Handler原理的整理及相关提问点
前言
有关handler的使用网上的教程有很多，并不是很难，在我的前几篇文章中也有简单的介绍过handler使用的整理，但是有关其实现的方式确实面试中非常喜欢问到的。不过以一次我被问到了handler的延迟的实现原理，当时我很郁闷并没有深入的去研究其delay的原理，然而对其handler的流程的学习也是跳过了延迟这部分，说明还是学的不精，后来再我深入学习这部分的时候，发现这里对handler的学习是很有帮助的。
MessageQueue
首先大家都知道，这个是消息队列，是我们管理发布消息的地方，然而我看到网上有人说这个是先进先出的，其实并不是这样，进确实是按照发布的顺序进入的，而出并不是。下面我详细介绍这个类。
Message的功能？ MessageQueue主要包含俩个操作，插入、读取，而读取会伴随着删除操作，对应的方法就是enqueueMessage和next。然而尽管我们叫它消息队列，而其内部的实现并不是队列而是单链表的数据结构。下面我们分析一下它的俩个主要方法的源码。
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) {
//插入消息到链表的头部：MessageQueue实例的头结点Message进行触发时间先后的比较，
//如果触发时间比现有的头结点Message短，或者现有的链表头部Message触发时间为0，
//亦或是当前MessageQueue中消息链表为空，则这个新的Message作为整个
//MessageQueue的头结点，如果阻塞着，则立即唤醒线程处理
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//插入消息到链表的中间：如果插入消息到链表头部的条件不具备，则依次
//循环消息链表比较触发时间的长短，然后将消息插入到消息链表的合适位置。接着
//如果需要唤醒线程处理则调用C++中的nativeWake()函数.
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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延迟进入队列？这里为了方便理解，我将官方的注释改为中文理解的注释。我们知道，我们所有的post，postDelay，sendMessage，sendMessageDelay都会最终调用MessageQueue的enqueueMessage方法，也就是插入队列，可以看到我们在插入队列时候并没有任何的阻塞操作和延迟操作，其中主要的内容只有一个if else和一个循环，和一个唤醒操作，也就是说网上说的延迟进入消息队列的说法是错误的。而这个方法的返回值是其进入到了队列中就会返回true。
先进先出？其message在发布后，会马上进入到消息队列中，并不会延迟进入。其中if和else可以参照注释，if中是有消息进入并不伴随着延迟，就是要立即looper的消息，我们的enqueueMessage会使用if将它插入到链表的头部，而else呢，根据代码和注释它是根据其delay的时间，在使用遍历将其插入到链表的合适的位置，所以说消息队列是不存在先进先出的。
唤醒什么？而needWake是什么呢，顾名思义，他是需要唤醒，也就是这个标识量为true的时候，队列处于阻塞状态，需要唤醒，在if的neekWake=mBlocked中就是将阻塞的标识给了需要唤醒这个标识。
所以，既然我们不是先进先出了，那么我先postDelay（，3000），然后马上post的话，那么后面的消息会阻塞3000ms吗？当然不会，有些人会误认为它是队列，先进先出，然而在后面的消息到达后，我们的队列根据他的唤醒标识，去唤醒队列的阻塞，将这个消息插入到消息队列的头部，如果没有延迟则立刻调用looper()。
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下面我们看下next方法的实现。
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;
}
}
怎么又有循环？next的代码主要功能是从队列中取出消息，然后再删除对应的消息，我们可以看到这里面有个循环，就是其实现取消息，整个循环是实现了取，而looper中的循环是让MessageQueue不停地取，也就是说这个next中如果取到了消息，并删除，然后返回message，这个循环就停止，也就是取消息成功！而我们知道looper中还有一个循环，便实现了不停的取，这个取成功了继续去取，一个轮询，俩个循环不能混淆。
这里有阻塞？next的调用就是在looper中。我们可以很亲切的官方注释说这个方法会阻塞，我们在next中看到了这样一个方法，nativePollOnce(ptr, nextPollTimeoutMillis);，这个方法就是实现了阻塞，具体的实现是在C层，介绍一下这个方法。
1.如果nextPollTimeoutMillis=-1，一直阻塞不会超时。
2.如果nextPollTimeoutMillis=0，不会阻塞，立即返回。
3.如果nextPollTimeoutMillis>0，最长阻塞nextPollTimeoutMillis毫秒(超时)，如果期间有程序唤醒会立即返回。
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这个真的是厉害，本身像wait一样但是还是sleep的功能，而且如果有新消息到来时，还可以在enqueueMessage中通过nativePollOnce进行唤醒，那么看到这里，我们应该知道了这个postDelay如何实现的延迟了吧。
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);
}
阻塞逻辑在nativePollOnce里面？是的，时间未到，next中则去改变这个nextPollTimeoutMillis，然后根据这个标识，在nativePollOnce方法内部去通过逻辑实现是否要去阻塞，阻塞多久。
sleep和postdelay的区别？那么网上有一种区分sleep和postDelay的说法看来是错误的，他们说sleep实现了阻塞而delay没有，实际上并不是，sleep是线程睡眠，而delay也是实现了阻塞，只不过为什么delay后面的代码还是立即执行，原因就是之前提到的enqueueMessage中，我们新的消息插入到了队列头，我们知道整个android的ui线程也是一个消息，我们在postdelay后立刻setText，而setText的这个消息边会唤醒这个队列，从而我们以为它只是延迟而没有阻塞。
主线程不是卡主了？那么这里有人提问了，比如我在一个线程中去阻塞，比如主线程，他不就卡主了嘛？当然不会，我们在主线程new Handler.postDelay（new MyRunable,3000）;后会有大量的操作进入我们主线程的这个looper当中，当然我们的主线程有且只有这一个loop（可以不在子线程创建looper就不会有了...），我们的手的各种操作都会触发这个looper，我们的屏幕的16ms的刷新也会触发，所以我们的这个looper这个循环不会停止，主线程停止了不就结束了嘛，在我们的ActivityThread中这个循环会不停的下去，去发消息，去触发生命周期，当然除非我们在生命周期去进行大量的耗时操作，这个ui线程才会真正意义上的阻塞，而5秒没有唤醒就是触发了ANR。然而我们的主线程的很多时候都阻塞的，不然它的开销是太大的，需要的时候唤醒就好了。
子线程呢？这里有人会问如果我在一个子线程，让它的looper也在这个子线程，它的enqueueMessage也在这个子线程，它阻塞了怎么办？我们可以试一下。
当然我们直接在子线程中去new Handler().postDelay是无法实现的，因为子线程的looper还来不及去创建便去通过looper去操作消息会抛出异常。
这里我们可以使用handlerThread，重写里面的run方法。
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
Log.i("zhou","!!!"+getThreadId());
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Log.i("zhou","3000");
Log.i("zhou","~~~"+getThreadId());
}
},3000);
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
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作者是不是有病？好嘛，真的是有病，不过我们试一下。他是可以打印出来内容的，但是不是阻塞了嘛？enqueue不像是主线程不停有新消息进入去唤醒，我们别忘了之前说的。如果nextPollTimeoutMillis>0，最长阻塞nextPollTimeoutMillis毫秒(超时)，如果期间有程序唤醒会立即返回。也就是说唤醒的方式有俩种，一种是在插入链表的时候，一种是延迟时间到达的时候。所以理解handler的阻塞对理解handler是很有帮助的。
Looper
looper是什么？网上说looper是一个泵，这么说挺有意思的，Looper在消息机制中扮演着消息循环的角色，具体说它就是不停地从MessageQueue中查看是否有新消息，通过轮询next方法去实现不停地查看，如果有就通过next返回，没有就通过标志量-1而阻塞。这里我们知道了，looper在哪个线程，里面的dispatchMessage就是哪个线程，其回调就是哪个线程，也是通过这种方式实现的线程通讯。
looper怎么退出？MessageQueue唯一跳出循环的方式是MessageQueue的next方法返回null，这样looper会通过quit或者quitSafely方法来退出。
public final class Looper {
/*
* API Implementation Note:
*
* This class contains the code required to set up and manage an event loop
* based on MessageQueue. APIs that affect the state of the queue should be
* defined on MessageQueue or Handler rather than on Looper itself. For example,
* idle handlers and sync barriers are defined on the queue whereas preparing the
* thread, looping, and quitting are defined on the looper.
*/
private static final String TAG = "Looper";
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;
private Printer mLogging;
private long mTraceTag;
/* If set, the looper will show a warning log if a message dispatch takes longer than time. */
private long mSlowDispatchThresholdMs;
/** 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));
}
/**
* Initialize the current thread as a looper, marking it as an
* application's 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();
}
}
/**
* Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
/**
* 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
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 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();
}
}
/**
* 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();
}
/**
* Return the {@link MessageQueue} object associated with the current
* thread. This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
/**
* Returns true if the current thread is this looper's thread.
*/
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}
/**
* Control logging of messages as they are processed by this Looper. If
* enabled, a log message will be written to <var>printer</var>
* at the beginning and ending of each message dispatch, identifying the
* target Handler and message contents.
*
* @param printer A Printer object that will receive log messages, or
* null to disable message logging.
*/
public void setMessageLogging(@Nullable Printer printer) {
mLogging = printer;
}
/** {@hide} */
public void setTraceTag(long traceTag) {
mTraceTag = traceTag;
}
/** {@hide} */
public void setSlowDispatchThresholdMs(long slowDispatchThresholdMs) {
mSlowDispatchThresholdMs = slowDispatchThresholdMs;
}
/**
* Quits the looper.
* <p>
* Causes the {@link #loop} method to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @see #quitSafely
*/
public void quit() {
mQueue.quit(false);
}
/**
* Quits the looper safely.
* <p>
* Causes the {@link #loop} method to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* However pending delayed messages with due times in the future will not be
* delivered before the loop terminates.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p>
*/
public void quitSafely() {
mQueue.quit(true);
}
/**
* Gets the Thread associated with this Looper.
*
* @return The looper's thread.
*/
public @NonNull Thread getThread() {
return mThread;
}
/**
* Gets this looper's message queue.
*
* @return The looper's message queue.
*/
public @NonNull MessageQueue getQueue() {
return mQueue;
}
/**
* Dumps the state of the looper for debugging purposes.
*
* @param pw A printer to receive the contents of the dump.
* @param prefix A prefix to prepend to each line which is printed.
*/
public void dump(@NonNull Printer pw, @NonNull String prefix) {
pw.println(prefix + toString());
mQueue.dump(pw, prefix + " ", null);
}
/**
* Dumps the state of the looper for debugging purposes.
*
* @param pw A printer to receive the contents of the dump.
* @param prefix A prefix to prepend to each line which is printed.
* @param handler Only dump messages for this Handler.
* @hide
*/
public void dump(@NonNull Printer pw, @NonNull String prefix, Handler handler) {
pw.println(prefix + toString());
mQueue.dump(pw, prefix + " ", handler);
}
/** @hide */
public void writeToProto(ProtoOutputStream proto, long fieldId) {
final long looperToken = proto.start(fieldId);
proto.write(LooperProto.THREAD_NAME, mThread.getName());
proto.write(LooperProto.THREAD_ID, mThread.getId());
proto.write(LooperProto.IDENTITY_HASH_CODE, System.identityHashCode(this));
mQueue.writeToProto(proto, LooperProto.QUEUE);
proto.end(looperToken);
}
@Override
public String toString() {
return "Looper (" + mThread.getName() + ", tid " + mThread.getId()
+ ") {" + Integer.toHexString(System.identityHashCode(this)) + "}";
}
}
它的构造方法中会创建一个MessageQueue，然后保存线程的对象。
子线程记得要创建looper looper.prepare就可以创建 looper可以开启循环，也可以通过HandlerThread它在run里面已经给我们创建了，并且还伴有wait方法在控制异常。
我们可以看到looper里面就是一个循环，去next()，然后去msg.target.dispatchMessage，在去回调handleMessage()。
