Andorid 執行緒間通訊Handler原理（原始碼解析）

摘要： *class LooperThread extends Thread { *public Handler mHandler; * *public void run() *//1.建立當前執行緒的Looper,初始化函式會同時建立...

*class LooperThread extends Thread {
*public Handler mHandler;
*
*public void run() 
*//1.建立當前執行緒的Looper,初始化函式會同時建立MessageQueue
*Looper.prepare();
*//2.建立當前執行緒的Handler,處理髮送來的訊息。
*mHandler = new Handler() {
*public void handleMessage(Message msg) {
*// process incoming messages here
*}
*};
*//3.Looper開啟迴圈，不斷訪問是否有新的訊息
*Looper.loop();
*}
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//4.在任意執行緒傳送一個訊息
mHandler.sendMessage(msg);
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Looper的建立和儲存

Looper.prepare() 建立當前執行緒的Looper。Looper初始化函式會同時建立當前執行緒的MessageQueue。Looper建立完成儲存到ThreadLocal裡。

Looper.java

/** 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");
}
//ThreadLocal儲存Looper物件，每個執行緒都有自己的Looper。ThreadLocal內部會區分不同執行緒進行儲存,每個執行緒有獨立的map, Looper的存取是存放在當前執行緒的map裡面的
sThreadLocal.set(new Looper(quitAllowed));
}
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Looper.java

Looper初始化函式會同時建立當前執行緒的MessageQueue

private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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ThreadLocal存取Looper

簡介：以執行緒為維度，可以拿到當前執行緒的map，進行資料儲存。這裡存放的是當前執行緒的Looper物件。

ThreadLocal.jva

/**
* 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();
//拿到當前執行緒的map. 所以不同執行緒呼叫，獲取到的是不同map
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}


/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @paramt the current thread
* @return the map
*/
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}


/**
* 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();
//從當前執行緒在後去map,從map裡面去取值（這裡最終從map獲取的是當前執行緒的Looper）
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();
}
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Handler傳送訊息

Handler簡介：傳送訊息（建立Handler的執行緒或者其他執行緒）和處理訊息（建立Handler的執行緒）

Handler.java

Handler初始化

///Handler的初始化，拿到當前的Looper和MessageQueue
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());
}
}
//獲取當前執行緒的Looper
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//獲取當前執行緒的MessageQueue
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}

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Looper.java

獲取當前的執行緒已經建立的Looper.

/**
* 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();
}
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Handler.java

mHandler.sendMessage();傳送訊息

/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @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.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @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 final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}


/**
* 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);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//Handler傳送訊息，最後其實是放入到了建立Handler執行緒對應的MessageQueue裡面
return queue.enqueueMessage(msg, uptimeMillis);
}


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MessageQueue 儲存和獲取訊息

MessageQueue 簡介：負責執行緒的訊息管理，內部儲存Message使用連結串列資料結構

MessageQueue.java

//向MessageQueue 放入一條訊息
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 do not 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;
}
}
//當前訊息存放到連結串列中
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MessageQueue獲取一條訊息
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;
//獲取訊息可能是阻塞的，會一直迴圈等待新的訊息。Looper.loop();
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();
//返回從連結串列裡獲取到的Message
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;
}
}

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Looper.loop()如何分發訊息

Looper進行無限迴圈，如果從MessageQueue獲取到訊息，則回撥到Handler的handleMessage（msg）去處理

Looper.java

Looper.loop()後都做做了什麼？

/**
* 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();

//進行無限迴圈，從MessageQueue中獲取資料
for (;;) {
//從MessageQueue中獲取Message，如果當前MessageQueue裡面沒有訊息，next()方法是阻塞的。
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 {
//獲取到Message則分發到Handler進行處理
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 was not 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();
}
}
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Handler.java

/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//4.回撥Handler的handleMessage(); 完整的訊息分發至此完成。
handleMessage(msg);
}
}
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Q&A

1. looper.loop()方法是阻塞的，主執行緒為什麼還可以執行其他函式（Activity生命週期回撥等等）？

通過主執行緒的handler傳送Message到主執行緒MessageQueue。Looper.loop（）獲取到Message，然後回撥handler.handleMessage()方法去執行各種事件回撥（Activity生命週期回撥等等）。詳細自行檢視ActivityThread.java類。
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2. 執行緒間通訊Handler核心點在哪裡？

生產者（Handler任意執行緒生產）消費者（建立Looper執行緒消費）模式
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