Android异步消息机制
Android的异步消息机制在Android系统中的重要性读者应该都很清楚，无论是在平时开发中，还是笔试、面试中，这方面的内容都是无法避免的。Android提供了Handler和Looper来满足线程间通信，而MessageQueue则是用来存放线程放入的消息。下面我们就结合源码分析一下这三者的关系。
（1）Handler的基本使用方式：
public class MainActivity extends AppCompatActivity {
private MyTextView title;
private static final String TAG = MainActivity.class.getSimpleName();
private static final int HANDLER1 = 1;
/**
* 在主线程中定义Handler，并实现对应的handleMessage方法
*/
public static Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == HANDLER1) {
Log.i(TAG, "接收到handler消息...");
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
title = (MyTextView) findViewById(R.id.title);
title.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread() {
@Override
public void run() {
// 在子线程中发送异步消息
mHandler.sendEmptyMessage(HANDLER1);
}
}.start();
}
});
}
}
相信上面的用法大家都很熟悉，但现在有个问题，如果想把Handler放到子线程中使用，是怎么样子的呢？下面我们看一段代码。
public class MainActivity extends AppCompatActivity {
private MyTextView title;
private static final String TAG = MainActivity.class.getSimpleName();
private static final int HANDLER1 = 1;
/**
* 在主线程中定义Handler，并实现对应的handleMessage方法
*/
Handler mHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
title = (MyTextView) findViewById(R.id.title);
title.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
mHandler.sendEmptyMessage(HANDLER1);
}
});
new Thread() {
@Override
public void run() {
// 在子线程中发送异步消息
mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == HANDLER1) {
Log.i(TAG, "接收到handler消息...");
}
}
};
}
}.start();
}
}
点击按钮运行后会报下面的错。
E/AndroidRuntime: FATAL EXCEPTION: Thread-95
Process: com.eyck.androidsample, PID: 2552
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
at android.os.Handler.(Handler.java:200)
at android.os.Handler.(Handler.java:114)
at com.eyck.androidsample.MainActivity$2$1.(MainActivity.java:0)
at com.eyck.androidsample.MainActivity$2.run(MainActivity.java:41)
根据提示信息我们可以很清晰的看到，在初始化Handler对象之前需要先调用Looper.prepare()方法，那么我们就加上这句代码再执行一次。
new Thread() {
@Override
public void run() {
// 在子线程中发送异步消息
Looper.prepare();
mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == HANDLER1) {
Log.i(TAG, "接收到handler消息...");
}
}
};
Looper.loop();
}
}.start();
这次运行就不会报错了，说明程序在初始化Handler之前需要调用Looper.prepare()，那么主线程为什么能直接初始化Handler？相信有的读者已经想到，是不是在初始化主线程时系统已经帮我们，我们来到应用程序的入口，在APP初始化时会执行ActivityThread的main()方法：
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, “ActivityThreadMain”);
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());
AndroidKeyStoreProvider.install();
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
可以看到这里调用了Looper.prepareMainLooper();和Looper.loop();两个方法。接下来我们看一下Handler的源码。
首先就是Looper.prepareMainLooper()；
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
可以看到在prepareMainLooper()中调用了prepare(false);方法。
接下来看一下Looper.prepare();
public static void prepare() {
prepare(true);
}
可以看到，上面两个方法都调用了prepare(boolean quitAllowed)；那接下来我们就来看一下这个方法。
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));
}
可以看到Looper中有一个ThreadLocal的成员变量，关于ThreadLocal在这里不做详细说明，它的主要作用是为每个使用该变量的线程提供一个独立的变量副本，所以每一个线程都可以独立的改变自己的副本，而不会影响其他线程所对应的副本。而且从中我们也可以看出在每个线程中Looper.prepare()只能调用一次，不然就会抛出”Only one Looper may be created per thread”异常。
接下来我们继续看new Looper(quitAllowed)；
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以清楚的看到在构造方法中，初始化了一个MessageQueue对象，以及获取了当前的线程。
按着代码的顺序，接下来我们看一下Handler的构造方法
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class 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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
在Handler的构造方法中，主要对一些相关的变量进行初始化。我们再看一下mLooper = Looper.myLooper();这句代码
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
通过上面的代码我们可以清晰看到Looper.myLooper();就是从sThreadLocal中得到我们之前保存的Looper对象，这样就和前面的sThreadLocal.set(new Looper(quitAllowed));联系起来了。
接下来我们再看一下sendEmptyMessage();
在Handler中有这么几个方法：
sendMessage(Message msg)；
sendEmptyMessage(int what)；
sendMessageDelayed(msg, 0);
sendEmptyMessageDelayed(what, 0);
最后都调用了sendMessageAtTime(Message msg, long uptimeMillis)这个方法
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);
}
接下来来到enqueueMessage(queue, msg, uptimeMillis);
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;//this==handler==msg.target
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看到这里的msg.target就是Handler对象，queue对象就是我们的Handler内部维护的Looper对象关联的MessageQueue对象。
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;
}
可以看到这里Message.next按照时间将所有的Message连接起来。
接下来继续看Looper.Loop()；
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
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;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}
这一段代码中有一个for的死循环，在死循环中不断获取MessageQueue中的消息，Message msg = queue.next(); // might block，接下来如果获取到不为空的消息，就会调用msg.target.dispatchMessage(msg);进行消息的分发，我们在前面已经介绍过msg.target就是一个Handler对象，也就是接下来就会调用Handler中的dispatchMessage(msg);
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
可以清晰的看到，这里调用了handleMessage(msg);，这个方法不正是我们在MainActivity里面调用的回调接口吗？
关于Android异步消息相关的就分析到这里，如果有什么不足的地方，希望读者指出，谢谢。