android handler异步处理机制详解（线程局部存储(TLS)知识点）

预备知识：

线程局部存储（TLS）

我们可以从变量作用域的角度来理解什么是线程局部存储：

　　　　函数内部变量 ：作用域为该函数，即每次调用该函数时该变量都会回到初始值。

　　　　类内部变量：其作用域为该类产生的对象，即只要对象没有被销毁，则对象的变量一直保持。

　　　　类内部静态变量：其作用域是整个进程，即只要在该进程中，即该变量的值就一直保持，无论该类构造过多少对象，该变量只有一个赋值，并一直保持。（编译器内部会为静态变量分配单独的内存空间）

　　　　线程局部存储：其作用域为同一个线程，当同一个线程中调用线程局部存储对象时，其值只有一个。 

ThreadLocal

java是用 ThreadLocal 类来存储线程局部存储对象的类，它的构造函数为空，是利用set（）和get（）方法来保存和获取线程局部存储对象的。该对象将所有同一个线程的线程局部存储对象保存在 values对象内（没有具体了解应该是链表结构）。　　

public void set(T value) {
        Thread currentThread = Thread.currentThread();
        Values values = values(currentThread);
        if (values == null) {
            values = initializeValues(currentThread);
        }
        values.put(this, value);
    }

　　set方法内首先根据线程对象获取values对象，若没有则初始化，将要保存的对象put进去。

public T get() {
        // Optimized for the fast path.
        Thread currentThread = Thread.currentThread();
        Values values = values(currentThread);
        if (values != null) {
            Object[] table = values.table;
            int index = hash & values.mask;
            if (this.reference == table[index]) {
                return (T) table[index + 1];
            }
        } else {
            values = initializeValues(currentThread);
        }

        return (T) values.getAfterMiss(this);
    }

　　get方法，首先是利用当前线程来获取存储“线程局部存储”对象的values对象，然后获取存储的所有对象的数组table，然后根据hash和value.mask得到获取的index值，最终获取相应的线程局部存储对象。

好了预备知识介绍完了，下面进入正题：

　　为什么要介绍线程局部存储呢？其实我们activity创建前activity所在的线程就会调用Looper.prepare()方法和Looper.loop（）方法：为该线程创建存储“线程局部存储”对象的对象sThreadLocal，并为activity所在的线程创建一个Looper对象，并将Looper保存为线程局部存储对象的values里面，并启动消息队列运行机制。

// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

 　　当不调用preprare（）时sTreadLocal为空

 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));
    }

　　该方法首先判断是否已经存储了Looper若存储了就抛出异常，因为每个线程只能有一个Looper对象，若没有保存就会保存Looper为线程局部存储对象。

 而上面的new Looper（）创建Looper的方法原型如下：

private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

Looper会创建一个消息队列。

　　下面我们看看创建了activity前调用loop()做的事情：

  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();
        }
    }

　　myLooper()是获取当前线程的Looper对象

　　msg.target.dispatchMessage(msg)，msg.target是获取发送msg消息对象，即handler；handler调用dispatchMessage来处理消息。handler 里面的dispatchMessage()里面实际是利用handleMessage（）处理的，因此，当我们想要异步处理消息的时候只需要构建自己handler，并覆写它们的消息处理的方法。因为Looper中msg会自动获取我们创建的handler，并调用我们自己覆写的处理方法。

　　msg.recycleUnchecked() Message对象里面用数据池保存message对象，因此为了频繁删除创建，当消息处理完后就标记为空闲状态回收重复利用。

　　queue.next() 是从消息队列MessageQueue中取出一条消息：　

Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        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);

            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);
            }

            // 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;
        }
    }

　　nativePollOnce（ptr , nextPollTimeoutMillis）是一个JNI函数，其作用是从消息队列中取出一个消息。MessageQueue内部本身没有保存消息队列，真正的消息队列保存在C代码中。C环境中创建了一个NativeMessageQueue数据对象，也就是nativePollOnce方法的第一个参数ptr，它是个long变量，在构造MessageQueue时候初始化，在C环境中利用它来获得NativeMessageQueue对象.在C环境中，若消息队列中无消息，会导致当前线程挂起（wait）,若队列有消息，则C代码会把当前消息复值给java环境中的mMessages变量。

　　synchronized (this) 同步代码块，

　　　　主要是判断执行消息的时间是否到了（有的消息本身指定了处理的时刻，消息是以队列形式存储，若读取没到执行时刻就执行下一个消息，下次读取仍然从队列头读取，每次都会判断是否到了执行时间），若到了就返回消息，并将mMessages置为空；若时间没有到则尝试读取下一个消息。

　　　　若mMessage为空，则队列里没有消息，调用mPendingIdleHandlers里的空闲回调函数。

MessageQueue是用enqueueMessage函数添加消息的:

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;
    }

　　该方法先将参数msg赋值给mMessage,然后利用nativeWake写入消息到C环境，若消息线程处于wait状态，则唤醒。