Android訊息機制（Handler、MessageQueue和Looper三者的工作原理）
阿新 • • 發佈：2018-12-14
Android的訊息機制主要是指Handler的執行機制以及Handler所附帶的MessageQueue和Looper的工作過程。messagequeue意思是訊息佇列，它內部儲存一組訊息，有插入和刪除的功能，其實內部是以單鏈表的形式來實現佇列功能的。looper的意思是迴圈，它的主要功能是迴圈讀取messagequeue裡面的訊息，然後加以處理，如果暫時還沒有訊息，則looper會一直等待。在android日常開發中，我們經常利用handler將工作執行緒切換到UI執行緒從而達到更新UI的目的。
在handler機制中，每個執行緒都會對應一個looper，但執行緒預設是沒有looper的，需要自己建立，而我們在android的UI執行緒中可以直接使用handler是因為UI執行緒在建立的時候已經初始化了looper，然後looper物件是通過ThreadLocal儲存在對應thread的threadlocalmap中的。ThreadLocal是什麼？ThreadLocal不是一個執行緒，它的作用是可以為每個不同的執行緒儲存資料，這些不同執行緒的資料互不干擾，像我們的looper就是通過它來儲存在各自執行緒中的，並且可以通過threadlocal來獲取每個執行緒的looper。下面我們來講解它們各自的工作原理。
ThreadLocl的工作原理
先來看一個例子：
public class ThreadLocalDemo { private java.lang.ThreadLocal<String> booleanThreadLocal = new ThreadLocal<>(); public void test() { new Handler(new Handler.Callback() { @Override public boolean handleMessage(Message msg) { return false; } }); booleanThreadLocal.set("主執行緒"); LogUtils.d(Thread.currentThread().getName() + "的值=" + booleanThreadLocal.get()); new Thread("執行緒01") { @Override public void run() { super.run(); booleanThreadLocal.set("執行緒01"); Log.d("tag", Thread.currentThread().getName() + "的值=" + booleanThreadLocal.get()); } }.start(); new Thread("執行緒02") { @Override public void run() { super.run(); //booleanThreadLocal.set("執行緒02"); Log.d("tag", Thread.currentThread().getName() + "的值=" + booleanThreadLocal.get()); } }.start(); } }
執行結果：
通過上面的例子執行結果來看，threadlocal確實可以為不同的執行緒儲存不同的結果，下面我們來看看它的原始碼是怎麼實現的：
public void set(T value) { //獲取當前執行緒 Thread t = Thread.currentThread(); //獲取當前執行緒的threadlocalmap ThreadLocalMap map = getMap(t); //如果threadlocalmap不為null，那麼將value值儲存，其中key值為當前threadlocal if (map != null) map.set(this, value); else createMap(t, value); } //返回執行緒的threadlocalmap物件 ThreadLocalMap getMap(Thread t) { return t.threadLocals; } //建立對應執行緒的threadlocalmap例項，並且儲存對應的值 void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }
其實這裡的關鍵是將value值儲存到當前執行緒的threadlocalmap中去，而且對應的key是threadlocal本身，下面我們來看看threadlocal的get方法：
public T get() {
//獲取對應的執行緒
Thread t = Thread.currentThread();
//獲取對應執行緒的threadlocalmap
ThreadLocalMap map = getMap(t);
//如果不為null，將取出key為當前threadlocal對應的value
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
//threadlocalmap為null的時候，返回setInitialValue的值，其實就是null
return setInitialValue();
}
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;
}
get方法其實就是返回我們儲存的value值，當然如果我們之前沒有對應set方法儲存對應值的話，就返回null，這就是為什麼上面的那個例子中，執行緒02會返回null的原因了。好了，其實threadlocal還是比較簡單的。
MessageQueue的工作原理
messagequeue的插入和讀取操作內部是由單鏈表結構來實現的，可能是因為單鏈表結構在插入和刪除上效率比較高吧。插入用enqueueMessage方法，讀取用next方法來實現，下面我們來看看他們的原始碼：
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;
}
上面是訊息的插入操作，注意引數when代表這個訊息何時取出來處理。下面我們看看，next方法原始碼：
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，next是一個無限迴圈的方法，當取到訊息的時候返回msg，沒有訊息的時候，會一直阻塞直到有訊息。注意上面的一個判斷，如果now<msg.when，就先不取msg，其實就是說這個訊息還沒到執行時間，所以暫時不會取出來。還有當looper執行退出的時候，即上面的mQuitting為true的時候，此時next方法返回null。理解了messagequeue，下面我們來看看Looper的工作原理。
Looper的工作原理
looper在訊息機制中扮演者訊息迴圈的角色，就是說它會不停地從訊息佇列中檢視有沒有新訊息，如果取出了新訊息就會馬上處理，否則就一直阻塞在那裡。我們知道，開啟looper的工作，需要在對應的執行緒中通過Looper.prepare()建立一個looper，然後通過loop（）方法來開啟訊息的迴圈，我們來看看這個兩個方法的原始碼：
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//一個執行緒只能建立一個looper
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//建立一個looper例項，通過threadlocal儲存起來
sThreadLocal.set(new Looper(quitAllowed));
}來看看looper的構造方法：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}可以看到，構造方法中會建立一個訊息佇列和獲取當前looper的執行緒。looper還提供了prepareMainLooper方法，這個方法是用來給主執行緒ActivityThread建立looper使用的，當然其本質也是通過prepare方法來實現的。由於主執行緒的looper比較特殊，所以其提供了getMainLooper方法，用來在任何地方都可以獲取主執行緒的looper物件。looper提供了兩個退出方法：quit和quitSafely，其中quit是立即退出，quitSafely是等訊息佇列中的訊息處理完畢以後再安全退出。如果我們手動建立了looper，在不用的時候需要我們手動去退出，否則這個子執行緒會一直處於等待狀態。下面我們來看看looper最關鍵的方法loop，只有呼叫了loop方法，訊息迴圈系統才會真正起作用，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();
//無限迴圈
for (;;) {
//呼叫訊息佇列的next方法取訊息
Message msg = queue.next(); // might block
//如果訊息為null，結束方法
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 {
//呼叫對應handler物件的方法，將訊息遞交給handler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 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();
}
}loop方法還是比較好理解的，首先這是一個死迴圈，唯一一個跳出迴圈的條件是取到的訊息為null，就是當我們呼叫了looper的退出方法的時候，同時也會去呼叫messagequeue的退出方法，此時next方法就會返回一個null值。所以說我們不用的時候必須要手動退出looper，不然這個迴圈會一直持續下去。如果我們取出了新訊息，後面就會呼叫msg.target.dispatchMessage(msg)，這裡的target就是傳送訊息的handler物件，這樣訊息又交給了handler來處理了。值得注意的是，我們取出訊息呼叫handler的dispatchMessage是在looper的loop方法中執行的，而looper物件是儲存在建立時候的那個執行緒中的，所以此時就成功地實現了執行緒切換。講解了訊息佇列和looper的工作原理，下面我們來看看Handler是怎麼工作的。
Handler的工作原理
handler的工作主要包括訊息的傳送和接收，訊息的傳送主要用post和send系列方法，接收一般是通過重寫對應的handleMessage方法。下面我們先來看看一個例子：
public class HandlerDemo {
//重寫handler的handleMessage方法
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
LogUtils.d("handler接收到訊息,what內容=" + msg.what);
}
};
//通過傳遞一個callback實現來處理訊息
Handler handler1 = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
public void sendMsg() {
//在一個子執行緒中通過主執行緒的handler傳送訊息給主執行緒處理
new Thread(new Runnable() {
@Override
public void run() {
try {
//延時兩秒
TimeUnit.SECONDS.sleep(2);
//傳送一個訊息給handler處理
Message msg = new Message();
msg.what = 01;
handler.sendMessage(msg);
} catch (InterruptedException e) {
}
}
}).start();
}
public void post() {
handler1.post(new Runnable() {
@Override
public void run() {
LogUtils.d("handler1的post方法執行了run方法");
}
});
}
}執行結果：
可以看到，例子中建立了主執行緒的handler物件，演示了在子執行緒中通過sendMessage方法傳送訊息給主執行緒處理還有post方法的使用，下面我們來看看它們的原始碼是怎麼實現的。
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);
}
//將訊息儲存到訊息佇列中
return queue.enqueueMessage(msg, uptimeMillis);
}其實上面的最終關鍵的就是把訊息儲存到對應的訊息佇列中，值得注意的時候，msg.target=this，這裡給target賦值為handler本身。我們來看看這個訊息佇列是怎麼來的
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());
}
}
//獲取對應的looper例項
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//獲取訊息佇列
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}可以看到，在handler的構造方法中，我們獲得了執行緒對應的looper，如果是主執行緒當然就是獲得主執行緒的looper了，然後通過looper獲得訊息佇列mQueue。
在講looper的時候，我們知道loop方法會一直去訊息佇列中取訊息，一旦取出新訊息，就會呼叫handler的dispatchMessage方法，下面我們來看看這個方法：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}其中上面的msg.callback就是post方法中的runnable物件，我們來看看：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}再來看看handleCallback(msg)
private static void handleCallback(Message message) {
message.callback.run();
}可以看到會呼叫runnable物件的run方法，這就是handler的post方法的原理。那mCallback 又是怎麼來的
public Handler(Callback callback) {
this(callback, 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;
//給mCallback賦值
mCallback = callback;
mAsynchronous = async;
}
//Callback介面
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public boolean handleMessage(Message msg);
}可以看到，其實這個mCallback就是我們在例子中建立handler1的時候傳遞的引數。如果這個回撥方法為null，我們就呼叫handleMessage(msg)方法處理，這是一個public方法裡面為空實現，需要我們自己重寫去處理，這個就是我們例子中建立handler的時候實現。
好了，關於android訊息機制的工作原理，通過上面的講解我們大概也清楚是怎麼一回事了。