概述
本篇文章将通过源码来分析Handler异步消息处理机制，主要包括Handler的使用方法、场景、线程安全等
Handler机制的简单介绍
Android的消息异步处理机制，是每个Android程序员都应该掌握的。下面是这几个类的官方介绍，简单的翻译下，大家都应该看得懂。
Handler
Handler对象允许你发送消息和Runnable对象到MessageQueue里面，也可以处理这些从MessageQueue里面取出来的消息和Runnable对象。每一个Handler实例都跟一个线程和MessageQueue绑定。当你创建一个新的Handler时，handler对象将会跟创建时所在的线程的线程实例和MessageQueue绑定。从这点可以知道，handler将会把消息和runnable实例发送到和它绑定的messagequeue，并且执行或处理从这些队列分发出来的消息和Runnable对象。
Looper
每个线程最多只有一个Looper对象。使用Looper时需要在所在的线程调用一下静态方法Looper.prepare()，这个方法会创建当前线程所属的Looper对象和Messagequeue对象。然后调一下Looper.loop方法开启一个死循环不断的从MessageQueue中取出消息交给Handler去处理。
MessageQueue
简单的说，MessageQueue本质上内部有个Message的单链表，通过Handler对象发送Message对象到链表里面，又Looper.loop()方法里面分发出来。可以通过静态方法Looper.myQueue拿到当前线程的MessageQueue。
Message
Message类描述了消息的类型以及它所承载的数据。创建Message最好的方法是调用 Message.obtain()或者Handler.obtainMessage()，这两个方法是从一个Message对象的回收池中获取对象。避免了频繁地创建Message对象产生额外的开销。
Handler机制大概原理如下图所示，首先Handler把Message发送到MessageQueue，同时Looper不断的从MessageQueue取出待处理的Message，然后回调Handler的dispatchMessage方法，把消息交给Handler去处理。
Handler机制的一般使用场景
public class HandlerActivity extends AppCompatActivity {
private TextView textView;
Handler handler = new MyHandler(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
textView = findViewById(R.id.text);
new Thread(new Runnable() {
@Override
public void run() {
//模拟从网络获取消息
String text = getTextFromNet();
Message message = Message.obtain();
message.what = 0;
message.obj = text;
handler.sendMessage(message);
}
}).start();
}
private void setText(String text){
textView.setText(text);
}
private String getTextFromNet(){
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return "text text text";
}
static class MyHandler extends Handler{
private WeakReference<HandlerActivity> activityRef;
private MyHandler(HandlerActivity activity){
this.activityRef = new WeakReference<>(activity);
}
@Override
public void handleMessage(Message msg) {
if (activityRef!=null) {
HandlerActivity activity = activityRef.get();
if (activity != null) {
switch (msg.what) {
case 0:
String text = (String) msg.obj;
activity.setText(text);
break;
}
}
}
}
}
}
以上是一个模拟在子线程去访问网络获取一个文本信息，然后通过主线程把它显示到UI上面。下面从源码分析Handler的工作原理。
源码分析
Looper
想要在一个线程中使用Handler机制处理消息，首先要调用Looper.prepare()方法
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
sThreadLocal是ThreadLocal的一个实例，它用于储存当前线程变量 。可以看到一个线程只能调用一次Looper.prepare()方法，否则将抛出RuntimeException。prepare()方法内部创建了一个Looper实例
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的构造方法里面创建了一个MessageQueue对象，并且通过给Looper的mThread对象赋值的方式绑定了当前线程。注意Looper的构造方法为私有方法，它只在prepare()方法内部调用，而一个线程只能调用一次prepare()方法，所以一个线程只有一个Looper实例和一个MessageQueue实例。
然后我们看Looper.Looper()方法。
public static void loop() {
final Looper me = myLooper();//获取当前线程的Looper对象
if (me == null) {//loop()要在prepare()之后调用
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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
for (;;) {//循环从MessageQueue中取出Message
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 traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
msg.target.dispatchMessage(msg);//这里分发消息
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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);
}
msg.recycleUnchecked();
}
}
Looper.loop()方法内部首先调用myLooper()方法拿到当前线程的Looper对象
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
从looper方法里面可以看到，如果当前线程looper对象为空就会抛出错误。所以loop()方法要在prepare()方法后面调用。接下来可以看到Loope()方法内23行有个for循环，不断MessageQueue对象的next方法中不断取出Message，该方法是一个阻塞方法，没有获取到Message对象就会阻塞，然后通过第57行语句msg.target.dispatchMessage(msg)分发Message对象。msg.target实际是Handler对象，后面会具体介绍到。
Handler
接着我们看Handler
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());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
在Handler的构造方法内会先拿到当前线程的Looper对象，然后判断是否等于空，如果等于空就抛出错误。在这里，handler对象会跟当前线程的Looper对象和MessageQueue对象绑定，即持有他们的引用。
在使用Handler对象发送Message的时候，我们一般会调用handler.sendMessage(msg)或者sendMessageDelayed()方法，不过在方法内部最终都会调用sendMessageAtTime()方法。
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);
}
在sendMessageAtTime()方法里面又调用enqueueMessage(queue, msg, uptimeMillis);
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看到msg.target = this;把当前对象赋给了msg.target，也就是我们在介绍Looper.loop()方法时候提到的msg.target是Handler对象。最后调用了MessageQueue的enqueueMessage()方法把消息存到MessageQueue对象里面，MessageQueue的enqueueMessage()方法
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;
}
从第22到25行代码可以知道，Message的入队方式是根据回调的时间决定入队的位置的，也就是when参数，when参数小的排在队列的前面，参数大的排在后面。通过这种方式实现了延时消息。
我们前面说过，Looper.loop()方法会不断的从MessageQueue里面取出待处理的Message对象然后分发给Handler对象去处理。也就是msg.target.dispatchMessage(msg)语句，那么我们看下Handler的dispatchMessage(msg)里面到底是怎样实现的。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);//情况一
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {//情况二
return;
}
}
handleMessage(msg);//情况三
}
}
可以看到，如果Message的callback不等于空，则优先处理交给它，该callback是Runnable对象；如果Message的callback等于空，判断handler的callback是否等于空，如果不等于空则回调handler.mCallback的handleMessage(msg)方法来处理消息，最后才会交给handler的handleMessage(msg)方法处理。这里需要注意一下，只有handler.mCallback的handleMessage(msg)返回false才会继续交给handler的handleMessage(msg)方法。所以我们可以通过handler的mCallback的返回值去拦截这个消息。
//if (msg.callback != null) 知识简单的调了callback的run方法
private static void handleCallback(Message message) {
message.callback.run();
}
那么什么情况下msg.callback才不会为空呢？
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
可以看到当调用handler的post(runnable)方法的时候，会调用getPostMessage()方法，在该方法里面，会把runnable参数赋给Message对象的callback属性，并返回Message对象。也就是调用handler的post(runnable)方法发送消息，在dispatchMessage方法里走的是情况一。
同样的，我们看下handler的callback
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
在构造handler对象的时候，给它传入callback就可以了，这时候走的是情况二。handler.mCallback.handleMessage(msg)如果返回true则表示该方法已经处理消息，就不在交给handler.handleMessage(msg)方法，否则交给它处理，就会到情况三。
public void handleMessage(Message msg) {
}
可以看到handler.handleMessage(msg)是个空方法，里面没有实现。所以使用的时候要重写该方法。
MessageQueue的线程安全
既然handler是做异步处理的，那MessageQueue在多线程情况下必然会出现线程安全问题。那么MessageQueue是如何保证线程安全的呢
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.");
}
//通过synchronized 关键字加锁来保证线程安全
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;
}
通过源码可以看到，MessageQueue的入队列操作是通过synchronized 关键字加锁来保证线程安全。同样的出队列也是同样的方法。
主线程Looper.prepare()
前面我们说到一个线程想要使用Handler机制处理消息，首先要调用Looper.prepare()方法。那么我们没有对主线程调用该方法，为什么还是可以使用handler呢。原因在于主线程的Looper.prepare()方法是在ActivityThread.main()方法里面调用的，main()方法是Android应用程序的入口方法。
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>");
Looper.prepareMainLooper();//这里调用了主线程的prepareMainLooper()方法
...
}
需要注意的地方
- 如果想要在子线程做更新UI操作，那么可以使用handler在子线程发送消息交给主线程处理，但是这里需要注意handler实例应该在主线程创建，或者创建handler给它传入Looper.getMainLooper()返回的主线程Looper对象。
Handler handler = new Handler(Looper.getMainLooper());
- 当要创建一个Message对象，可以调用new Message()的构造方法创建。但最好的办法是使用Message.obtain() 或者 Handler.obtainMessage() 方法，因为两个方法是从一个message对象回收池里面获取message对象的，节省了创建新的message对象所需要的消耗。
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