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Handler源码解析

时间:2023-02-13 11:15:30浏览次数:57  
标签:null MessageQueue Handler Looper msg Message 解析 源码

Handler源码解析

一、基本原理回顾

在android开发中,经常会在子线程中进行一些操作,当操作完毕后会通过handler发送一些数据给主线程,通知主线程做相应的操作。

探索其背后的原理:子线程 handler 主线程 其实构成了线程模型中的经典问题 生产者-消费者模型。 生产者-消费者模型:生产者和消费者在同一时间段内共用同一个存储空间,生产者往存储空间中添加数据,消费者从存储空间中取走数据。

好处: 保证数据生产消费的顺序(通过MessageQueue,先进先出) - 不管是生产者(子线程)还是消费者(主线程)
都只依赖缓冲区(handler),生产者消费者之间不会相互持有,使他们之间没有任何耦合

二、Handler机制的相关类

  • Hanlder:发送和接收消息
  • Thread(ThreadLocal):存储Looper对象的
  • Looper:用于轮询消息队列,一个线程只能有一个Looper
  • Message: 消息实体
  • MessageQueue: 消息队列用于存储消息和管理消息

1、Thread(ThreadLocal)

Handler机制用到的跟Thread相关的,而根本原因是Handler必须和对应的Looper绑定,而Looper的创建和保存是跟Thread一一对应的,也就是说每个线程都可以创建唯一一个且互不相关的Looper,这是通过ThreadLocal来实现的,也就是说是用ThreadLocal对象来存储Looper对象的,从而达到线程隔离的目的。

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
 
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));
}

2、Handler

Handler()
 
Handler(Callback callback)
 
Handler(Looper looper)
 
Handler(Looper looper, Callback callback)
 
Handler(boolean async)
 
Handler(Callback callback, boolean async)
 
Handler(Looper looper, Callback callback, boolean async)

2.1 创建Handler大体上有两种方式:

一种是不传Looper

这种就需要在创建Handler前,预先调用Looper.prepare来创建当前线程的默认Looper,否则会报错。

最常见的创建handler

Handler handler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
    }
};

在内部调用this(null, false);

public Handler(@Nullable Callback callback, boolean async) {
    ......
    mLooper = Looper.myLooper();;//获取Looper,**注意不是创建Looper**!
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;//创建消息队列MessageQueue
    mCallback = callback;//初始化了回调接口
    mAsynchronous = async;
}

Looper.myLooper()

//这是Handler中定义的ThreadLocal ThreadLocal主要解多线程并发的问题
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

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

sThreadLocal.get() will return null unless you’ve called prepare(). 这句话告诉我们get可能返回null除非先调用prepare()方法创建Looper。下面Looper里会介绍。

一种是传入指定的Looper

这种就是Handler和指定的Looper进行绑定,也就是说Handler其实是可以跟任意线程进行绑定的,不局限于在创建Handler所在的线程里。

2.2 async参数

这里Handler有个async参数,通过这个参数表明通过这个Handler发送的消息全都是异步消息,因为在把消息压入队列的时候,会把这个标志设置到message里.这个标志是全局的,也就是说通过构造Handler函数传入的async参数,就确定了通过这个Handler发送的消息都是异步消息,默认是false,即都是同步消息。至于这个异步消息有什么特殊的用途,我们在后面讲了屏障消息后,再联系起来讲。

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

2.3 callback参数

这个回调参数是消息被分发之后的一种回调,最终是在msg调用HandlerdispatchMessage时,根据实际情况进行回调:

// Looper.loop()中会调用
msg.target.dispatchMessage(msg);

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

3、Looper

用于为线程运行消息循环的类。默认线程没有与它们相关联的Looper;所以要在运行循环的线程中调用prepare(),然后调用loop()让它循环处理消息,直到循环停止。

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));
}
     
public static void loop() {
    ...
 
    for (;;) {
    ...
    }
     
    ...
}
 
class LooperThread extends Thread {
    public Handler mHandler;
    
    public void run() {
        Looper.prepare();  
         
        mHandler = new Handler() { 
            public void handleMessage(Message msg) {
                 
                Message msg=Message.obtain();
            }
        };
        Looper.loop(); 
    }
}

既然在使用Looper前,必须调用prepare创建Looper,为什么我们平常在主线程里没有看到调用prepare呢?这是因为Android主线程创建的时候,在ActivityThread的入口main方法里就已经默认创建了Looper

public static void main(String[] args) {
    ...
    Looper.prepareMainLooper();//初始化Looper以及MessageQueue
    ...
    Looper.loop();// 开始轮询操作
    ...
}

public static void prepareMainLooper() {
    prepare(false);//消息队列不可以quit
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

prepare有两个重载的方法,主要看prepare(boolean quitAllowed) quitAllowed的作用是在创建MessageQueue时标识消息队列是否可以销毁, 主线程不可被销毁 下面有介绍

public static void prepare() {
    prepare(true);//消息队列可以quit
}

private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {//不为空表示当前线程已经创建了Looper
        throw new RuntimeException("Only one Looper may be created per thread");
        //每个线程只能创建一个Looper
    }
    sThreadLocal.set(new Looper(quitAllowed));//创建Looper并设置给sThreadLocal,这样get的 时候就不会为null了
}

4、MessageQueue

MessageQueue是一个消息队列,HandlerMessage发送到消息队列中,消息队列会按照一定的规则取出要执行的MessageMessage并不是直接加到MessageQueue的,而是通过Handler对象和Looper关联到一起。

4.1、创建MessageQueue

创建MessageQueue以及Looper与当前线程的绑定

// Looper.java
private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);//创建了MessageQueue
    mThread = Thread.currentThread();//当前线程的绑定
}

MessageQueue的构造方法

// MessageQueue.java
MessageQueue(boolean quitAllowed) {
//mQuitAllowed决定队列是否可以销毁 主线程的队列不可以被销毁需要传入false, 在MessageQueue的quit()方法 就不贴源码了
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
}

4.2、Looper.loop()

同时是在main方法中Looper.prepareMainLooper()Looper.loop(); 开始轮询

// Looper.java
public static void loop() {
    final Looper me = myLooper();//里面调用了sThreadLocal.get()获得刚才创建的Looper对象
    if (me == null) {//如果Looper为空则会抛出异常
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;

    ......

    for (;;) {//这是一个死循环,从消息队列不断的取消息
        Message msg = queue.next(); // might block
        if (msg == null) {
            //由于刚创建MessageQueue就开始轮询,队列里是没有消息的,等到Handler sendMessage enqueueMessage后 //队列里才有消息
            // 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);
        }

        ......
      
        try {
            msg.target.dispatchMessage(msg);//msg.target就是绑定的Handler,详见后面Message的部分,Handler开始
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        ......
        msg.recycleUnchecked();
    }
}

5、Message

MessageQueue里的message是按时间排序的,越早加入队列的消息放在队列头部,优先执行,这个时间就是sendMessage的时候传过来的,默认是用的当前系统从启动到现在的非休眠的时间SystemClock.uptimeMillis()

5.1、创建Message

可以直接new Message但是有更好的方式Message.obtain。因为可以检查是否有可以复用的Message,用过复用避免过多的创建、销毁Message对象达到优化内存和性能的目地

// Message.java
public static Message obtain(Handler h) {
    Message m = obtain();//调用重载的obtain方法
    m.target = h;//并绑定的创建Message对象的handler

    return m;
}

public static Message obtain() {
    synchronized (sPoolSync) {//sPoolSync是一个Object对象,用来同步保证线程安全
        if (sPool != null) {//sPool是就是handler dispatchMessage 后 通过recycleUnchecked 回收用以复用的Message
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

5.2、Message和Handler的绑定

创建Message的时候可以通过Message.obtain(Handler h)这个构造方法绑定。当然可以在Handler中的enqueueMessage()也绑定了,所有发送Message的方法都会调用此方法入队,所以在创建Message的时候是可以不绑定的

// Handler.java
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this; //绑定
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

三、Handler发送消息

Handler发送消息的重载方法很多,但是主要只有2种。
sendMessage(Message) sendMessage方法通过一系列重载方法的调用,sendMessage调用sendMessageDelayed,继续调用sendMessageAtTime,继续调用enqueueMessage,继续调用messageQueueenqueueMessage方法,将消息保存在了消息队列中,而最终由Looper取出,交给HandlerdispatchMessage进行处理

// Handler.java
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;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

// MessageQueue.java
boolean enqueueMessage(Message msg, long when) {
    ......
    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;
}

// Looper.java
public static void loop() {

        ......
      
        try {
            msg.target.dispatchMessage(msg);//msg.target就是绑定的Handler,详见后面Message的部 分,Handler开始
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        
        ......
    }
}

我们可以看到在dispatchMessage方法中,messagecallback是一个Runnable对象,如果callback不为空,则直接调用callbackrun方法,否则判断mCallback是否为空,mCallbackHandler构造方法中初始化,在主线程通直接通过无参的构造方法new出来的为null,所以会直接执行后面的handleMessage()方法。

// Handler.java
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {//callback在message的构造方法中初始化或者使用 handler.post(Runnable)时候才不为空
        handleCallback(msg);
    } else {
        if (mCallback != null) {//mCallback是一个Callback对象,通过无参的构造方法创建出来的handler, 该属性为null,此段不执行
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

private static void handleCallback(Message message) {
    message.callback.run();
}

四、线程同步问题

Handler是用于线程间通信的,但是它产生的根本并不只是用于UI处理,而更多的是handler是整个app通信的框架,大家可以在ActivityThread里面感受到,整个App都是用它来进行线程间的协调。Handler既然这么重要,那么它的线程安全就至关重要了,那么它是如何保证自己的线程安全呢?

Handler机制里面最主要的类MessageQueue,这个类就是所有消息的存储仓库,在这个仓库中,我们如何的管理好消息,这个就是一个关键点了。消息管理就2点:1)消息入库(enqueueMessage),2)消息出库(next),所以这两个接口是确保线程安全的主要档口。

MessageQueue.java源码如下:

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.");
    }

    // 锁开始的地方
    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);
        }
    }// 锁结束的地方
    return true;
}

synchronized锁是一个内置锁,也就是由系统控制锁的lock unlock时机的。

synchronized (this)

这个锁,说明的是对所有调用同一个MessageQueue对象的线程来说,他们都是互斥的,然而,在我们的Handler里面,一个线程是对应着一个唯一的Looper对象,而Looper中又只有一个唯一的MessageQueue(这个在上文中也有介绍)。所以,我们主线程就只有一个MessageQueue对象,也就是说,所有的子线程向主线程发送消息的时候,主线程一次都只会处理一个消息,其他的都需要等待,那么这个时候消息队列就不会出现混乱。

另外,在看next函数

Message next() {
    ......
    
    for (;;) {
        
        ......
        
        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }

            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }

            // If first time idle, then get the number of idlers to run.
            // Idle handles only run if the queue is empty or if the first message
            // in the queue (possibly a barrier) is due to be handled in the future.
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }

            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }}//synchronized 结束之处

        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = null; // release the reference to the handler

            boolean keep = false;
            try {
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }

            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }

        // Reset the idle handler count to 0 so we do not run them again.
        pendingIdleHandlerCount = 0;

        // While calling an idle handler, a new message could have been delivered
        // so go back and look again for a pending message without waiting.
        nextPollTimeoutMillis = 0;
    }
}

next函数很多同学会有疑问:我从线程里面取消息,而且每次都是队列的头部取,那么它加锁是不是没有意义呢?答案是否定的,我们必须要在next里面加锁,因为,这样由于synchronized(this)作用范围是所有this正在访问的代码块都会有保护作用,也就是它可以保证next函数enqueueMessage函数能够实现互斥。这样才能真正的保证多线程访问的时候messagequeue的有序进行。

小结: 这个地方是面试官经常问的点,而且他们会基于这个点来拓展问你多线程,所以,这个地方请大家重视。

源码深度解析 Handler 机制及应用

标签:null,MessageQueue,Handler,Looper,msg,Message,解析,源码
From: https://www.cnblogs.com/zuojie/p/16913249.html

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