做過android開發的同學，相信對android的消息處理都不陌生。在我們開發應用中也是無時不刻不用到它，相信大家對handler的消息處理都能夠熟練使用了，那么本文將從源碼角度帶領讀者完成Handler，MessageQueue，Looper，ThreadLocal之間的聯系以及以及從消息發送到完成接收，系統為我們做了些什么。
handler相信大家都不陌生了，要完成線程間的消息通信一定會用到它，而使用handler無非是用到他的sendMessage或post方法，以及handleMessage回調。其實，在整個消息流中也確實是只有這些處理了。主要依賴的還是MessageQueue,Looper與handler之間的聯系。
在消息發送與消息處理完成，MessageQueue對于我們一直是不可見的，但我們卻必須用到它。MessageQueue作為消息隊列，顧名思義是用來存儲消息的。而MessageQueue作為消息隊列，主要完成消息的插入與刪除操作。即enqueueMessage與next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;
}
通過enqueueMessage方法，將message插入到合適的位置，從這里我們可以看到，MessageQueue并非使用隊列作為消息存儲的，其實使用了單向鏈表的結構。
接下來我們看MessageQueue是如何獲取消息與刪除消息的：
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
可以看到在next方法中，一開始就進入了一個死循環 for (;;)，在循環中主要完成對message的查找并返回的操作，當MessageQueue為空時，那么就會一直處于block狀態，直到MessageQueue不為空或mQuitting為true.
分析完MessageQueue我們接下來看Looper，其實Looper在消息通信中主要扮演了消息循環的角色，這一點可以從Looper.loop方法中看出：
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 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();
}
}
在loop中仍然是進行了死循環操作 for (;;)，在循環中不斷從獲取MessageQueue中獲取消息：
Message msg = queue.next();
唯一跳出循環的條件是msg == null。其實當我們調用Looper.quit()時，就是改變MessageQueue的mQuitting為true，使其返回null退出的。
當獲取到新消息時，會調用到
msg.target.dispatchMessage(msg);
而msg.target又是一個handler對象，咦，是不是有些熟悉呢？其實一點也沒錯，這里的handler就是我們一開始在代碼里面定義的handler,這一點可以在handler中找到：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
不管你是通過send還是post方式，最終都會調用到這里的，不信自己可以去跟蹤下代碼。
接下來我們繼續跟蹤下msg.target.dispatchMessage(msg)；方法
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
到這里我們又回到了handler方法中，又回到了最初的起點：這里會進行條件判斷，如果msg.callback不為null,處理handleCallback(msg)，而msg.callback是我們通過handler的post方法傳進來的Runnable對象，這一點源碼中可以看到：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
繼續跟蹤getPostMessage(r)方法：
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
可以看到，通過getPostMessage方法，將Runnable對象賦給了Message的callback回調：
而handleCallback最終對調用到Runnalbe的run方法：
private static void handleCallback(Message message) {
message.callback.run();
}
而第二個判斷mCallback != null是作為構造參數傳給handler，是我們自己定義實現的callback回調；最后一種實現是handleMessage(msg);是一個空實現，是由我們自己繼承handleMessage(msg)來實現的。
到這里我們從消息發送到消息接收并處理整個流程都跑通了，是不是該結束了呢？
不，因為還有一個核心的東西我們還沒有提到，而它正是我們可以完成跨線程通信的核心機制，就是我們開始說到的ThreadLocal，ThreadLocal并不是本地線程，甚至連一個新線程也不算，為什么要叫ThreadLocal？他跟Thread有什么關系？這里我們要從Looper的生命周期開始看起：
在我們創建handler時，如果我們沒有在構造中傳入Looper，那么handler會在構造方法中初始化Looper.
mLooper = Looper.myLooper();
在Looper.myLooper()中：
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
這里調用到了sThreadLocal.get()方法，而sThreadLocal正是ThreadLocal對象，而在sThreadLocal.get()方法中：
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();
}
這里用到了ThreadLocalMap類，主要用作數據存儲的，而在getMap(t)方法中，
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
返回了Thread的threadLocals對象，也是ThreadLocalMap的對象，其實ThreadLocalMap是用作數據存儲的，而ThreadLocalMap既是ThreadLocal的內部類，又是Thread的成員變量，不過通過ThreadLocal的set和get方法我們可以得知其實ThreadLocal操作的就是當前線程的ThreadLocalMap對象，通過調用Looper.parpare()為當前線程的ThreadLocalMap對象設置looper對象：
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sThreadLocal.set(new Looper(quitAllowed))方法中：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
當第一次創建Map時，
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
將Looper對象最終傳給了當前線程的ThreadLocalMap對象并緩存，這就是為什么Looper和線程為什么可以綁定了，因為每一個Thread對應了一個Looper對象，當我們通過調用當前線程的Looper.parpare時，其實就已經將 Looper與當前線程綁定在了一起，而Looper持有了MessageQueue的引用，所以接下來的操作都綁定在了Looper所關聯的線程中了。所以handler處理消息回調時所在的線程并不一定是在handler的創建線程。
不信我們可以試試在子線程中創建handler，但在handler構造中傳入主線程的Looper,并在回調中打印當前線程名。
你發現規律了嗎？
