Android Handler 具体解释
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android开发中常常使用Handler来实现“跨越线程(Activity)更新UI”。本文将从源代码角度回答:为什么使用Handler可以跨线程更新UI?为什么跨线程更新UI一定要用Handler?
Demo
Demo1. 用Handler更新UI
以下这个Demo全然是为了演示“跨线程更新UI”而写的。
界面上仅仅有一个TextView和一个Button。按下Button创建一个后台线程。该后台线程每隔一秒更新一次TextView。连续更新10次。结束。
Activity的代码例如以下:
public class MainActivity extends Activity { static final String TAG = "MainActivity"; Handler handler = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); final TextView text = (TextView)findViewById(R.id.txtHello); Button button = (Button)findViewById(R.id.btnRun); button.setOnClickListener(new OnClickListener(){ @Override public void onClick(View v) { Log.d(TAG, "clicked!"); new Thread() { public void run() { for(int i=0; i<10; i++) { Message msg = new Message(); msg.what = 1; msg.obj = "item-"+i; handler.sendMessage(msg); Log.d(TAG, "sended "+"item-"+i); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } }.start(); } }); handler = new Handler() { @Override public void handleMessage(Message msg) { String str = "unknow"; switch(msg.what) { case 1: str = (String)msg.obj; break; default: break; } Log.d(TAG, "recv " + str); text.setText(str); super.handleMessage(msg); } }; } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } }
布局文件较为简单:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" tools:context=".MainActivity" > <TextView android:id="@+id/txtHello" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/hello_world" /> <Button android:id="@+id/btnStart" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Start" /> </RelativeLayout>
这里展示的是Handler的典型使用方法——用来更新UI控件。
以下再展示一个非典型使用方法。不过为了后面的分析方便。
Demo2. 自制ActivityThread模拟Activity
本例是为了分析方便而创建的;使用一个线程LooperThread来模拟Activity。
后面阐述为什么要这么做,代码例如以下:
package com.example.handlerdemo; import android.os.Bundle; import android.os.Message; import android.app.Activity; import android.util.Log; import android.view.Menu; import android.view.View; import android.widget.Button; import android.widget.TextView; public class MainActivity extends Activity { static final String TAG = "MainActivity"; ActivityThread acitivityThread = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); setupViews(); } private void setupViews() { TextView tv = (TextView)findViewById(R.id.txtHello); Button bt = (Button)findViewById(R.id.btnStart); Log.d(TAG, String.format("[MainActivity] Thread %s(%d)", Thread.currentThread().getName(), Thread.currentThread().getId())); acitivityThread = new ActivityThread(); acitivityThread.start(); acitivityThread.waitForHandlerReady(); bt.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { new Thread() { @Override public void run() { for(int i=0; i<10; i++) { Message msg = new Message(); msg.what = i; acitivityThread.mHandler.sendMessage(msg); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } }.start(); } }); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } }MainActivity.java
package com.example.handlerdemo; import android.os.Handler; import android.os.Looper; import android.os.Message; import android.util.Log; public class ActivityThread extends Thread { static final String TAG = "LooperThread"; public Handler mHandler = null; public ActivityThread() { super("LooperThread"); } @Override public void run() { Looper.prepare(); synchronized(this) { mHandler = new Handler() { @Override public void handleMessage(Message msg) { Log.d(TAG, String.format("recv msg.what: %d in Thread: %s(%d)", msg.what, Thread.currentThread().getName(),Thread.currentThread().getId())); } }; this.notify(); } Looper.loop(); } public void waitForHandlerReady() { try { synchronized(this) { while(mHandler == null) this.wait(); } } catch (InterruptedException e) { e.printStackTrace(); } } }ActivityThread.java
这个Demo的布局文件非常easy。就不贴出来了。
为什么使用Handler可以跨线程更新UI?
概览
以Demo2为例,这个Demo至少涉及三个线程:GodActivity线程。ActivityThread线程(模拟UI)。匿名线程(GodActivity创建的。叫他aThread)。暂且把GodActivity当做上帝。把ActivityThread看做Demo1里的Activity。如今,我们先预览一下为什么aThread能够通过Handler来更新ActivityThread的UI(纯属虚构)。这两个线程的交互关系例如以下图所看到的:
(PS:此前的版本号画了非常多对象的生命线。结果非常混乱,删了一堆无关紧要的之后,立马清晰了,^_^)
这个序列图(Sequence Diagram)已经简洁明了地给出了答案:
- Activity线程的幕后另一个MessageQueue;MessageQueue故名思议是一个Message组成的Queue。
- aThread仅仅是将数据以Message的形式挂到了Activity幕后的MessageQueue上了;
- Activity线程从MessageQueue上取Message并调用Handler.handlerMessage,所以实际的“更新动作”还是发生在Activity线程内;
具体解释
以下将从Android 4.4.4源代码的角度分析Handler的“幕后黑手”。
几个关键类
Demo2中和Handler有关的类除了MessageQueue还有Message和Looper,这几个类的关系例如以下:
关键点:
- MessageQueue通过Message.next维护链表结构(java引用即指针);
- ActivityThread的消息循环被封装在Looper.loop()内,Looper.prepare()用于创建属于当前线程的Looper和MessageQueue。
- 每一个Message能够通过target指向一个Handler,Handler实际上就是一个用来处理Message的callback。
接下来的代码,仅仅贴代码片段(方法),假设对各类的属性有所疑惑。能够回头查看此图。
Looper.prepare()
依据Looper的凝视能够看到,Looper线程“三部曲”:
- Looper.prepare()
- new Handler() { /* override handleMessage() */ }
- Looper.loop();
以下逐渐切入Looper.prepare():
public static void prepare() { prepare(true); }Looper.java
无參数版本号调用了有參数版本号:
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)); // 放入“单例”中 }Looper.java
这段代码中引用了sThreadLocal。它被定义为ThreadLocal类型。即线程私有数据类型(或者叫做线程级别单例)
ThreadLocal<T>可以理解为Map<Thread,T>的一层包包装(实际上Android,JVM都是按Map实现的,感兴趣的同学可自行研究;set(value)时,以当前线程对象为key,所以每一个线程可以保存一份value。)
可见Looper.prepare()调用使得AcitivityThread通过Looper.sThreadLocal<Looper>持有了一个Looper对象。
继续看Looper的构造方法Looper(quitAllowed):
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); // 和当前线程关联 }Handler.java
能够看到Looper的构造函数中创建了一个MessageQueue。
流程又转到了MessageQueue的构造函数MessageQueue(quitAllowed):
MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; mPtr = nativeInit(); }MessageQueue.java
Handler()
首先看上面调用的默认构造方法:
/** * Default constructor associates this handler with the {@link Looper} for the * current thread. 将当前线程的Looper与此handler关联。 * 假设当前线程没有looper,这个handler将不能接收消息,从而导致异常抛出 * 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); }Handler.java
默认构造方法又调用了还有一版本号的构造方法,例如以下:
public Handler(Callback callback, boolean async) { if (FIND_POTENTIAL_LEAKS) { // FIND_POTENTIAL_LEAKS 为 false; 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(); // 获取当前线程(调用者)的Looper if (mLooper == null) { // 假设当前线程没有Looper。则抛异常 throw new RuntimeException( "Can‘t create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; // 这里引用的MessageQueue是Looper()中创建的 mCallback = callback; mAsynchronous = async; }Handler.java
Handler()调用了Looper.myLooper():
public static Looper myLooper() { return sThreadLocal.get(); // 从该线程的“单例”中取出Looper对象 }Looper.java
Looper.loop()
Looper.loop()封装了消息循环。所以我们如今看看Looper.loop()的“真面目”:
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, 取出消息 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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } // mLatencyLock is only initialized for non USER builds // (e.g., USERDEBUG and ENG) if ((!sLatencyEnabled) || (me != sMainLooper)) { msg.target.dispatchMessage(msg); // 通过msg.target分派消息 } else { // 记录性能数据 long t1 = SystemClock.uptimeMillis(); // 获得当前毫秒数(自启动) msg.target.dispatchMessage(msg); long t2 = SystemClock.uptimeMillis() - t1; // t2就是dispatchMessage(msg)所用时间 if (t2 < 50) { // We don‘t care about these from a latency perspective } else if (t2 < 250) { // Fast response that usually has low impact on user experience sLatencyCountFast++; sLatencySumFast += t2; if (sLatencyCountFast >= 100) { String name = getProcessName(); long avg = sLatencySumFast / sLatencyCountFast; EventLog.writeEvent(2731, "mainloop2_latency1", name, avg); sLatencyCountFast = 0; sLatencySumFast = 0; } } else if (t2 < 1000) { sLatencyCountSlow++; sLatencySumSlow += t2; if (sLatencyCountSlow >= 10) { String name = getProcessName(); long avg = sLatencySumSlow / sLatencyCountSlow; EventLog.writeEvent(2731, "mainloop2_latency2", name, avg); sLatencyCountSlow = 0; sLatencySumSlow = 0; } } else { String name = getProcessName(); EventLog.writeEvent(2731, "mainloop2_bad", name, t2); } } 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.recycle(); } }Looper.java
能够看到。Looper.loop()的for循环实际上就是“消息循环”,它负责从消息队列(MessageQueue)中不断地取出消息(MessageQueue.next),然后通过msg.target来派发(dispatch)消息。
How to dispatch?
以下看看Message究竟是怎样被dispatch的:public void dispatchMessage(Message msg) { if (msg.callback != null) { // 方法 1 handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { // 方法 2 return; } } handleMessage(msg); // 方法 3 } }Handler.java
从这段代码能够看出,实现正常的Message处理有三种方式:
- 为Message.callback注冊一个Runnable实例。
- 为Handler.mCallback注冊一个Handler.Callback实例。
- 重写Handler的handleMessage方法。
另外。这三种方法优先级依次减少。且一个Message仅仅能有一种处理方式。
Message的发送与获取
对于一个后台线程,它要发出消息(Handler.sendMessage)。对于Activity线程,它要得到其它线程发来的消息(MessageQueue.next);而这两种工作都是以MessageQueue为基础的。以下。分别分析发送和接收的详细流程:
Handler.sendMessage()
Demo中后台线程正是通过Handler.sendMessage实现向Activity发消息的。Handler.sendMessage方法的代码例如以下:
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }Handler.java
public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }Handler.java
当中,当中SystemClock.uptimeMillis()返回自启动以来CPU经过的毫秒数。
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); }Handler.java
Handler.enqueMessage事实上仅仅是对MessageQueue.enqueueMessage的简单包装:
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; // 将当前Handler(通常已重写handleMessage方法)与该Message绑定(通过target) if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); // 调用MessageQueue.enqueueMessage }Handler.java
这里看到了Looper.loop()里引用的target的来源。
流程转到了MessageQueue.enqueueMessage(),看命名基本知道它是入队操作。代码例如以下:
boolean enqueueMessage(Message msg, long when) { if (msg.isInUse()) { throw new AndroidRuntimeException(msg + " This message is already in use."); } if (msg.target == null) { throw new AndroidRuntimeException("Message must have a target."); } synchronized (this) { // 临界区 if (mQuitting) { RuntimeException e = new RuntimeException( msg.target + " sending message to a Handler on a dead thread"); Log.w("MessageQueue", e.getMessage(), e); return false; } msg.when = when; Message p = mMessages; // 链表头 boolean needWake; if (p == null || when == 0 || when < p.when) { // p == null 队列为空 // when == 0 由 Handler.sendMessageAtFrontOfQueue() 发出 // when < p.when 新消息的when比队头要早 // New head, wake up the event queue if blocked. msg.next = p; // 将msg放到队头,step 1 mMessages = msg; // 将msg放到队头,step 2 needWake = mBlocked; } else { // Inserted within the middle of the queue. Usually we don‘t have to wake 插到队列中间。MessageQueue.java通常我们不必唤醒 // up the event queue unless there is a barrier at the head of the queue 事件(event)队列。除非队头有一个barrier, // and the message is the earliest asynchronous message in the queue.且消息是队列中最早的同步消息。
needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { // 遍历链表 prev = p; p = p.next; if (p == null || when < p.when) { // 到“尾部”了 或 新消息比当前消息更早 break; } if (needWake && p.isAsynchronous()) { needWake = false; } } // 下面两行将msg插入prev和p之间 msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); // 通知前台线程“有消息来啦” } } return true; }
依据这段代码可知,MessageQueue上的Message是依照when大小排列的。唯一可能让人疑惑的是最后的nativeWake。稍后讨论。
MessageQueue.next()
前文的Looper.loop方法通过MessageQueue.next()取出消息,如今看看它是怎样实现的:
Message next() { int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } // We can assume mPtr != 0 because the loop is obviously still running. // The looper will not call this method after the loop quits. nativePollOnce(mPtr, nextPollTimeoutMillis); // 等待通知,可能堵塞 synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; // 链表头 if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. do { // 遍历链表 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; // 将msg节点摘下 } else { // prevMsg == null, msg是链表头 mMessages = msg.next; } msg.next = null; // msg与MessageQueue“断绝关系” if (false) Log.v("MessageQueue", "Returning message: " + msg); msg.markInUse(); return msg; // 退出点1 到这为止。是常规逻辑 } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; // 退出点2 } // 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("MessageQueue", "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; } }
MessageQueue.java
MessageQueue.next()相同让人疑惑的是nativePollOnce,稍后也将见分晓。
小结
MessageQueue.next()和MessageQueue.sendMessage()分别被Activity线程、后台线程调用,而他们两个线程可能同一时候在调用这两个方法,所以他们共享并改动的成员变量须要加锁,这就是synchronized (this)出现的原因。
至此,已经可以完整的回答“为什么用Handler可以实现跨线程更新UI”。简单的说,Activity线程的背后都有一个消息队列(MessageQueue)。后台线程通过Handler的sendMessage方法向这个消息队列上放消息。Activity线程将消息从消息队列上取下来之后,通过详细Handler的handleMessage方法处理消息,而更新UI的代码就在这个handleMessage中。所以。后台线程并没有做实际的“更新”,仅仅是将要更新的内容以借助MessageQueue告诉了Activity线程,Activity线程才是实际做“更新”动作的人。
简言之。Handler并没有真正的实现“跨线程”更新UI,而是将要更新的数据(Message携带)和怎样更新(Handler携带)通过消息队列告诉了UI线程,UI线程才是真正的“幕后英雄”。
真正的ActivityThread
Demo2中的ActivityThread全然是虚构出来的,以下来看看Android的Activity究竟是不是想我虚构的那样有一个Looper。
经过上面的分析,能够从双方面验证:
- 看看Activity源代码中运行onCreate之前是否调用了Looper.prepare()。
- 运行onXXX方法时的CallStack上是否有Looper.loop();
第二点非常easy验证。仅仅需在随意onXXX方法中打一个断点。然后看程序的CallStack,就一面了然了:
依据这个调用栈。能够非常明显的看到有Looper.loop;同一时候还能看到是ActivityThread.main调用它的。所以能够看看ActivityThread.main的源代码:
public static void main(String[] args) { SamplingProfilerIntegration.start(); // 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()); Security.addProvider(new AndroidKeyStoreProvider()); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); // 它和Looper.prepare相似 ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } AsyncTask.init(); if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }ActivityThread.java
所以。上面提到的双方面都得到了验证。
即真正的ActivityThread是有Looper的。
Native浮云
细心的朋友可能会发现。上面MessageQueue的代码中还遗留几个native开头方法:nativeInit,nativePollOnce,nativeWake。
以下就来扫清这些“遮眼”的浮云。和这几个native方法直接相应的是:
static JNINativeMethod gMessageQueueMethods[] = { /* name, signature, funcPtr */ { "nativeInit", "()I", (void*)android_os_MessageQueue_nativeInit }, { "nativeDestroy", "(I)V", (void*)android_os_MessageQueue_nativeDestroy }, { "nativePollOnce", "(II)V", (void*)android_os_MessageQueue_nativePollOnce }, { "nativeWake", "(I)V", (void*)android_os_MessageQueue_nativeWake }, { "nativeIsIdling", "(I)Z", (void*)android_os_MessageQueue_nativeIsIdling } };
android_os_MessageQueue.cpp
nativeInit
以下从adnroid_os_MessageQueue_nativeInit開始,顾名思义,nativeInit当然是完毕一些初始化工作的。
static jint android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) { NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue(); // 创建了NativeMessageQueue if (!nativeMessageQueue) { jniThrowRuntimeException(env, "Unable to allocate native queue"); return 0; } nativeMessageQueue->incStrong(env); return reinterpret_cast<jint>(nativeMessageQueue); }android_os_MessageQueue.cpp
看看NativeMessageQueue的声明:
class NativeMessageQueue : public MessageQueue { public: NativeMessageQueue(); virtual ~NativeMessageQueue(); virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj); void pollOnce(JNIEnv* env, int timeoutMillis); void wake(); private: bool mInCallback; jthrowable mExceptionObj; };android_os_MessageQueue.cpp
NativeMessageQueue继承了MessageQueue,再来看看MessageQueue的声明:
class MessageQueue : public RefBase { public: /* Gets the message queue‘s looper. */ inline sp<Looper> getLooper() const { return mLooper; } /* Checks whether the JNI environment has a pending exception. * * If an exception occurred, logs it together with the specified message, * and calls raiseException() to ensure the exception will be raised when * the callback returns, clears the pending exception from the environment, * then returns true. * * If no exception occurred, returns false. */ bool raiseAndClearException(JNIEnv* env, const char* msg); /* Raises an exception from within a callback function. * The exception will be rethrown when control returns to the message queue which * will typically cause the application to crash. * * This message can only be called from within a callback function. If it is called * at any other time, the process will simply be killed. * * Does nothing if exception is NULL. * * (This method does not take ownership of the exception object reference. * The caller is responsible for releasing its reference when it is done.) */ virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj) = 0; protected: MessageQueue(); virtual ~MessageQueue(); protected: sp<Looper> mLooper; };android_os_MessageQueue.h
如今看看NativeMessageQueue的构造函数:
NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) { mLooper = Looper::getForThread(); if (mLooper == NULL) { mLooper = new Looper(false); Looper::setForThread(mLooper); } }android_os_MessageQueue.cpp
NativeMessageQueue的构造函数又调用了Looper::getForThread(),Looper::Looper()和Looper::setThread(),当中getForThread和setForThread都是静态函数:
sp<Looper> Looper::getForThread() { int result = pthread_once(& gTLSOnce, initTLSKey); LOG_ALWAYS_FATAL_IF(result != 0, "pthread_once failed"); return (Looper*)pthread_getspecific(gTLSKey); }Looper.cpp
这段代码中,在第一次运行pthread_once时将调用initTLSKey。
void Looper::initTLSKey() { int result = pthread_key_create(& gTLSKey, threadDestructor); LOG_ALWAYS_FATAL_IF(result != 0, "Could not allocate TLS key."); }Looper.cpp
void Looper::threadDestructor(void *st) { Looper* const self = static_cast<Looper*>(st); if (self != NULL) { self->decStrong((void*)threadDestructor); } }Looper.cpp
void Looper::setForThread(const sp<Looper>& looper) { sp<Looper> old = getForThread(); // also has side-effect of initializing TLS if (looper != NULL) { looper->incStrong((void*)threadDestructor); } pthread_setspecific(gTLSKey, looper.get()); if (old != NULL) { old->decStrong((void*)threadDestructor); } }Looper.cpp
Looper::setForThread和getForThread中分别使用了pthread_setspecific,pthread_getsepcific,pthread_key_create,实现了线程私有的looper引用,这和Java层Looper类似。
Looper的构造函数例如以下:
Looper::Looper(bool allowNonCallbacks) : mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) { int wakeFds[2]; int result = pipe(wakeFds); LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe. errno=%d", errno); mWakeReadPipeFd = wakeFds[0]; mWakeWritePipeFd = wakeFds[1]; result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK); LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking. errno=%d", errno); result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK); LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake write pipe non-blocking. errno=%d", errno); mIdling = false; // Allocate the epoll instance and register the wake pipe. mEpollFd = epoll_create(EPOLL_SIZE_HINT); // 用epoll实现IO多路复用,EPOLL_SIZE_HINT定义为8 LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno); struct epoll_event eventItem; memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union eventItem.events = EPOLLIN; eventItem.data.fd = mWakeReadPipeFd; result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem); // 将Wake管道的读端加入到mEpollFd上 LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake read pipe to epoll instance. errno=%d", errno); }Looper.cpp
从Looper的构造函数能够看到,Looper的Wake是由管道+epoll实现的,且管道的两端fd都被设置为NONBLOCK的,并通过epoll实现IO多路复用。Looper的数据成员(data member)声明例如以下:
struct Request { int fd; int ident; sp<LooperCallback> callback; void* data; }; struct Response { int events; Request request; }; struct MessageEnvelope { MessageEnvelope() : uptime(0) { } MessageEnvelope(nsecs_t uptime, const sp<MessageHandler> handler, const Message& message) : uptime(uptime), handler(handler), message(message) { } nsecs_t uptime; sp<MessageHandler> handler; Message message; }; const bool mAllowNonCallbacks; // immutable int mWakeReadPipeFd; // immutable int mWakeWritePipeFd; // immutable Mutex mLock; Vector<MessageEnvelope> mMessageEnvelopes; // guarded by mLock bool mSendingMessage; // guarded by mLock // Whether we are currently waiting for work. Not protected by a lock, // any use of it is racy anyway. volatile bool mIdling; int mEpollFd; // immutable // Locked list of file descriptor monitoring requests. KeyedVector<int, Request> mRequests; // guarded by mLock // This state is only used privately by pollOnce and does not require a lock since // it runs on a single thread. Vector<Response> mResponses; size_t mResponseIndex; nsecs_t mNextMessageUptime; // set to LLONG_MAX when noneLooper.h
Looper数据成员涉及的类型还有有:作为callback的LooperCallback,MessageHandler,以及Message:
class MessageHandler : public virtual RefBase { protected: virtual ~MessageHandler() { } public: /** * Handles a message. */ virtual void handleMessage(const Message& message) = 0; };Looper.h
class LooperCallback : public virtual RefBase { protected: virtual ~LooperCallback() { } public: /** * Handles a poll event for the given file descriptor. * It is given the file descriptor it is associated with, * a bitmask of the poll events that were triggered (typically ALOOPER_EVENT_INPUT), * and the data pointer that was originally supplied. * * Implementations should return 1 to continue receiving callbacks, or 0 * to have this file descriptor and callback unregistered from the looper. */ virtual int handleEvent(int fd, int events, void* data) = 0; };Looper.h
struct Message { Message() : what(0) { } Message(int what) : what(what) { } /* The message type. (interpretation is left up to the handler) */ int what; };Looper.h
至此,android_os_MessageQueue_nativeInit分析完成。
nativeWake
接下来看看android_os_MessageQueue_nativeWake和android_os_MessageQueue_nativePollOnce。
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jint ptr) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); return nativeMessageQueue->wake(); }android_os_MessageQueue.cpp
android_os_MessageQueue_nativeWake调用了NativeMessageQueue::wake:
void NativeMessageQueue::wake() { mLooper->wake(); }android_os_MessageQueue.cpp
NativeMessageQueue::wake直接将工作转交给了Looper::wake:
void Looper::wake() { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ wake", this); #endif ssize_t nWrite; do { nWrite = write(mWakeWritePipeFd, "W", 1); // 向pipe的写段写入一个字节 } while (nWrite == -1 && errno == EINTR); if (nWrite != 1) { if (errno != EAGAIN) { ALOGW("Could not write wake signal, errno=%d", errno); } } }Looper.cpp
能够看到nativeWake很easy,仅仅是向pipe上写一个字节。
但这是怎样唤醒等待的线程的呢?猜想:等待线程必定通过epoll_wait等在mEpollFd上,稍后将得到验证。
nativePollOnce
static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jclass clazz, jint ptr, jint timeoutMillis) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->pollOnce(env, timeoutMillis); // 调用NativeMessageQueue::pollOnce() }android_os_MessageQueue.cpp
android_os_MessageQueue_nativeWake调用了NativeMessageQueue::pollOnce:
void NativeMessageQueue::pollOnce(JNIEnv* env, int timeoutMillis) { mInCallback = true; mLooper->pollOnce(timeoutMillis); mInCallback = false; if (mExceptionObj) { env->Throw(mExceptionObj); env->DeleteLocalRef(mExceptionObj); mExceptionObj = NULL; } }
android_os_MessageQueue.cpp
NativeMessageQueue::pollOnce调用了Looper::pollOnce:
inline int pollOnce(int timeoutMillis) { return pollOnce(timeoutMillis, NULL, NULL, NULL); }Looper.h
Looper::pollOnce(int)调用了还有一版本号的Looper::pollOnce:
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) { int result = 0; for (;;) { while (mResponseIndex < mResponses.size()) { const Response& response = mResponses.itemAt(mResponseIndex++); // 取出一个response int ident = response.request.ident; if (ident >= 0) { int fd = response.request.fd; int events = response.events; void* data = response.request.data; #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - returning signalled identifier %d: " "fd=%d, events=0x%x, data=%p", this, ident, fd, events, data); #endif if (outFd != NULL) *outFd = fd; if (outEvents != NULL) *outEvents = events; if (outData != NULL) *outData = data; return ident; } } if (result != 0) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - returning result %d", this, result); #endif if (outFd != NULL) *outFd = 0; if (outEvents != NULL) *outEvents = 0; if (outData != NULL) *outData = NULL; return result; } result = pollInner(timeoutMillis); } }Looper.cpp
pollOnce的for(;;)循环里先查看是否还有没有取出的response,若有,取出一个马上返回;否则,调用Looper::pollInner,poll出一个IO事件(wake通知,后面可以看到):
int Looper::pollInner(int timeoutMillis) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis); #endif // Adjust the timeout based on when the next message is due. if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) { nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC); int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime); if (messageTimeoutMillis >= 0 && (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) { timeoutMillis = messageTimeoutMillis; } #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - next message in %lldns, adjusted timeout: timeoutMillis=%d", this, mNextMessageUptime - now, timeoutMillis); #endif } // Poll. int result = ALOOPER_POLL_WAKE; mResponses.clear(); mResponseIndex = 0; // We are about to idle. mIdling = true; struct epoll_event eventItems[EPOLL_MAX_EVENTS]; int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis); // 关键。等待wake通知 // No longer idling. mIdling = false; // Acquire lock. mLock.lock(); // Check for poll error. if (eventCount < 0) { if (errno == EINTR) { goto Done; } ALOGW("Poll failed with an unexpected error, errno=%d", errno); result = ALOOPER_POLL_ERROR; goto Done; } // Check for poll timeout. if (eventCount == 0) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - timeout", this); #endif result = ALOOPER_POLL_TIMEOUT; goto Done; } // Handle all events. #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount); #endif for (int i = 0; i < eventCount; i++) { // 处理全部事件 int fd = eventItems[i].data.fd; uint32_t epollEvents = eventItems[i].events; if (fd == mWakeReadPipeFd) { if (epollEvents & EPOLLIN) { awoken(); // 调用Looper::awoken(),运行实际的wake通知 } else { ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents); } } else { ssize_t requestIndex = mRequests.indexOfKey(fd); if (requestIndex >= 0) { int events = 0; if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT; if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT; if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR; if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP; pushResponse(events, mRequests.valueAt(requestIndex)); // push到mRequest上 } else { ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is " "no longer registered.", epollEvents, fd); } } } Done: ; // Invoke pending message callbacks.调用等待的消息回调 mNextMessageUptime = LLONG_MAX; while (mMessageEnvelopes.size() != 0) { nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC); const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0); if (messageEnvelope.uptime <= now) { // Remove the envelope from the list. // We keep a strong reference to the handler until the call to handleMessage // finishes. Then we drop it so that the handler can be deleted *before* // we reacquire our lock. { // obtain handler sp<MessageHandler> handler = messageEnvelope.handler; Message message = messageEnvelope.message; mMessageEnvelopes.removeAt(0); mSendingMessage = true; mLock.unlock(); #if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d", this, handler.get(), message.what); #endif handler->handleMessage(message); // 调用Message回调(MessageHandler) } // release handler mLock.lock(); mSendingMessage = false; result = ALOOPER_POLL_CALLBACK; } else { // The last message left at the head of the queue determines the next wakeup time. mNextMessageUptime = messageEnvelope.uptime; break; } } // Release lock. mLock.unlock(); // Invoke all response callbacks.调用全部响应回调 for (size_t i = 0; i < mResponses.size(); i++) { Response& response = mResponses.editItemAt(i); if (response.request.ident == ALOOPER_POLL_CALLBACK) { int fd = response.request.fd; int events = response.events; void* data = response.request.data; #if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p", this, response.request.callback.get(), fd, events, data); #endif int callbackResult = response.request.callback->handleEvent(fd, events, data); // 调用事件回调(LooperCallback) if (callbackResult == 0) { removeFd(fd); } // Clear the callback reference in the response structure promptly because we // will not clear the response vector itself until the next poll. response.request.callback.clear(); result = ALOOPER_POLL_CALLBACK; } } return result; }Looper.cpp
void Looper::awoken() { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ awoken", this); #endif char buffer[16]; ssize_t nRead; do { nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer)); // 读到暂时的buffer, } while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer)); }Looper.cpp
Looper::awoken的read从mWakeReadFd上读出的消息被放在一个暂时的buffer上,这再次表明了这个pipe之作唤醒通知之用,并不关心实际内容。
nativeIsIdling 和 nativeDestroy
剩下的两个native方法的实现都很easy,先看nativeIdling:
static jboolean android_os_MessageQueue_nativeIsIdling(JNIEnv* env, jclass clazz, jint ptr) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); return nativeMessageQueue->getLooper()->isIdling(); }android_os_MessageQueue.cpp
NativeMessageQueue::getLooper:
inline sp<Looper> getLooper() const { return mLooper; }android_os_MessageQueue.cpp
bool Looper::isIdling() const { return mIdling; }Looper.cpp
再看nativeDestroy:
static void android_os_MessageQueue_nativeDestroy(JNIEnv* env, jclass clazz, jint ptr) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->decStrong(env); }android_os_MessageQueue.cpp
nativeDestroy将nativeMessageQueue的强引用减1,引用计数减为0时,对象会自己主动被析构并回收。
小结
隐藏在nativePollOnce和nativeWake背后起着重要作用的事实上是pipe。nativeWake向pipe的写端写一个字节,通知前台线程“有消息来了”。
总结
后台线程使用Handler更新UI的本质上是“生产者消费者问题”。后台线程扮演生产者,生产消息(Message),并放到消息队列上。前台线程扮演消费者。从消息队列上取消息。并处理(消费)它。
在这个过程中Handler扮演了两个角色:
- 消息队列的窗体,后台线程通过Handler.sendMessage()向消息队列放消息;
- 处理消息的回调,前台线程通过Handler.handleMessage()处理从队列上取下来的消息。
引申
本文开头所给的两个Demo都是“单生产者单消费者问题”。
这个问题中须要指出的是。消费者必定唯一。由于每一个线程最多仅仅能仅仅有一个Looper(通过Looper.prepare创建)。而MessageQueue是由Looper的构造方法创建的。所以每一个Looper相应一个MessageQueue;所以不可能有多个消费者线程共享一个MessageQueue。
但生产者能够不必唯一,比方本文开头的Demo1,按下Button之后,会创建一个后台线程。这个线程每一个1秒更新一次TextView,更新10次后结束。当你点下Button后不到10秒(比方5秒)时。再次点下Button,此时又创建了一个后台线程;这时两个后台线程都是生产者。感兴趣的朋友能够自己试试,看看实际执行的效果。
pipe是仅仅有两个端的结构。多生产者时,有多个线程向写端write,但始终仅仅有一个线程从读端read。
所以,nativePollOnce能够实现为堵塞的,即pipe的读端mWakeReadPipeFd能够不设为NONBLOCK(当然也就不须要要用epoll了)。
但因为可能存在多个生产者,所以pipe的写端设为NONBLOCK还是非常有必要的。
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