMessage() 函数。
多谢阅读