Android訊息機制原理解析
Android訊息機制:主要指Handler的執行機制以及Handler所附帶的MessageQueue和Looper的工作過程。
Handler：主要作用是將一個任務切換到指定的執行緒中去執行。Android中UI控制元件不是執行緒安全的，所以在Android中不允許子執行緒訪問UI，而當子執行緒需要訪問UI時，Android提供了Handler來解決這個問題。
MessageQueue訊息佇列：儲存訊息列表，內部儲存結構是單鏈表的資料結構
Looper迴圈：以無限迴圈的形式去查詢是否有新訊息，如果有就處理訊息，如果沒有就一直等待。
ThreadLocal的工作原理：
ThreadLocal是一個執行緒內部資料儲存類，通過它可以在指定執行緒中儲存資料，資料儲存以後，只有在指定執行緒中可以獲取到儲存的資料，對於其他執行緒則無法獲取到資料。
Handler為什麼要用到這個呢？因為對於Handler來說，需要獲取到當前執行緒的Looper，而Looper的作用域就是當前執行緒，並且在不同執行緒有不同的Looper，這個時候可以方便的通過ThreadLocal對Looper進行存取。
大概瞭解ThreadLocal後，下面分析下ThreadLocal的內部實現，ThreadLocal是一個泛型類，只要弄清楚ThreadLocal的get()和set()方法就可以明白它的工作原理。
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}set()方法中，首先是獲取到當前使用這個ThreadLocal物件的執行緒，然後根據當前執行緒獲取對應的ThreadLocalMap 物件，如果沒有ThreadLocalMap 物件，則建立一個，如果有則把資料存入ThreadLocalMap 物件中。
getMap()方法返回的是Thread.threadLocals，在Thread內部我們發現threadLocals變數是用來儲存ThreadLocal的資料，所以getMap()直接返回這個屬性，當該屬性為空時，通過createMap(t, value)來給Thread.threadLocals賦值。
/* ThreadLocal values pertaining to this thread. This map is maintained by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;ThreadLocalMap類是ThreadLocal類的一個靜態內部類，在ThreadLocalMap內部有一個Entry[]陣列，ThreadLocal儲存的值就儲存在這個數組裡。Entry類是一個弱引用類，節省了記憶體。下面是table的儲存規則
private void 接下來看get()方法
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}get()方法仍然是通過當前執行緒獲取到ThreadLocalMap物件，然後獲取之前儲存的值，如果ThreadLocalMap物件為空，則使用預設值null
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
protected T initialValue() {
return null;
}從ThreadLocal的set 和get 方法，它們所操作的物件都是當前執行緒ThreadLocalMap物件的Entry[] table陣列，因此在不同執行緒中訪問同一個ThreadLocal的set 和get 方法，它們對ThreadLocal所做的操作僅限於各自執行緒的內部。這也是為什麼ThreadLocal可以在多個執行緒中互不干擾地儲存和修改資料。
MessageQueue工作原理
MessageQueue訊息佇列，主要包含兩個操作：插入和讀取。enqueueMessage(Message msg, long when)往訊息佇列中插入一條訊息，next()從訊息佇列中去除一條訊息並將其從訊息佇列中一處。
boolean enqueueMessage(Message msg, long when) {
//判斷Message是否繫結一個Handler物件(msg.target)
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//判斷當前訊息是否正在使用
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
// 判斷該訊息是否正在退出，如果正在退出則回收當前訊息並返回false
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
//回收當前訊息
msg.recycle();
return false;
}
//設定當前訊息正在使用
msg.markInUse();
//配置當前訊息的一些資訊
msg.when = when;
Message p = mMessages; //第一次執行到這裡時，mMessages為null
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 {
//當訊息佇列有訊息時
// 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;
//迴圈找到最後一個message
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;
}next()，當訊息佇列裡有訊息時，則會取出這個message，即mMessages。當滿足條件時，則取出這個mMessages（通過Message msg = mMessages賦值後返回msg），然後把mMessages賦值為msg.next，即把下一個訊息賦值成mMessages。通過這個過程就把當前訊息處理完了，並且把處理過的訊息刪除掉了。
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;
}
}Looper的工作原理
Looper不斷地從MessageQueue中檢視是否有新訊息，如果有新訊息就立刻處理，否則就一直阻塞在哪裡。Looper的建構函式中建立了一個MessageQueue，並且儲存了當前執行緒。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}Handler工作需要Looper，沒有Looper執行緒會報錯，那麼如何為一個執行緒建立Looper呢？其實通過Looper.prepare()就可以為當前執行緒建立一個Looper，接著通過Looper.loop()方法來開啟訊息迴圈。
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));
}prepare方法，就是把一個Looper物件儲存到了ThreadLocal
public static void loop() {
//獲取當前執行緒的Looper物件
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//把Looper物件繫結到MessageQueue
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();
//死迴圈，
for (;;) {
Message msg = queue.next(); // might block
//當MessageQueue為空時，則跳出迴圈loop結束
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 {
//如果有訊息，則把這個訊息通過Handler(即msg.target)的dispatchMessage方法來處理這個訊息，這樣就成功將程式碼邏輯切換到制定執行緒中
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();
}
}上述通過prepare方法和loop方法只是對普通執行緒來說的，對於主執行緒來說，由於主執行緒情況比較複雜，所以提供了prepareMainLooper來給ActivityThread建立Looper物件，但是其本質也是通過prepare來實現的，這個可以自己去看原始碼。同時，也可以通過getMainLooper方法在其他任何地方獲取到主執行緒的Looper物件。
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}Handler的工作原理
Handler的主要工作包含訊息的傳送和接收過程。訊息的傳送可通過一系列post、send方法來實現。
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);
}可以看出傳送訊息其實就是在MessageQueue中插入了一條訊息。MessageQueue的next()方法就會返回這條訊息給Looper,Looper接收訊息後就開始處理，最終訊息由Looper交由Handler處理，即呼叫Handler的dispatchMessage方法，這是Handler就進入處理訊息階段。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
-----------------------------------------------------
private static void handleCallback(Message message) {
message.callback.run();
}
--------------------------------------------------------Handler處理訊息過程；
首先檢查Messager的callback是否為null,，不為null就通過handleCallback來處理訊息。 message.callback是一個Runnable物件，實際上就是Handler的post方法所傳遞的Runnable物件。其次，檢查mCallback是否為null,不為null呼叫mCallback.handleMessage(msg)來處理訊息，最後，呼叫handleMessage來處理訊息。
總結
Android的訊息機制的總體流程就是：Handler向MessageQueue傳送一條訊息（即插入一條訊息），MessageQueue通過next方法把訊息傳給Looper，Looper收到訊息後開始處理，然後最終交給Handler自己去處理。換句話說就是：Handler給自己傳送了一條訊息，然後自己的handleMessage方法處理訊息，只是中間過程經過了MessageQueue和Looper。呼叫的方法過程如下：Handler.sendMessage方法–>Handler.enqueueMessage–>MessageQueue.next–>Looper.loop–>handler.dispatchMessage–>Handler.handleMessage(或者Runnable的run方法或者Callback.handleMessage)。