前言
handler是所有Android工程師都十分常用的工具，功能豐富，既可以用於執行緒間的訊息傳遞、元件間通訊，也可以實現定時任務、重複任務。本文將從原始碼角度理解handler的實現。
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1.handler的建立
//frameworks/base/core/java/android/os/Handler.java
//建立方法1
Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//do Something
}
};
//建立方法2 通常用於子執行緒中操作UI時使用 傳入主執行緒的loop
Handler handler = new Handler(Looper.getMainLooper());
複製程式碼除了上面兩種建立方式之外handler還有好幾種構造方法，但是最終呼叫的只有如下兩個方法。
//frameworks/base/core/java/android/os/Handler.java
//上面建立方法1最終所呼叫的方法
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
//上面建立方法2最終所呼叫的方法
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
複製程式碼可以看到帶有loop的構造方法大致只是做了一個賦值，其中loop是傳入的已存在的looper，callback是Handler中的一個interface。
//frameworks/base/core/java/android/os/Handler.java
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);
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
/**
* 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);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
複製程式碼可以看到在Handler在處理msg的時候首先判斷msg裡面的callback是否為空，不為空就直接handleCallback了，為空再走callback的handleMessage和Handler本身的成員方法handleMessage，再看handleCallback方法，是不是恍然大悟，我們常用的handler.post(Runnable r)方法。我們在原始碼裡再驗證下我們的想法。
//frameworks/base/core/java/android/os/Handler.java
//handler中的post方法
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
//通過Message.obtain()拿到了Message物件m，並將r賦值給m.callback
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
複製程式碼終於到最後一個引數了，async，中文是非同步的。看看賦值之後在哪裡被使用
//frameworks/base/core/java/android/os/Handler.java
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
複製程式碼最終還是到了Message身上，msg.setAsynchronous(true);設定message是否是非同步的，這是message的一個屬性。同一個Thread只有一個Looper，一個MessageQueue，但是可以有很多個Handler，如果Handler初始化的時候async引數是true，那麼這個Handler所post的所有的message都會帶上非同步的屬性。可以通過MessageQueue的postSyncBarrier(long when)來向佇列中插入一個同步分割欄，同步分割欄是一個特殊的message，這種message的target=null，就像一個卡子，當他被插入時，會卡住在這之後的所有的同步的message，只會摘取非同步的message。當然也可以通過MessageQueue的removeSyncBarrier(int token)來移除這個同步分割欄，token就是postSyncBarrier方法的返回值。但是目前這兩個方法都被hide了。所以大家一般用到的都只是普通的Message。
到現在終於可以看一眼比較複雜的建立方法1了。
//frameworks/base/core/java/android/os/Handler.java
//上面建立方法1最終所呼叫的方法
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
複製程式碼第一個if，找到潛在的洩漏，看看判斷的條件，是否是匿名內部類、成員內部類、區域性內部類並且不是static。想想看，有點意思，Java的特性：非靜態的內部類和匿名內部類都會隱式的持有一個外部類的引用，所以這才是導致可能發生記憶體洩漏的關鍵，在我們日常程式設計中要注意這點，不然很可能會GG。繼續往下，由於沒有傳Looper進來，所以拿到自己的Looper，這裡可能丟擲一個異常，大家應該遇到過，當你在一個新的執行緒中使用handler的時候，要先Looper.prepare()，不然就會丟擲上面的異常，我們再去看下Looper.prepare()幹了啥。
//frameworks/base/core/java/android/os/Looper.java
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));
}
/**
* Initialize the current thread as a looper, marking it as an
* application is main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
/**
* 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();
}
複製程式碼可以看到就是新建了一個Looper然後設定到了sThreadLocal中，其中new Looper(quitAllowed)時傳入了一個引數quitAllowed。
//frameworks/base/core/java/android/os/Looper.java
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
//frameworks/base/core/java/android/os/MessageQueue.java
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
複製程式碼可以看到這個引數控制這MessageQueue是否可以清空，如果呼叫了quitAllowed=true的Looper的quitSafely()方法，將清空所有Message，並且拒絕接收新的Message。prepareMainLooper()方法中quitAllowed引數為false，所以我們沒辦法讓主執行緒的MessageQueue清空並拒絕插入Message，這也符合Android主執行緒的設計。
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2.handler傳送Message
//frameworks/base/core/java/android/os/Handler.java
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(Runnable r, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean postAtFrontOfQueue(Runnable r)
{
return sendMessageAtFrontOfQueue(getPostMessage(r));
}
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
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);
}
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}
複製程式碼上面是所有Handler傳送訊息的方法，不管是什麼方法，最終都是來到了enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)方法。
//frameworks/base/core/java/android/os/Handler.java
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
//frameworks/base/core/java/android/os/MessageQueue.java
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;
}
}
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第一個方法中 msg.target = this;將當前Handler賦值給了msg.target,這是區分由哪個handler處理的關鍵。再看Message Queue中的方法，首先判斷msg的target是否為空以及當前msg是否已經被使用，接著一個大的synchronized塊，首先判斷mQuitting的值，如果true則釋放這個message並且return false。mQuitting這個值的設定就在在之前MessageQueue的quit(boolean safe),這裡也說明了一旦呼叫了這個方法，MessageQueue則不會再接收任何訊息。接下來第一個if的判斷，p == null代表當前沒有要處理的Message、when == 0代表立馬插入、when < p.when代表傳入msg的when比當前要處理的Message的時間還要提前，所以滿足上面的條件之一的都會被插入到訊息佇列的首部。那下面else部分就是判斷傳入的msg該插入到佇列中的哪個部分，裡面的for迴圈就是完成了這麼一件事情。但是其中有個needWake，字面意思是需要被喚醒，兩個賦值的地方：if塊裡 needWake = mBlocked，直接賦值，如果當前狀態是blocked，需要喚醒，沒毛病。 else塊裡needWake = false，如果需要喚醒並且p是非同步的，注意一點能走到這裡的代表p不是第一個訊息。說明即便msg是非同步的，也不是連結串列中第一個非同步訊息，所以沒必要喚醒了。
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3.Message的處理
在前面只說到了Message被插入到了訊息佇列中，那麼Message又是怎麼被取出來，又是怎麼處理的呢，上面絲毫未提，但是回想一下，我們在Looper.prepare()之後是不是有個必做的方法Looper.loop()。看來應該都在這裡面了。
//frameworks/base/core/java/android/os/Looper.java
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
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 {
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();
}
}
複製程式碼大部分都是Log，可以看到一個死迴圈，通過Message msg = queue.next()，拿到了要處理的msg，這個next()可能會引起中斷，通過 msg.target.dispatchMessage(msg);實現了處理。msg.target就是之前設定進去的handler，所以就是呼叫handler的dispatchMessage，這個方法已經在上面分析過了。那麼現在就簡單了，我們看下MessageQueue的next()方法。
//frameworks/base/core/java/android/os/MessageQueue.java
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;
//第一個地方
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) {
//第二個地方
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 {
//第三個地方
// No more messages.
nextPollTimeoutMillis = -1;
}
//第四個地方
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
//第五個地方
// 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);
}
//第六個地方
// 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);
}
}
}
//第七個地方
// 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;
}
}
複製程式碼看這程式碼，不出所料（沒有其他可能了） nativePollOnce(ptr, nextPollTimeoutMillis);這句就是中斷用的了，然後我們看第一個地方，msg != null && msg.target == null，是不是同步分割欄，看程式碼，如果當前的msg是同步分割欄，那麼就找到後面是有非同步屬性的msg。第二個地方，首先判斷msg應該執行的時間，有dealy就計算delay的時間，delay最大不超過Integer.MAX_VALUE。沒有delay就拿到了msg，就直接return回去了。第三個方法說明此時已經沒了msg，nextPollTimeoutMillis= -1，當執行nativePollOnce時就代表一直阻塞。第四個地方說明呼叫了quit()方法，丟棄msg並返回空。第五個地方，看註釋，如果第一次空閒，則獲取要執行的idlers數量。僅當佇列為空或第一條訊息在佇列中（可能是屏障）將在將來處理。要是沒有要執行的就直接設定mBlocked為true，然後continue了，接著就阻塞了。然後把mIdleHandlers拷貝到mPendingIdleHandlers裡，就到了第六個地方，就開始執行idler.queueIdle()了，根據idler的返回值判斷要不要從mIdleHandlers中移除，如果不移除那麼以後每次空閒都就會執行。第七個地方，將pendingIdleHandlerCount賦值為0，避免再執行（在這一次的MessageQueue的next（）方法中最多隻執行一次）。將nextPollTimeoutMillis賦值為0，因為不知道在所有的mIdleHandlers都執行完成之後msg的when到了沒了，所以設定成0，直接再來一次，看到這裡終於理解如何利用handler來做延遲載入了，其間奧祕全在mIdleHandlers裡。