參考資料：gityuan的知乎回答，http://gityuan.com，weibo.com/gityuan(微博)
1.基礎知識
1.android系統不能在子線程中更新UI；因爲系統的UI控件不是線程安全的，如果使用加鎖使其線程安全，則會讓UI控件變得複雜和低效，並可能阻塞某些進程的執行。
2.ThreadLocal類用來提供線程內部的局部變量。這些變量在多線程環境下訪問(通過get或set方法訪問)時能保證各個線程裏的變量相對獨立於其他線程內的變量，ThreadLocal實例通常來說都是private static類型。
3.一個線程中只有一個Looper實例和一個MessageQueue實例，可以有多個handler對象。2.Looper筆記
一般先調用Looper.prepare，再調用Looper.loop()方法
Looper.prepare()方法中，根據sThreadLocal.get()是否爲空，爲空時才調用sThreadLocal.set(new Looper(quitAllowed))，不爲空則拋出異常，從而保證了一個線程中只有一個Looper。
private static void prepare(booleanquitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only oneLooper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}Looper的構造函數中新建了MessageQueue對象，每個線程中都有一個MessageQueue對象：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}通過myLooper()方法可獲得當前線程的looper實例：
public static Looper myLooper() {
return sThreadLocal.get();
}Looper中有static對象sMainLooper，可通過static方法getMainLooper()來獲得主線程的looper實例。
3.Handler筆記
Handler的構造函數，把handler和looper關聯起來:
public Handler(Callback callback, boolean async) {
//省略一部分
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;
}
handler的dispatchMessage()方法中分發message：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
post(Runnable r)方法也是通過sendMessage實現的，post()會將該runable放入message，並把message的callback指向該runnable，並通過handleCallback最終運行該可執行代碼：
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
private static void handleCallback(Message message) {
message.callback.run();
}4.MessageQueue筆記
MessageQueue中的mMessages指向隊頭的message，對於一個新的message，會在enqueueMessage()方法中，根據msg.when等信息，將該message插入到適當的位置：
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("MessageQueue", 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;
}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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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("MessageQueue", "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;
}
}5.發送message的過程
handler.sendMessage()——hanler.enqueueMessage()——queue.enqueueMessage()
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//把將要發送的message.target指向自己
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}6.處理message的過程
Looper.loop()——queue.next()——msg.target.dispatchMessage()
Looper.loop()代碼如下：
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
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();
}
}7.主線程的消息機制
ActivityThread實際上並非線程，並沒有真正繼承Thread類，只是往往運行在主線程，承載ActivityThread的主線程是由Zygote fork而創建的進程。
ActivityThread的main()代碼如下：
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 libcore
EventLogger.setReporter(new EventLoggingReporter());
AndroidKeyStoreProvider.install();
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}其中：
main()方法中調用了prepareMainLooper()，所以主線程中可以直接新建handler，而不用先調用prepare()方法；此外，ActivityThread中有一個內部類Activity.H(繼承自handler)的對象mH，用來處理Activity生命週期相關的message。
thread.attach(false)會創建一個Binder線程（具體是指ApplicationThread，Binder的服務端，用於接收系統服務ApplicationManagerService發送來的事件），該Binder線程通過Activity.H將Message發送給主線程。
Looper.loop()方法會循環讀取主線程的MessageQueue中的message，當MessageQueue中沒有message時，message.next()方法會發生阻塞(通過nativePollOnce()方法)，此時主線程會釋放CPU資源進入休眠狀態，直到下個消息到達或者有事務發生，通過往pipe管道寫端寫入數據來喚醒主線程工作。
當有新的事件發生時，系統進程會通過binder方式調用ApplicationThreadd的對應方法(比如啓動服務時調用scheduleCreateService()方法)，在該方法內調用了sendMessage()方法，通過mH對象sendMessage()向主線程的消息隊列中發送了消息，進而喚醒了主線程(MessageQueue的enqueueMessage()方法會調用nativeWake(mPtr)方法進行喚醒)。
public final void scheduleCreateService(IBinder token, ServiceInfo info, CompatibilityInfo compatInfo, int processState) {
updateProcessState(processState, false);
CreateServiceData s = new CreateServiceData();
s.token = token;
s.info = info;
s.compatInfo = compatInfo;
sendMessage(H.CREATE_SERVICE, s);
}
private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
+ ": " + arg1 + " / " + obj);
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
if (async) {
msg.setAsynchronous(true);
}
mH.sendMessage(msg);
}