Android源码学习 Handler、Looper和MessageQueue

源码使用Android Q

知识储备

知道Handler是干什么的,怎么使用的就可以了,如果不会可以看度娘。

获取Handler的方法

以下方法为Google官方文档提供的说明

构造方法 说明
Handler() 默认构造函数将Handler与当前线程的Looper关联。
Handler(Handler.Callback callback) 构造函数将Handler与当前线程的Looper关联,并接受一个回调接口,在该接口中可以处理消息。
Handler(Looper looper) 使用传入的Looper
Handler(Looper looper, Handler.Callback callback) 结合2、3

Handler源码

构造方法
public Handler() {
        this(null, false);
}

public Handler(@Nullable Callback callback) {
        this(callback, false);
}

public Handler(@NonNull Looper looper) {
        this(looper, null, false);
}

public Handler(@NonNull Looper looper, @Nullable Callback callback) {
        this(looper, callback, false);
}

前两个构造方法调用了以下方法:

    public Handler(@Nullable 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();//说明1
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

构造方法里说明1处获取了Looper,然后创建了队列。

后面两个构造方法则使用了构造时提供的方法

    public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

这个就不多说了。

以上我们可以了解到Handler创建时绑定了Looper。

SendMessage方法

//1
public final boolean sendMessage(@NonNull Message msg) {
        return sendMessageDelayed(msg, 0);
    }
//2
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
//3
public boolean sendMessageAtTime(@NonNull 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);
    }

第一步:调用下一个方法,忽略

第二部:设置默认delayMillis,调用下一个方法

第三步:获取当前的MessageQueue,然后将当前消息入列调用 enqueueMessage方法

    private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        msg.target = this;
        msg.workSourceUid = ThreadLocalWorkSource.getUid();

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

上述代码关键点:

1、msg的target设置为this(当前Handler)

2、设置workSouceUid

3、将当前msg加入MessageQueue里

Post方法

//1 
public final boolean post(@NonNull Runnable r) {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
//2
//第二部之后和sendMessage方法相同

post方法其实和sendMessage一样,只不过是调用了getPostMessage将Runnable包装成Message

private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

Handler的方法到此就差不多了,接下来看看Looper的。

获取Looper的方法

获取方法 说明
getMainLooper() 返回应用程序的主looper,它位于应用程序的主线程中。
prepare() 初始化当前线程的Looper

getMainLooper

    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }

一个App只有一个主线程,也就意味着只有一个主线程Looper。主线程的Looper是App启动的时候创建的。

prepare

//1 
public static void prepare() {
        prepare(true);
    }
//2
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));
    }
//3
private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

通过prepare方法可以发现:

1、一个线程只有一个Looper在2代码里进行了限制

2、Looper在构造方法里创建mQueue,也就是mQueue实在Looper里构建的

loop

    public static void loop() {
        final Looper me = myLooper();//1
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;
        ...

        for (;;) {
            Message msg = queue.next(); // 2
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
            ...
            try {
                msg.target.dispatchMessage(msg);//3
                if (observer != null) {
                    observer.messageDispatched(token, msg);
                }
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } catch (Exception exception) {
                if (observer != null) {
                    observer.dispatchingThrewException(token, msg, exception);
                }
                throw exception;
            } finally {
                ThreadLocalWorkSource.restore(origWorkSource);
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            ...
            msg.recycleUnchecked();
        }
    }

loop方法调用需要注意以下几点

1、当前线程必须有Looper绑定

2、调用MessageQueue的next方法拿到队列中的msg

3、msg.target.dispatchMessage(msg),最终调用handler的dispathchMessage方法

先看看MessageQueue的next方法

Message next() {
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);//1

            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;
                }
        }
    }

这里最重要的就是nativePollOnce(ptr, nextPollTimeoutMillis);这个native方法

1、如果nextPollTimeoutMillis=-1,一直阻塞不会超时。

2、如果nextPollTimeoutMillis=0,不会阻塞,立即返回。

3、如果nextPollTimeoutMillis>0,最长阻塞nextPollTimeoutMillis毫秒(超时),如果期间有程序唤醒会立即返回。

nativePollOnce关键源码
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

Java层的阻塞是通过native层的epoll监听文件描述符的写入事件来实现的

再来看看入栈方法

    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;
    }

以上代码可以得出两个结论

1、MessageQueue是一个单列表

2、有消息插入时会调用nativeWake方法

3、入队是根据时间戳的顺序入队的

nativeWake关键源码

ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));

nativeWake调用write文件写入方法,重点是write(mWakeEventFd, &inc, sizeof(uint64_t)),写入了一个inc,这个时候epoll就能监听到事件,也就被唤醒了

下面看看Handler的dispatchMessage源码

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

调用了我们最初传进来的Msg的Runnable,最终回调了我们自己写的处理方法

获取Message的方法

获取方法 说明
Message.obtain Message提供的默认实例获取方法(使用msg池缓存推荐使用)

Message.obtain()源码

public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

这里维护了一个Message的池,使用一个单链表,我们用过的message进过recycle方法就会进入这个池加以复用,如果池里没有Msg就会new一个

总结

1、创建Handler时绑定了Looper,主线程使用默认的Looper,其他线程需要调用prepare或者使用主线程的Looper

2、调用Handler的sendMessage或者post方法时,设置msg的target为自己,然后将消息加入messageQueue

3、Looper创建后调用loop方法会一直循环,为了防止ANR采用了Linux的epoll监听文件描述符的写入事件来实现loop的阻塞和唤醒,实现无线循环

4、最终Looper会调用msg.target.dispatchMessage(msg)来最终回调Handler处理事件

5、Message获取时采用了回收复用的机制,建议使用obtain获取Message

6、Looper的prepare方法会创建MessageQueue

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