带你从源码看Android Handler 异步消息处理机制完全解析

android studio 2.2.2 SDK版本 25.0.3
在2.2.2的版本中修改代码调试 真的是超级快，特别爽 两三秒就运行了。
写了一个小demo 分别在主线程 子线程中都创建了handler 最后都实现textView 内容的更新。
上代码：

import android.os.Handler;import android.os.Looper;import android.os.Message;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.util.Log;import android.widget.TextView;public class MainActivity extends AppCompatActivity {    private static final int UPDATE_TEXT = 1;    private static final int UPDATE_TEXT_2 = 2;    private static final String TAG = "MainActivity";    private TextView mTextContentTextView;    private TextView mSecondTextContent;    private int anInt = 0;    private int mRes = 0;    public Handler mHandler; @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mTextContentTextView = (TextView) findViewById(R.id.tv_content);        mSecondTextContent = (TextView) findViewById(R.id.tv_content_2);        Log.d(TAG, "onCreate  thread id :" + Thread.currentThread().getId());        changeContent();    }    /**     * handler     */    private Handler handler = new Handler() {        @Override        public void handleMessage(Message msg) {            switch (msg.what){                case UPDATE_TEXT:                    Log.d(TAG, "handler  thread id :" + Thread.currentThread().getId());                    mTextContentTextView.setText(String.valueOf(anInt));                    break;                case UPDATE_TEXT_2:                    mSecondTextContent.setText(String.valueOf(mRes));                    break;                default:                    break;            }        }    };    private void changeContent() {        new Thread() {            @Override            public void run() {                super.run();                while (true){                    Log.d(TAG, "changeContent  thread id :" + Thread.currentThread().getId());                    try {                        sleep(1000);                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                    if (anInt == 1000) {                        anInt = 0;                    }                    anInt++;                    Message message = new Message();                    message.what = UPDATE_TEXT;                    handler.sendMessage(message);                }    private void getContent(){        new Thread(new Runnable() {            private int aa = 0;            @Override            public void run() {                while (true){                    Log.d(TAG, "getContent  thread id :" + Thread.currentThread().getId());                    try {                        Thread.sleep(500);                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                    if (aa >= 100){                        aa = 0;                    }                    aa+=2;                    Message mMessage = new Message();                    mMessage.what=1;                    mMessage.arg1=aa;                    mHandler.sendMessage(mMessage);                }            }        }).start();    }}

Log 输出结果：
MainActivity: onCreate thread id :1 主线程id
D/MainActivity: changeContent thread id :814
D/MainActivity: changeContent anotherThread thread id :815
D/MainActivity: getContent thread id :816
D/MainActivity: getContent thread id :816
D/MainActivity: changeContent mHandler thread id :815 处理结果在子线程中运行
D/MainActivity: changeContent thread id :814
D/MainActivity: handler thread id :1 处理结果在主线程中运行
D/MainActivity: getContent thread id :816
D/MainActivity: changeContent mHandler thread id :815
D/MainActivity: getContent thread id :816
D/MainActivity: changeContent mHandler thread id :815
D/MainActivity: changeContent thread id :814
D/MainActivity: handler thread id :1

在看源码之前要知道，handler 是用于线程之间传递数据的。
首先看demo 我在主线程和子线程都创建了handler。 仔细观察会发现稍有不同，在子线程创建handler的时候会先执行Looper.prepare(); 如果不写这段这段代码就会提示Can’t create handler inside thread that has not called Looper.prepare()。 主线程中创建的时候并不要啊。为什么呢，那我们就去看看handler的源码，handler的无参构造函数如下：

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;    }

源码中 mLooper = Looper.myLooper(); 去获取了Looper对象 如果为空就会抛出异常。那具体是怎么为空的呢，继续往下看：

    public static @Nullable Looper myLooper() {        return sThreadLocal.get();    }

那这个sThreadLocal 是什么，，，继续看：
static final ThreadLocal sThreadLocal = new ThreadLocal();
ThreadLocal 是什么，进去看看，注释提示该类提供线程的一些值的存储。可以在指定线程内存储数据，存储以后，只有指定线程可以得到存储数据。那就要找在哪儿set数据的。那好肯定是在looper 中的prepare() 中了。代码如下：

    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));    }

new Looper()的代码如下：

    private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed); //创建消息队列        mThread = Thread.currentThread();    }

Looper中消息怎么进入怎么处理，后面再说，先看看主线程中 创建handler的时候好像没有执行Looper.prepare() 方法啊，怎么没有报错。 其实主线程 Activity的入口在这，在Activity 的源码找到ActivityThread 这里面定义的东西很多，包括 handler消息的处理 ApplicationThread 中的sendMessage。看main() 函数 如下：

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());        // 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();        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");    }

是不是跟子线程的创建有点差不多，先看这一句：Looper.prepareMainLooper(); 进Looper看

 public static void prepareMainLooper() {        prepare(false);        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }

prepareMainLooper(),创建当前线程的looper ，它会在应用环境创建的时候就会创建。

sMainThreadHandler = thread.getHandler(); 在 sMainThreadHandler 管理着activity的生命周期，以及UI的一些操作 具体的代码就不贴出来了太长了。
然后代码就运行到了Looper.loop(); 在我这个SDK 版本中的代码如下：（不同的SDK 会有不同）

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();        }    }

可以看到在for (;;) 这一句程序进入了死循环（永真循环）然后 Message msg = queue.next(); 这个就是将消息从消息队列取出的方法。代码在MessageQueue类中 next()，主要实现的功能就是将当前MessageQueue消息队列中的mMessages 出队 ，然后将下一个消息成为mMessages。取出消息后执行 msg.target.dispatchMessage(msg); 这一句，这里的msg.target 是什么呢，这个要看消息发送的时候代码如下（在Handler类中）:

public final boolean sendMessage(Message msg)    {        return sendMessageDelayed(msg, 0);    }    public final boolean sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }   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);    }    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {        msg.target = this; //将当前的handler 给msg的target         if (mAsynchronous) {            msg.setAsynchronous(true);        }        return queue.enqueueMessage(msg, uptimeMillis);    }

好了知道msg.target 是什么 那就看看dispatchMessage(msg)做了什么。

public void dispatchMessage(Message msg) {        if (msg.callback != null) {            handleCallback(msg);        } else {            if (mCallback != null) {                if (mCallback.handleMessage(msg)) {                    return;                }            }            handleMessage(msg);           }    }

在dispatchMessage(Message msg) 中对创建handler时重写的handleMessage()，在这里进行了回调。

最后总结一下：
每个线程最多只能有一个Looper对象，Looper内部有一个消息队列，当一个线程启用Looper后它就变成了一个Looper线程，loop() 方法中会不断从消息队列中取出消息并执行；Handler 线程之间传递数据，实现向MessageQueue中添加消息，在loop()到消息后执行消息，整个过程是异步的，从上面的构造方法中可以看到，handler会关联当前所创建的looper；在一个线程中是可以创建多个Handler的；在Handler中有多种发送消息的方法 比如post(Runnable r ) 但是最终都是调用sendMessageAtTime();
继续深挖 这个线程不是进入了loop()死循环吗，，那这个Handler是怎么运行的，，，这里就涉及到Android中采用Binder作为IPC机制，，，，用于进程间的通信。。。后面在仔细看看吧。就到这里了。

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