Handler消息机制相关类介绍
- Message：是在线程之间传递的消息，它可以在内部携带少量的消息，用于在不同线程之间交换数据。
- Handler：处理者的意思，它主要是用于发送和处理消息。发送消息一般使用Handler的sendMessage()方法，接收消息使用Handler的handleMessage()方法。
- MessageQueue：消息队列的意思，它主要用于存放所有通过Handler发送的消息，每个线程中只会有一个MessageQueue对象。
- Looper：是每个线程的MessageQueue管家，调用Looper的Loop方法，就会进入到一个无限循环当中，然后每当发现MessageQueue中存在一条消息，就会将它取出并传递到Handler的handleMessage()方法中，每一个线程也只会有一个Looper对象。
- ThreadLocal：是一个线程内部的数据存储类，通过它可以在指定的线程中存储数据，数据存储以后，只有在指定线程中可以获取到存储的数据，对于其他线程来说则无法获取到数据。线程本地变量，存放Looper对象。每个线程对应自己的Looper。
Message的生成
- new Message()
- Message.obtain()
- Handler.obtain()
Handler 类
Handler handler = new Handler() { // 匿名子类对象()，调用父类默认的构造函数public void handleMessage(Message msg) {// 更新UI操作}}// Handler类的默认构造函数public Handler() {this(null, false);}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());}}// Looper 的静态方法myLooper()，获取到本线程的Looper对象，如果是在主线程中，则设置Looper对象是在ActivityThread类的main()方法中设置。mLooper = Looper.myLooper();if (mLooper == null) {throw new RuntimeException("Can't create handler inside thread that has not called Looper.prepare()");}// 获取MessageQueue对象mQueue = mLooper.mQueue;mCallback = callback;mAsynchronous = async;}
ActivityThread 类中的main方法如下：
public static final void main(String[] args) {...//Looper对象的初始化,放到线程本地变量中Looper.prepareMainLooper();if (sMainThreadHandler == null) {sMainThreadHandler = new Handler();}....//轮询器轮询取消息Looper.loop();....}}
Looper 类
// 创建线程本地变量，存放Looper对象static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();// 初始化主线程的Looper对象public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {// 只能初始化一次，否则会报异常throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}// 参数quitAllowed false：不允许中断；true：允许中断private static void prepare(boolean quitAllowed) {if (sThreadLocal.get() != null) {throw new RuntimeException("Only one Looper may be created per thread");}// 把Looper对象保存到ThreadLocal中sThreadLocal.set(new Looper(quitAllowed));}// Looper的构造方法private Looper(boolean quitAllowed) {// 创建MessageQueue对象，并设置是否可以中断mQueue = new MessageQueue(quitAllowed);mThread = Thread.currentThread();}// 调用此方法创建Looper，是可以中断的public static void prepare() {prepare(true);}
Handler 发送消息
// 1、handler对象 sendMessage()public final boolean sendMessage(Message msg) {return sendMessageDelayed(msg, 0);}// 2、public final boolean sendMessageDelayed(Message msg, long delayMillis) {if (delayMillis < 0) {delayMillis = 0;}// SystemClock.uptimeMillis() 获取系统的时间，从开机时间算起return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}// 3、public boolean sendMessageAtTime(Message msg, long uptimeMillis) {// 获取MessageQueue对象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);}// 4、private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {// this 就是当前Handler对象，把Handler对象给msg对象的target属性msg.target = this;if (mAsynchronous) {msg.setAsynchronous(true);}// 消息队列按照消息的进入顺序和发送时间排序，唤醒looper轮询器return queue.enqueueMessage(msg, uptimeMillis);}
MessageQueue 插入消息
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;// 如果消息为空，或者获取当前消息的时间为0，或者当前消息的时间比消息队列的第一个消息的时间小，则把当前消息插入到消息队列的头部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();// 保存当前消息队列Message prev;for (;;) {prev = p;// 获取下一个消息p = p.next;if (p == null || when < p.when) {break;}if (needWake && p.isAsynchronous()) {needWake = false;}}// 插入消息，Message是一个单链表形式，要找到插入消息的前一个位置和后一个位置msg.next = p; // invariant: p == prev.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;}
Looper 轮询器取消息
public static void loop() {// 获取当前的Looper对象final Looper me = myLooper();if (me == null) {throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");}// 获取MessageQueue对象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 (;;) {// 没有消息就阻塞，有消息就唤醒Message msg = queue.next(); // might blockif (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 loggerPrinter logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}// 1、msg.target对象就是前面设置的handler对象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);}// 2、消息回收利用msg.recycleUnchecked();}}
MessageQueue next()方法取消息
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 iterationint 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.// 如果当前时间，小于msg的时间，则等待并计算等待时间nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);} else {// Got a message.mBlocked = false;if (prevMsg != null) {prevMsg.next = msg.next;} else {// 从消息队列中取出消息头mMessages = msg.next;}// 把取出的消息的next设置为null，取消与下一个节点的关联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 handlerboolean 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;}}
Handler dispatchMessage()处理消息
public void dispatchMessage(Message msg) {if (msg.callback != null) {// 如果设置了消息的callback，则处理callbackhandleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}// 走handleMessage方法处理消息handleMessage(msg);}}// 回调callback的run方法private static void handleCallback(Message message) {message.callback.run();}// handler的handleMessage空方法，让子类去实现，处理业务逻辑public void handleMessage(Message msg) {}
Message 消息回收
// 消息回收方法void recycleUnchecked() {// Mark the message as in use while it remains in the recycled object pool.// Clear out all other details.// 消除message消息对象里的属性值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) { // 就是Object对象满足synchronized语法if (sPoolSize < MAX_POOL_SIZE) { // MAX_POOL_SIZE=50next = sPool; // 设置新的回收消息的下一个为sPoolsPool = this; // 设置该消息为新的消息头sPoolSize++; // 回收的消息+1}}}
Handler消息机制流程图
在子线程中使用Handler机制
* class LooperThread extends Thread {* public Handler mHandler;** public void run() {* Looper.prepare(); // 创建Looper对象** mHandler = new Handler() {* public void handleMessage(Message msg) {* // process incoming messages here* }* };** Looper.loop(); // 开启Looper轮询器，读取消息，没有消息则等待* }* }