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。
Android异步消息机制_第1张图片

（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<? 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 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。

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