从源码去理解Handler
handler是所有Android工程师都十分常用的工具，功能丰富，既可以用于线程间的消息传递、组件间通讯，也可以实现定时任务、重复任务。本文将从源码角度理解handler的实现。
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1.handler的创建
//frameworks/base/core/java/android/os/Handler.java
//创建方法1
Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//do Something
}
};
//创建方法2 通常用于子线程中操作UI时使用 传入主线程的loop
Handler handler = new Handler(Looper.getMainLooper());
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除了上面两种创建方式之外handler还有好几种构造方法，但是最终调用的只有如下两个方法。
//frameworks/base/core/java/android/os/Handler.java
//上面创建方法1最终所调用的方法
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;
}
//上面创建方法2最终所调用的方法
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
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可以看到带有loop的构造方法大致只是做了一个赋值，其中loop是传入的已存在的looper，callback是Handler中的一个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();
}
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可以看到在Handler在处理msg的时候首先判断msg里面的callback是否为空，不为空就直接handleCallback了，为空再走callback的handleMessage和Handler本身的成员方法handleMessage，再看handleCallback方法，是不是恍然大悟，我们常用的handler.post(Runnable r)方法。我们在源码里再验证下我们的想法。
//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);
}
//通过Message.obtain()拿到了Message对象m，并将r赋值给m.callback
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
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终于到最后一个参数了，async，中文是异步的。看看赋值之后在哪里被使用
//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);
}
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最终还是到了Message身上，msg.setAsynchronous(true);设置message是否是异步的，这是message的一个属性。同一个Thread只有一个Looper，一个MessageQueue，但是可以有很多个Handler，如果Handler初始化的时候async参数是true，那么这个Handler所post的所有的message都会带上异步的属性。可以通过MessageQueue的postSyncBarrier(long when)来向队列中插入一个同步分割栏，同步分割栏是一个特殊的message，这种message的target=null，就像一个卡子，当他被插入时，会卡住在这之后的所有的同步的message，只会摘取异步的message。当然也可以通过MessageQueue的removeSyncBarrier(int token)来移除这个同步分割栏，token就是postSyncBarrier方法的返回值。但是目前这两个方法都被hide了。所以大家一般用到的都只是普通的Message。
到现在终于可以看一眼比较复杂的创建方法1了。
//frameworks/base/core/java/android/os/Handler.java
//上面创建方法1最终所调用的方法
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;
}
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第一个if，找到潜在的泄漏，看看判断的条件，是否是匿名内部类、成员内部类、局部内部类并且不是static。想想看，有点意思，Java的特性：非静态的内部类和匿名内部类都会隐式的持有一个外部类的引用，所以这才是导致可能发生内存泄漏的关键，在我们日常编程中要注意这点，不然很可能会GG。继续往下，由于没有传Looper进来，所以拿到自己的Looper，这里可能抛出一个异常，大家应该遇到过，当你在一个新的线程中使用handler的时候，要先Looper.prepare()，不然就会抛出上面的异常，我们再去看下Looper.prepare()干了啥。
//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();
}
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可以看到就是新建了一个Looper然后设置到了sThreadLocal中，其中new Looper(quitAllowed)时传入了一个参数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);
}
}
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可以看到这个参数控制这MessageQueue是否可以清空，如果调用了quitAllowed=true的Looper的quitSafely()方法，将清空所有Message，并且拒绝接收新的Message。prepareMainLooper()方法中quitAllowed参数为false，所以我们没办法让主线程的MessageQueue清空并拒绝插入Message，这也符合Android主线程的设计。
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2.handler发送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);
}
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上面是所有Handler发送消息的方法，不管是什么方法，最终都是来到了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;
}
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可以看到第一个方法中 msg.target = this;将当前Handler赋值给了msg.target,这是区分由哪个handler处理的关键。再看Message Queue中的方法，首先判断msg的target是否为空以及当前msg是否已经被使用，接着一个大的synchronized块，首先判断mQuitting的值，如果true则释放这个message并且return false。mQuitting这个值的设置就在在之前MessageQueue的quit(boolean safe),这里也说明了一旦调用了这个方法，MessageQueue则不会再接收任何消息。接下来第一个if的判断，p == null代表当前没有要处理的Message、when == 0代表立马插入、when < p.when代表传入msg的when比当前要处理的Message的时间还要提前，所以满足上面的条件之一的都会被插入到消息队列的首部。那下面else部分就是判断传入的msg该插入到队列中的哪个部分，里面的for循环就是完成了这么一件事情。但是其中有个needWake，字面意思是需要被唤醒，两个赋值的地方：if块里 needWake = mBlocked，直接赋值，如果当前状态是blocked，需要唤醒，没毛病。 else块里needWake = false，如果需要唤醒并且p是异步的，注意一点能走到这里的代表p不是第一个消息。说明即便msg是异步的，也不是链表中第一个异步消息，所以没必要唤醒了。
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3.Message的处理
在前面只说到了Message被插入到了消息队列中，那么Message又是怎么被取出来，又是怎么处理的呢，上面丝毫未提，但是回想一下，我们在Looper.prepare()之后是不是有个必做的方法Looper.loop()。看来应该都在这里面了。
//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();
}
}
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大部分都是Log，可以看到一个死循环，通过Message msg = queue.next()，拿到了要处理的msg，这个next()可能会引起中断，通过 msg.target.dispatchMessage(msg);实现了处理。msg.target就是之前设置进去的handler，所以就是调用handler的dispatchMessage，这个方法已经在上面分析过了。那么现在就简单了，我们看下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;
//第一个地方
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) {
//第二个地方
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 {
//第三个地方
// No more messages.
nextPollTimeoutMillis = -1;
}
//第四个地方
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
//第五个地方
// 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);
}
//第六个地方
// 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);
}
}
}
//第七个地方
// 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;
}
}
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看这代码，不出所料（没有其他可能了） nativePollOnce(ptr, nextPollTimeoutMillis);这句就是中断用的了，然后我们看第一个地方，msg != null && msg.target == null，是不是同步分割栏，看代码，如果当前的msg是同步分割栏，那么就找到后面是有异步属性的msg。第二个地方，首先判断msg应该执行的时间，有dealy就计算delay的时间，delay最大不超过Integer.MAX_VALUE。没有delay就拿到了msg，就直接return回去了。第三个方法说明此时已经没了msg，nextPollTimeoutMillis= -1，当运行nativePollOnce时就代表一直阻塞。第四个地方说明调用了quit()方法，丢弃msg并返回空。第五个地方，看注释，如果第一次空闲，则获取要运行的idlers数量。仅当队列为空或第一条消息在队列中（可能是屏障）将在将来处理。要是没有要运行的就直接设置mBlocked为true，然后continue了，接着就阻塞了。然后把mIdleHandlers拷贝到mPendingIdleHandlers里，就到了第六个地方，就开始执行idler.queueIdle()了，根据idler的返回值判断要不要从mIdleHandlers中移除，如果不移除那么以后每次空闲都就会运行。第七个地方，将pendingIdleHandlerCount赋值为0，避免再执行（在这一次的MessageQueue的next（）方法中最多只执行一次）。将nextPollTimeoutMillis赋值为0，因为不知道在所有的mIdleHandlers都执行完成之后msg的when到了没了，所以设置成0，直接再来一次，看到这里终于理解如何利用handler来做延迟加载了，其间奥秘全在mIdleHandlers里。
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