生活有度，人生添寿 — 书摘
写在前面
对于Android开发者来说，Handler一定不陌生，在日常开发中会经常用到它，我们都知道Android主线程不能处理耗时任务，否则会容易发生ANR，但是在子线程执行完耗时任务，要想更新UI怎么办呢？不要慌，Android提供了一个消息机制Handler来解决这个问题。
Handler的作用是跨线程通信，当子线程中进行耗时操作后需要更新UI时，通过Handler将有关UI的操作切换到主线程中执行。
分别介绍下四要素：
- Message(消息)：需要被传递的消息，其中包含了消息Id，消息处理对象以及消息处理的数据等，由MessageQueue统一列队，最终由Handler处理。
- MessageQueue(消息队列)：用来存放Handler发送过来的消息，内部通过单链表的数据结构来维护消息队列，等待Looper的抽取。
- Handler(处理者)：负责Message的发送及处理，通过Handler的sendMessage向MessageQueue发送消息，通过handleMessage处理相应的消息事件。
- Looper(消息泵)：通过Looper.prepare()创建实例，通过Looper.loop()不断的从MessageQueue中抽取消息，按分发机制将消息分发给目标处理者。
用法
下面用一个简单的Demo来演示一下Handler该如何使用，这里开启了一个子线程sleep一秒来模拟任务，在子线程中调用sendEmptyMessage发送一条消息，当Handler的handleMessage收到该条消息时，弹出一个Toast。
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private Button mStartTask;
private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if (msg.what == 1) {
Toast.makeText(MainActivity.this, "更新UI", Toast.LENGTH_SHORT).show();
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initView();
}
private void initView() {
mStartTask = findViewById(R.id.btn_start_task);
mStartTask.setOnClickListener(this);
}
@Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.btn_start_task:
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(1000);
mHandler.sendEmptyMessage(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
break;
}
}
}
学会了使用，那么问题来了，让我们带着问题去分析源码：
1.明明是在子线程发送的，是如何切换至主线程的呢？
2.sendEmptyMessage发送的消息，handleMessage是如何接收到的呢？
源码
一.首先来看Handler是如何创建的，初始化的时候都做了什么？
1.从Handler的构造函数开始
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Callback callback) {
this(callback, false);
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
/**
* Use the {@link Looper} for the current thread
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(boolean async) {
this(null, async);
}
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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());
}
}
// 得到当前Looper，但有一点需要注意，只有先Looper.prepare()，才会创建新的Looper,
// 也就是说Looper.prepare()一定在这之前执行过了，否则会直接抛出异常
// Looper.prepare()是在哪里调用的呢？
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
// 创建Looper实例时，Looper会创建一个MeesageQueue，
// 直接把Looper的MessageQueue拿出来使用
mQueue = mLooper.mQueue;
// 为Callback赋值
mCallback = callback;
mAsynchronous = async;
}
从上面代码可以看出，无论是调用无参数的构造函数还是带参数的构造函数，最终都会调用带两个参数的构造函数，带两个参数的构造函数内部做了一些初始化的事情，为其一些变量赋值。
2.何处调用Looper.prepare()
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// 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>");
// 划重点
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
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);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
以上代码是ActivityThread类中的main函数，在这个函数中Android系统已经帮我们调用了Looper.prepare()，就是Looper.prepareMainLooper()。
3. Looper.prepareMainLooper()做了什么
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
/**
* 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();
}
}
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));
}
/**
* 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();
}
从以上代码看到prepareMainLooper内部其实先调用了prepare，prepare内部会创建一个新的Looper对象，并放入ThreadLocal；然后又调用了myLooper，myLooper内部会从ThreadLocal得到Looper对象，并赋值给sMainLooper。
那么Looper的构造函数做了什么呢？创建了一个新的MessageQueue实例，并得到当前所在线程。
既然用到了ThreadLocal，就介绍一下它是用来干什么的，ThreadLocal 为解决多线程程序的并发问题提供了一种新的思路。使用这个工具类可以很简洁地编写出优美的多线程程序。当使用ThreadLocal 维护变量时，ThreadLocal 为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。实则ThreadLocal.set设置的值是与当前线程进行绑定了的。
所以就能得出结论，Looper.prepare()函数就是将Looper与当前线程进行绑定，ThreadLocal就是负责绑定。
二.Handler如何发送和接收消息
1.Handler开始sendMessage
/**
* 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);
}
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it 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. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @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. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
// 这行代码非常重要
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
从以上代码看出，最终调到了enqueueMessage，内部将当前的Handler赋值给了Message中的target变量，这样就将每个调用sendMessage的Handler与Message进行绑定；返回值为queue.enqueueMessage()，也就是说最终调到了MessageQueue的enqueueMessage，将这个消息加入到MessageQueue中。
2.MessageQueue添加消息
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;
}
上面的代码就是把消息放入消息队列，如果添加消息成功会返回true，失败则返回false，等待Looper来读取。
如果是delay的消息并不会先等待一段时间在放入消息队列，而是直接进入并阻塞当前线程，然后将其delay的时间与队头进行比较，按照触发时间进行排序，如果触发时间更近则放入队头，保证队头的时间最小，队尾的时间最大，此时，如果对头的Message正是被delay的，则将当前线程堵塞一段时间，等待足够的时间再唤醒执行该Message，否则唤醒后直接执行。
3.Looper.loop()读取消息
众所周知，Handler内部维护了Looper的，通过Looper.loop()开启消息循环，不断的从MessageQueue中读取消息，最后调用handleMessage来处理消息。
不知细心的同学有没有发现Looper.loop()是何时调用的，其实是在ActivityThread类的main函数中调用的，在这个函数中Android系统也已经帮我们调用好了Looper.loop()。
那么就来看看Looper.loop()都做了什么吧。
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
// 得到与主线程绑定的Looper
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();
// 开始死循环
for (;;) {
// 从MessageQueue中读取消息
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 {
// 这行代码非常重要，这里的dispatchMessage则又回到了调用sendMessage的Handler中。
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();
}
}
在这个函数中，首先拿到与主线程绑定的Looper，然后开始死循环，并在死循环中不断调用MessageQueue的next函数读取消息，当得到消息后会通过Message的target调用dispatchMessage派发消息。Message的target是Handler的sendMessage最后调用enqueueMessage中将当前Handler赋值给target的，所以也就是调用的Handler的dispatchMessage。
4.Handler处理消息
现在再将视线回到Handler，通过Looper.loop()读取出来的消息投递给了Handler的dispatchMessage。
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
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) {
// msg.callback就是Runnable对象
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
// 其实就是让Runnable执行
message.callback.run();
}
首先来看dispatchMessage，msg.callback就是Runnable对象，如果不为null，就会调用handleCallback让Runnable跑起来；mCallback是Callback接口，如果不为null就会调用Callback的handleMessage，否则就会执行Handler自己的handleMessage。
5.回到主线程
到这里Handler的工作机制就算分析完了，在子线程中Handler发送消息的时候就已经和Message进行了绑定，在通过Looper.loop()开启消息轮询的时候，当调用MessageQueue的next得到Message的时候，就会调用与Message绑定的Handler对象执行dispatchMessage，最终调用handleMessage，由于Looper对象是在主线程创建的，Handler也是在主线程中创建的，所以自然就从子线程切换到了主线程。
6.分析Message
Message作为消息，也必要了解一下，它其实可以理解为一个Bean。
在实际项目中会用到大量的Message，为了避免重复创建Message，Message内部提供了一个obtain()方法，它会从消息池中返回一个新的Message实例。
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
/**
* 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++;
}
}
}
从以上代码可以看到，sPool就是一个静态的变量，若为空则创建一个新的Message实例，否则会直接返回sPool，在回收Message时，会将Message重置并赋值给sPool，这样就有效的避免了重复创建Message，从而达到复用的目的。
总结
具体流程.png
- Handler.sendMessage()时会通过MessageQueue.enqueueMessage()向MessageQueue中添加一条消息，MessageQueue会按照delay时间进行排序，保证队头时间最小，队尾时间最大，如果队头的时间是delay的，则将当前线程堵塞一段时间，等待足够的时间唤醒当前线程继续执行。
- 通过Looper.loop()开启消息循环后，不断轮询调用MessageQueue.next()，当读取的消息不为空时，就会调用目标Handler.dispatchMessage()去传递消息。
- 目标Handler收到消息后调用Handler.handleMessage()处理消息。
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