handler分析

面试时候经常被问到 handler是什么,说说你的理解...被问到这些心里就发毛,因为不知道从什么深度来说,说的深了,自己也没读过源码,细节的东西说不清反而不好,

说的浅了,这问题也没啥意义了..

从java层的代码来看看,handler和相关的一系列的类到底是怎么回事

先从handlerthread来看,handler也是作用在thread上的,看看handlerthread有啥不一样的

public class HandlerThread extends Thread {    int mPriority;    int mTid = -1;    Looper mLooper;    public HandlerThread(String name) {        super(name);        mPriority = Process.THREAD_PRIORITY_DEFAULT;    }        /**     * Constructs a HandlerThread.     * @param name     * @param priority The priority to run the thread at. The value supplied must be from      * {@link android.os.Process} and not from java.lang.Thread.     */    public HandlerThread(String name, int priority) {        super(name);        mPriority = priority;    }        /**     * Call back method that can be explicitly overridden if needed to execute some     * setup before Looper loops.     */    protected void onLooperPrepared() {    }    @Override    public void run() {        mTid = Process.myTid();        Looper.prepare();        synchronized (this) {            mLooper = Looper.myLooper();            notifyAll();        }//同下面的wait块,创建好了looper,notify一下wait的那边        Process.setThreadPriority(mPriority);        onLooperPrepared();        Looper.loop();        mTid = -1;    }        /**     * This method returns the Looper associated with this thread. If this thread not been started     * or for any reason is isAlive() returns false, this method will return null. If this thread      * has been started, this method will block until the looper has been initialized.       * @return The looper.     */    public Looper getLooper() {        if (!isAlive()) {            return null;        }                // If the thread has been started, wait until the looper has been created.        synchronized (this) {            while (isAlive() && mLooper == null) {                try {                    wait();                } catch (InterruptedException e) {                }            }        }//java里面典型的wait块,wait的时候会把thread moniter放弃,直到looper创建好了,执行notify.        return mLooper;    }    }

我们可以看到handlerthread就是把looper给loop了,具体的干活的都是在looper里面了.不过从这个简单的类里面咱们可以复习一下java多线程的知识,看代码的注释..

看looper这个类

先要prepare,就是要创建looper的类,用到了threadlocal变量,因为是线程独有的.

    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));    }

loop的话 会调用messagequeue.next来得到下一个message,得到的话 会调用message包含的target,也就是handler的dispatchmessage函数.

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            Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            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);            }            msg.recycleUnchecked();        }    }

messagequeue就不贴了,使用epoll的nio技术

看看handler的

    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;    }

看一下构造函数,如果我们在activity也就是主线程使用new handler(),那么这个handler的handlemessage函数就是运行在主线程,就是UI线程

而如果是在其他handlerthread的话,那么就是这个线程的looper.否则的话 就需要注明参数具体是哪个looper了

在看一下发送消息的函数

    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);    }

就是把message往messagequeue里面enqueue一下.

还有经常会问的问题就是handler和asynctask的区别.

aysnctask的默认的executor是串行执行的,也就是说会一个任务一个任务执行,如果一个app里面有N多图片要加载,而每个图片都是运行一个asynctask的话,那么就会非常的慢.

当然也可以使用并行的executor来.当然如果doinbackground之后有需要往UI线程写点东西,那么asynctask当然很方便了

可以看一下这个博客,图很清楚 http://www.cnblogs.com/anee/archive/2012/09/24/2699829.html