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.javaHandler.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。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;
}
MessageQueue.java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.cppvoid 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.cppNativeMessageQueue::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.cppvoid 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。
