Handler：發送和接收消息
Looper：消息(循環)輪詢器
Message:消息池
MessageQueue：消息隊列。雖然名為隊列，但事實上它的内部存儲結構并不是真正的隊列，而是采用單連結清單的資料結構來存儲消息清單的
先來看Handler，其實系統很多東西都是通過Handler消息來實作的，其中也包括activity的生命周期，應用程式的退出等；在ActivityThread類中的main方法中可以很清楚的看到，同時main方法也是android應用程式的主入口；
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());
// 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>");
//執行個體化一個main Looper
Looper.prepareMainLooper();
//執行個體化ActivityThread
ActivityThread thread = new ActivityThread();
//調用attach方法
thread.attach(false);
//通過ActivityThread擷取Handler對象
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);
//開啟消息輪詢器
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
在執行個體化一個main looper的時候調用的是prepareMainLooper();方法；prepareMainLooper();方法是Looper類中的方法；
public static void prepareMainLooper() {
//調用prepare方法
prepare(false);
synchronized (Looper.class) {
//這裡需要注意隻能執行個體化一個main Looper對象，多次執行個體化會報錯
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
prepare();方法這裡就暫時不講了，後面會講到，執行個體化完mian looper後，就會去執行個體化ActivityThread對象并調用其中的attach()方法及getHandler()方法去執行個體化一個Handler對象；
final Handler getHandler() {
//mH extends Handler 在加載ActivityThread的時候就已經初始化了
return mH;
}
private class H extends Handler {
public static final int LAUNCH_ACTIVITY = 100;
public static final int PAUSE_ACTIVITY = 101;
public static final int DESTROY_ACTIVITY = 109;
public static final int EXIT_APPLICATION = 111;
public static final int STOP_SERVICE = 116;
String codeToString(int code) {
if (DEBUG_MESSAGES) {
switch (code) {
case LAUNCH_ACTIVITY: return "LAUNCH_ACTIVITY";
case PAUSE_ACTIVITY: return "PAUSE_ACTIVITY";
case DESTROY_ACTIVITY: return "DESTROY_ACTIVITY";
case EXIT_APPLICATION: return "EXIT_APPLICATION";
case STOP_SERVICE: return "STOP_SERVICE";
}
}
return Integer.toString(code);
}
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
case PAUSE_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
SomeArgs args = (SomeArgs) msg.obj;
handlePauseActivity((IBinder) args.arg1, false,
(args.argi1 & USER_LEAVING) != 0, args.argi2,
(args.argi1 & DONT_REPORT) != 0, args.argi3);
maybeSnapshot();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
case DESTROY_ACTIVITY:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDestroy");
handleDestroyActivity((IBinder)msg.obj, msg.arg1 != 0,
msg.arg2, false);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
case EXIT_APPLICATION:
if (mInitialApplication != null) {
mInitialApplication.onTerminate();
}
Looper.myLooper().quit();
break;
case STOP_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStop");
handleStopService((IBinder)msg.obj);
maybeSnapshot();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
}
}
H類就是一個内部類，看到LAUNCH_ACTIVITY、PAUSE_ACTIVITY、EXIT_APPLICATION、STOP_SERVICE等很多消息辨別，這樣通過handler消息來控制activity的生命周期以及一些其他東西；當msg.what為EXIT_APPLICATION時，應用程式就會退出，并不是activity的生命周期都會走，當消息辨別為EXIT_APPLICATION應用程式就已經退出了，而應用程式的退出時調用Looper中的quit方法；
public void quit() {
mQueue.quit(false);
}
這裡就粗略而過，後面會詳細講；這是一些系統handler的使用，還是看看平時開發中的使用吧；
public class MainActivity extends AppCompatActivity {
Handler mHanlder;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//開啟子線程
new MyThread().start();
}
class MyThread extends Thread implements Runnable {
@Override
public void run() {
super.run();
//執行個體化消息輪詢器
Looper.prepare();
//執行個體化handler對象
mHanlder = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
//擷取message對象
Message message = mHanlder.obtainMessage();
message.what = 1;
//發送消息
mHanlder.sendMessage(message);
//開啟looper輪詢器
Looper.loop();
}
}
}
這是一段在子線程中執行個體化handler的代碼，當然了也可以不線上程中執行個體化handler，這樣子就不需要執行個體化一個Looper，系統已經實作了的，如果在子線中執行個體化handler就需要同樣執行個體化一個Looper，確定handler所線上程和Looper所在的線程一緻，否則會報錯；
先來看第一步Looper.prepare()執行個體化一個Looper；
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//調用prepare方法quitAllowed就為true，調用的是prepareMainLooper方法quitAllowed就是false
//sThreadLocal.get()是直接從ThreadLocal池中擷取 這裡同樣隻能執行個體化一個Looper，多次執行個體化會報錯
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//直接new 一個Looper，并将執行個體化的Looper對象添加到ThreadLocal池中
sThreadLocal.set(new Looper(quitAllowed));
}
在new 一個looper的時候會去執行個體化一個MessageQueue對象；
private Looper(boolean quitAllowed) {
//直接執行個體化一個MessageQueue對象
mQueue = new MessageQueue(quitAllowed);
//擷取目前線程
mThread = Thread.currentThread();
}
第二步handler的執行個體化
handler提供多個構造方法，可以傳入一個Looper，Callback回調等，這裡直接用了無參構造；
public Handler() {
this(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());
}
}
//擷取執行個體化好的looper對象，這裡是從ThreadLocal池中直接擷取的
mLooper = Looper.myLooper();
//如果擷取到的looper對象為null就會報錯，是以要保證執行個體化好的looper對象和handler對象所在的線程要一緻，
//也就是平時如果在子線程中執行個體化handler沒有執行個體化looper報錯的原因
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//擷取MessageQueue對象，MessageQueue對象在執行個體化looper的構造方法中就已經執行個體化好了
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public static @Nullable Looper myLooper() {
//myLooper是looper類中的方法，從ThreadLocal池中直接擷取looper對象
return sThreadLocal.get();
}
第三步Message對象的執行個體化，通過obtainMessage();方法擷取的Message對象，當然也可以直接new 一個Message，不過建議使用obtainMessage();方法擷取；
public final Message obtainMessage()
{
//調用Handler類中的obtainMessage方法，傳回一個Message對象，不過調用的是Message中的obtain方法，并将Handler本身做參數傳入
return Message.obtain(this);
}
public static Message obtain(Handler h) {
//這裡又調用了Message中的obtain()方法
Message m = obtain();
//将傳入的handler對象指派給Message中的target變量
m.target = h;
return m;
}
/**
* Return a new Message instance from the global pool. Allows us to
傳回一個Message執行個體，從全局池中
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
//判斷Message池是否為null
if (sPool != null) {
//不為null就将sPool對象指派個m
Message m = sPool;
//又将m中的指派給sPool 這裡是連結清單結構
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
//将擷取到的message執行個體傳回
return m;
}
}
//sPool為null的情況下直接new 一個Message對象并傳回
return new Message();
}
Message類中還提供了recycleUnchecked()方法，用于handler消息的回收，這裡暫不說；Message對象有了，調用sendMessage()發送消息，當然了Handler類中不隻這一個方法可以發送消息，還有其他的方法，具體可以看Handler源碼；
/**
* 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);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
//delayMillis延遲發送的時間 毫秒值
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
//擷取MessageQueue對象，mQueue是在執行個體化handler時，通過looper對象擷取，最初是在執行個體化looper對象的構造方法中執行個體化的
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) {
//将目前的handler指派給Message中的handler
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
走到這裡就會去調用MessageQueue中的enqueueMessage()方法；
boolean enqueueMessage(Message msg, long when) {
//msg.target就是handler對象，上面已經進行了指派操作
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) {
//已經取消了
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
//調用recycle方法，如果沒有被回收就會調用recycleUnchecked()方法進行handler消息的回收
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
//建立新的Message對象，并指派
Message p = mMessages;
boolean needWake;
//建立的message對象為null或者沒有延遲發送消息或者延遲發送消息的時間小于發送的時間
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
//添加到新的消息頭部，如果阻塞，喚醒事件隊列
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;
}
這樣子就将消息添加到隊列中了，通過Looper.loop();開啟輪詢去取消息；
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//擷取looper對象，如果為null說明沒有執行個體化looper對象，會報錯
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//擷取MessageQueue對象，MessageQueue對象是在執行個體化looper對象的構造方法中執行個體化的
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();
//進行消息輪詢 這裡是一個for死循環，不管是ActivityThread中的void main方法，還是平時使用loop()方法都是在最後面調用，這樣不會阻塞
for (;;) {
//這裡是連結清單結構，通過next()去擷取每一個message對象
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
//沒有消息訓示消息隊列正在退出。 直接退出該方法，
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 traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
//将擷取到的message消息通過handler中的dispatchMessage方法進行分發
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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);
}
//通過messsage中的recycleUnchecked方法對消息進行回收
msg.recycleUnchecked();
}
}
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;
//消息循環
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
//嘗試檢索下一條消息。如果找到，傳回。
//擷取時間 從開機到現在的毫秒數（手機睡眠的時間不包括在内）；
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 {
//将msg指派給prevMsg
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
//擷取到的時間消息消息的時間
// Next message is not ready. Set a timeout to wake up when it is ready.
//下一條消息尚未準備好。設定一個逾時，以便在準備就緒時喚醒。
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;
}
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//設定了callback就會走這裡
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//這裡是沒有設定callback
handleMessage(msg);
}
}
/**
* Subclasses must implement this to receive messages.
*執行個體化handler的時候都會去重寫該方法，在該方法中進行消息的處理
*/
public void handleMessage(Message msg) {
}
在消息處理完畢後，還會對消息進行回收的；
/**
* Recycles a Message that may be in-use.
* Used internally by the MessageQueue and Looper when disposing of queued Messages.
*/
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
Looper.loop()；中是一個死循環，是以在使用handler消息的時候需要注意當頁面銷毀後要将消息移除掉，避免造成記憶體洩漏什麼的；handler提供一些移除消息的方法；
/**
* Remove any pending posts of callbacks and sent messages whose
* <var>obj</var> is <var>token</var>. If <var>token</var> is null,
* all callbacks and messages will be removed.
*/
public final void removeCallbacksAndMessages(Object token) {
//token為null的時候所有的callback和messages都會被移除掉
mQueue.removeCallbacksAndMessages(this, token);
}
當然也可以移除指定的消息或者callback；上面就是對handler消息的一些源碼分析。
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