Android原始碼解析之一 非同步訊息機制
1，handler的常規使用方式
/**
* 測試Activity，主要用於測試非同步訊息（Handler）的使用方式
*/
public class MainActivity extends AppCompatActivity {
public static final String TAG = MainActivity.class.getSimpleName();
private TextView texttitle = null;
/**
* 在主執行緒中定義Handler，並實現對應的handleMessage方法
*/
public static Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "接收到handler訊息...");
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
texttitle = (TextView) findViewById(R.id.texttitle);
texttitle.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread() {
@Override
public void run() {
// 在子執行緒中傳送非同步訊息
mHandler.sendEmptyMessage(101);
}
}.start();
}
});
}
}
複製程式碼根據上述程式碼可以看出，一般handler的使用方式都是在主執行緒中定義handler，然後在子執行緒中呼叫mHandler.sendEnptyMessage();方法，那麼這裡就有一個疑問了，我們可以在子執行緒中定義handler 嗎？
2，如何在子執行緒中定義Handler ？
我們在子執行緒中定義Handler,看看測試結果：
/**
* 定義texttitle的點選事件處理
*/
texttitle.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
/**
* 定義在子執行緒中執行handler的建立
*/
new Thread() {
@Override
public void run() {
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "在子執行緒中定義Handler，並接收到訊息。。。");
}
}
};
}
}.start();
}
});
複製程式碼
可以看出在子執行緒中定義的Handler物件報錯了，難道Handler物件的定義或者出事中能在主執行緒中？其實不是這樣，錯誤資訊中提示的很明顯了，在初始化Handler物件之前需要呼叫Looper.prepare()方法。
/**
* 定義texttitle的點選事件處理
*/
texttitle.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread() {
@Override
public void run() {
Looper.prepare();
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "在子執行緒中定義Handler，並接收到訊息。。。");
}
}
};
}
}.start();
}
});
複製程式碼再次點選按鈕執行此段程式碼，程式就不會報錯了，那麼這說明初始化Handler物件的時候我們是需要呼叫Looper.prepare()方法的，那麼主執行緒中為什麼可以直接初始化Handler呢？
再次查閱一下原始碼發現，APP初始化的時候會執行ActivityThread 的main方法：
public static void More ...main(String[] args) {
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);
// 初始化應用中需要使用的系統路徑
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
//增加一個儲存key的provider
Security.addProvider(new AndroidKeyStoreProvider());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
//為應用設定當前使用者的CA證書儲存的位置
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
//設定程式的名稱
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
//建立ActivityThread 物件
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
複製程式碼首先Looper.prepareMainLooper();是為主執行緒建立了Looper，然後thread.getHandler();是儲存了主執行緒的Handler，最後Looper.loop();進入訊息迴圈。
3, 檢視Handler原始碼
① 檢視Looper.prepare()方法
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
/** 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");
}
sThreadLocal.set(new Looper(quitAllowed));
}
複製程式碼可以看到Looper中有一個ThreadLocal成員變數，熟悉JDK的同學應該知道，當使用ThreadLocal 維護變數時，ThreadLocal為每個使用該成員的執行緒提供獨立的變數副本，所以每一個執行緒都可以獨立地改變自己的副本，而不會影響其他執行緒所對應的副本。
由此可以看出在每個執行緒中Looper.prepare() 能且只能呼叫一次。
繼續看Looper物件的構造方法，可以看到在其構造方法中初始化了一個MessageQueue物件：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
複製程式碼綜上小結①：Looper.prepare()方法初始話了一個Looper物件並關聯在一個MessageQueue物件，並且一個執行緒中只有一個Looper物件，只有一個MessageQueue物件。
② 檢視Handler物件的構造方法
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;
mCallback = callback;
mAsynchronous = async;
}
複製程式碼可以看出在Handler中的構造方法中，主要初始化了一下變數，並判斷Handler物件的初始化不應在內部類，靜態類，匿名類中，並且儲存了當前執行緒中的Looper物件。
綜上小結② ：Looper.prepare()方法初始話了一個Looper物件並關聯在一個MessageQueue物件，並且一個執行緒中只有一個Looper物件，只有一個MessageQueue物件。而Handler的構造方法則在Handler內部維護了當前執行緒的Looper物件
③檢視handler.setMesssage(msg)方法 一般情況下，我們傳送非同步訊息的時候會這樣呼叫：
mHandler.sendMessage(new Message());
通過不斷的跟進原始碼，其最後會呼叫：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
複製程式碼原來msg.target就是Handler物件本身；而這裡的queue物件就是我們的Handler內部維護的Looper物件關聯的MessageQueue物件。檢視messagequeue物件的enqueueMessage方法：
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;
}
複製程式碼可以看到這裡MessageQueue並沒有使用列表將所有的Message儲存起來，而是使用Message.next儲存下一個Message,從而按照時間將所有的Message排序。
④ 檢視Looper.loop()方法
/**
* 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();
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();
}
}
複製程式碼可以看到Looper.loop()方法裡起了一個死迴圈，不斷的判斷MessageQueue中的訊息是否為空，如果為空則直接return掉，然後執行queue.next()方法：
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;
}
}
複製程式碼可以看到其大概的實現邏輯就是Message的出棧操作，裡面可能對執行緒，併發控制做了一些限制等。獲取到棧頂的Message物件之後開始執行：
msg.target.dispatchMessage(msg);
那麼 msg.target 是什麼呢？ 痛過追蹤可以知道就是我們定義的Handler物件，然後我們檢視一下Handler類的dispatchMessage 方法：
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
複製程式碼可以看到，如果我們設定了callback(Runnable物件)的話，則會直接呼叫handlerCallback方法：
private static void handleCallback(Message message) {
message.callback.run();
}
複製程式碼即，如果我們在初始化Handler的時候設定callback （Runnable）物件，則直接呼叫run方法。 比如我們經常寫的runOnUiThread 方法：
runOnUiThread(new Runnable() {
@Override
public void run() {
}
});
複製程式碼看起內部實現：
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
複製程式碼而如果msg.callback為空的話，會直接呼叫我們的mCallback.handleMessage(msg)，即handler的handlerMessage方法。由於Handler物件是在主執行緒中建立的，所以handler的handlerMessage方法的執行也會在主執行緒中。
總結
- 主執行緒中定義Handler,直接執行：
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
複製程式碼而如果想要在子執行緒中定義Hanlder,則標準寫法為：
// 初始化該執行緒Looper，MessageQueue，執行且只能執行一次
Looper.prepare();
// 初始化Handler物件，內部關聯Looper物件
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
// 啟動訊息佇列出棧死迴圈
Looper.loop();
複製程式碼-
一個執行緒中只存在一個Looper物件，只存在一個MessageQueue物件，可以存在N個Handler物件，Handler物件內部關聯了本執行緒中唯一的Looper物件，Looper物件內部關聯著唯一的一個MessageQueue物件。
-
MessageQueue訊息佇列不是通過列表儲存訊息列表的，而且通過Message物件next屬性關聯下一個Message 從而實現列表的功能，同時所有的訊息都是按時間來排序的。
-
Android中兩個子執行緒相互互動同樣可以通過Handler的非同步訊息機制實現，可以線上程A中定義Handler物件，而線上程B中獲取handler的引用並呼叫sendMessage方法。
-
activity內部預設存在一個handler的成員變數，Android中的一些其他的非同步訊息機制實現方法：
Handler的post方法：
mHandler.post(new Runnable() {
@Override
public void run() {
}
});
複製程式碼檢視其內部實現：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
複製程式碼可以發現其內部呼叫就是sendMessage系列方法。
view的post方法：
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Assume that post will succeed later
ViewRootImpl.getRunQueue().post(action);
return true;
}
複製程式碼可以發現其呼叫的就是activity中預設儲存的handler物件的post方法。
activity的runOnUiThread方法：
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
複製程式碼判斷當前執行緒是否是UI執行緒，如果不是，則呼叫handler的post方法，否則直接執行run方法。