一、消息隊列的工作原理
消息隊列在Android中指的是MessageQueue，MessageQueue主要包含兩個操作：插入和讀取。讀取操作本身會伴隨着刪除操作，插入和讀取對應的方法分別爲enqueueMessage和next，其中enqueueMessage的作用是往消息隊列中插入一條消息，而next的作用是從消息隊列中取出一條消息並將其從消息隊列中移除。儘管MessageQueue叫消息隊列，但是它的內部實現並不是用的隊列，實際上它是通過一個單鏈表的數據結構來維護消息隊列，單鏈表的插入和刪除上比較有優勢。下面主要看一下他的enqueueMessage和next方法的實現：
android.os.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;
}
從enqueueMessage的實現中可以看出，它的主要操作就是單鏈接的插入操作，這裏就不再過多解釋了，下面看一下next方法的實現，next的主要邏輯：
android.os.MessageQueue#next
Message next() {
...
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;//可以看到這裏取了一個消息出來，就斷開了這個消息的指向。
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
...
}
...
}
}
可以發現next方法是一個無限循環的方法，如果消息隊列中沒有消息，那麼next方法會一直阻塞在這裏。當有新消息到來時，next方法會返回這條消息並將其從單鏈表中移除。
二、Lopper的工作原理
Looper在Android消息機制中扮演着消息循環的角色，具體來說就是它會不停的從MessageQueue中查看是否有新消息，如果有新消息就會立即處理，否則就會一直阻塞在那裏。首先來看下他的構造方法，在構造方法中它會創建一個MessageQueue即消息隊列，然後將當前線程的對象保存起來如下所示。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
我們都知道，Handler的工作需要Looper，沒有Looper的線程就會報錯， 那麼如何爲一個線程創建Looper呢？其實很簡單，就是通過Looper.prepare()即可爲當前線程創建一個Looper，接着通過Looper.loop()來開啓循環，如下所示。
new Thread("Thread #2") {
@Override
public void run() {
Looper.prepare();
Handler mHandler = new Handler();
Looper.loop();
}
}.start();
Looper除了prepare方法外，還提供了parpareMainLooper方法，這個方法主要是給主線程也就是ActivityThread創建Looper使用的，其本質也是通過prepare方法實現的。由於主線程的Looper比較特殊，所以Looper提供了一個getMainLooper方法，通過它可以在任何地方獲取到主線程的Looper。Looper也是可以退出的，提供了quit和quitSafely來退出一個Looper，二者的區別是：quit會直接退出，而quitSafely只是設定一個退出標記，然後把消息隊列中已有消息處理完畢後才安全退出。
Looper退出後，通過Handler發送的消息會失敗，這個時候Handler的send方法會返回false。在子線程中，如果手動爲其創建了Looper，那麼在所有的事情完成以後應該調用quit方法來終止循環，否則子線程會一直處於等待狀態，而如果退出Looper以後，這個線程就會立刻終止，因此建議不需要的時候終止Looper。
Looper最重要的一個方法是loop，只有調用了loop後，消息循環系統纔會真正地起作用，它的實現如下：
android.os.Looper#loop
/**
* 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 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();
}
}
三、Handler的工作原理
Handler的工作主要是包含消息的發送和接收過程。消息的發送是通過post的一系列方法以及send的一系列方法來實現的，post的一系列方法最終是通過send的一系列方法來實現的。發送一條消息的經典過程如下所示。
android.os.Handler#sendMessage
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以發現，Handler發消息的過程僅僅是向消息隊列裏插入一條消息，MessageQueue的next方法就會返回這條消息給Looper，Looper收到消息後就開始處理了，最終消息由Looper交由Handler處理，即Handler的dispatchMessage方法就會被調用，這時Handler就進入了處理消息的階段，dispatchMessage的實現如下：
android.os.Handler#dispatchMessage
/**
* 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);
}
}
最後，調用Handler的handlerMessage方法來處理消息。Handler處理消息的過程可以歸納爲一個流程圖：
Handler還有一個特殊的構造方法，那就是通過一個特定的Looper來構造Handler，它的實現如下所示。通過這個構造方法可以實現一些特殊的功能。
public Handler(Looper looper) {
this(looper, 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());
}
}
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;
}
四、主線程的消息循環
Android的主線程就是ActivityThread，主線程的入口方法爲main，在main方法中系統會通過Looper.prepareMainLooper()來創建主線程的Looper和MessageQueue，並且通過Looper.loop()來開啓主線程的消息循環，如下所示：
android.app.ActivityThread#main
public static void main(String[] args) {
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());
Security.addProvider(new AndroidKeyStoreProvider());
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
}
主線程開始循環之後，ActivityThread還需要一個Handler來和消息隊列交互，這個Handler就是ActivityThreadH，它內部定義了一組消息類型，主要包含了四大組件的啓動和停止等過程，如下所示：
private class H extends Handler {
public static final int LAUNCH_ACTIVITY = 100;
public static final int PAUSE_ACTIVITY = 101;
public static final int PAUSE_ACTIVITY_FINISHING= 102;
public static final int STOP_ACTIVITY_SHOW = 103;
public static final int STOP_ACTIVITY_HIDE = 104;
public static final int SHOW_WINDOW = 105;
public static final int HIDE_WINDOW = 106;
public static final int RESUME_ACTIVITY = 107;
public static final int SEND_RESULT = 108;
public static final int DESTROY_ACTIVITY = 109;
public static final int BIND_APPLICATION = 110;
public static final int EXIT_APPLICATION = 111;
public static final int NEW_INTENT = 112;
public static final int RECEIVER = 113;
public static final int CREATE_SERVICE = 114;
public static final int SERVICE_ARGS = 115;
public static final int STOP_SERVICE = 116;
...
}
ActivityThread通過ApplicationThread和AMS進程進程間通信，AMS以進程間通信的方法完成ActivityThread的請求後回調ApplicationThread中的Binder方法，然後ApplicationThread會向H發送消息，H收到消息後將ApplicationThread中的邏輯切換到ActivityThread去執行，即切換到主線程去執行，這個過程就是主線程的消息循環模型。
到這裏消息機制就分析完了，不涉及很深的原理，但是設計思想很好。