android 主线程 进程 android 主线程handler

 当启动一个apk应用程序的时候，Android会开启一个主线程（UI线程），由于主线程是非线程安全，当我们需要在主线程中操作大数据或者联网等这些耗时的操作时，会影响到Android UI的显示并且会出现假死状态，这对用户的体验来说是很不乐观的。因此，我们需要把那些耗时的操作交给另外一个线程来处理，子线程将处理的结果返回给主线程，主线程根据得到的数据作出相应的操作。Handler就实现了这一机制。在Android中实现消息处理的主要有：Handler、Message、Looper、MessageQueue这几个类。我们来看看这几个类内部到底是怎么运转的。

 

 Looper.prepare初始化当前线程，看看都初始化了一些什么。

public class Looper {
    private static final String TAG = "Looper";

   
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();//线程TLS
    private static Looper sMainLooper; 
    final MessageQueue mQueue;//消息队列
    final Thread mThread;//当前线程
    volatile boolean mRun;
    private Printer mLogging;

     /** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static void prepare() {
        prepare(true);
    }
	
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mRun = true;
        mThread = Thread.currentThread();
    }

    private static void prepare(boolean quitAllowed) {
	
        if (sThreadLocal.get() != null) {
		//当前线程如果已经有Looper的时候则创建失败，由此可见，一个线程只能有一个Looper
            throw new RuntimeException("Only one Looper may be created per thread");
        }
		//创建Looper对象并存储到当前线程的TLS
        sThreadLocal.set(new Looper(quitAllowed));
    }
}

 

就这样Looper的初始工作已经完成了，然后就是循环监听消息。

public static void loop() {
        final Looper me = myLooper();//获取当前线程的Looper
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;//获取当前线程的消息队列

        
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

		//这里就是开始循环的地方,这是一个死循环，我记得2.x的版本是while(true),也许是考虑效率问题嘛。for(;;)效率要高些
        for (;;) {
            Message msg = queue.next(); //获取消息
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
			//这里就是把消息交给handler来处理。由于一个线程可能有多个Handler对象发送消息（Message）。
			//所以，每个Message对象保存了自己的Handler。这样Looper才知道到底要交给哪一个Handler来处理。
            msg.target.dispatchMessage(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);
            }
			//释放message资源
            msg.recycle();
        }
    }

接下看，在看看Handler的实现过程。构造函数

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());
            }
        }
		//在这里获取当前线程的Looper
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
		//获取当前线程消息队列
        mQueue = mLooper.mQueue;
        mCallback = callback;//回调函数接口
        mAsynchronous = async;
    }

 

Handler.sendMessage发送数据,最终使用的时enqueueMessage方法

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
	//设置message的target为当前Handler
	//与在Looper.loop() 里面的 msg.target.dispatchMessage(msg) 相对应
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

 

Handler.dispatchMessage接收数据

public void dispatchMessage(Message msg) {
	//msg.calllbak 是一个Runable,如果不为空就执行run方法来处理这个message
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
			//否则调用我们自己现实handler的回调函数
            handleMessage(msg);
        }
    }

 整个消息的处理就到这里了，一些细节的方面就在以后的项目中再来慢慢体会。