netty源码深入研究（从客户端入手）第三篇（详解写消息的管道处理流程）

假如我们的netty已经连接上了服务器，那么怎么写数据呢，如果从来没用过netty的话，该怎么办呢，有人会说百度啊，谷歌啊，其实最直接有效的方法就是看文档，不管做什么开发，都要熟读人家的文档，老是百度得来的答案，虽然快速，但是对自己后期成长不利。

netty的引导文档

从文档中找到

Channel ch = ...;ch.writeAndFlush(message);

这些方法，从中推测，最后写也是通过通道，ok跟进NioSocketChannel里面，写方法没有在NioSocketChannel中实现，而在其父类AbstractChannel中实现

 @Override    public ChannelFuture writeAndFlush(Object msg) {        return pipeline.writeAndFlush(msg);    }

这里可以看到，最终写方法是管道实现的，那么o了，管道默认为DefaultChannelPipeline进入其方法

 public final ChannelFuture writeAndFlush(Object msg, ChannelPromise promise) {        return tail.writeAndFlush(msg, promise);    }

look最终又进了我们的队列，从tail开始分发输入消息，tail是上一篇所讲的，放在管子最后的管道

protected DefaultChannelPipeline(Channel channel) {        this.channel = ObjectUtil.checkNotNull(channel, "channel");        succeededFuture = new SucceededChannelFuture(channel, null);        voidPromise =  new VoidChannelPromise(channel, true);        tail = new TailContext(this);        head = new HeadContext(this);       //head在前        head.next = tail;       //tail在尾          tail.prev = head;    }

在tail的父类AbstractChannelHandlerContext（既管道的包装类）找到writeAndFlusht的实现方法

 @Override    public ChannelFuture writeAndFlush(Object msg) {        return writeAndFlush(msg, newPromise());    }

其中newPromise（）创建一个new DefaultChannelPromise(channel(), executor())

  @Override    public ChannelFuture writeAndFlush(Object msg, ChannelPromise promise) {    //判断输入的消息不为空        if (msg == null) {            throw new NullPointerException("msg");        }         //检查ChannelPromise是否执行完或者取消        if (isNotValidPromise(promise, true)) {            ReferenceCountUtil.release(msg);            // cancelled            return promise;        }       //开始写消息        write(msg, true, promise);        return promise;    }

跟着源码接着进入写方法，我们只关心怎么写的，其他代码暂放一边

 private void write(Object msg, boolean flush, ChannelPromise promise) {    //向读消息一样，找到匹配的管道        AbstractChannelHandlerContext next = findContextOutbound();        //是否包装msg        final Object m = pipeline.touch(msg, next);        EventExecutor executor = next.executor();        if (executor.inEventLoop()) {            if (flush) {            //管道开始刷新缓冲区写                next.invokeWriteAndFlush(m, promise);            } else {            //写入缓冲区                next.invokeWrite(m, promise);            }        } else {        //加入写队列写消息            AbstractWriteTask task;            if (flush) {                task = WriteAndFlushTask.newInstance(next, m, promise);            }  else {                task = WriteTask.newInstance(next, m, promise);            }            safeExecute(executor, task, promise, m);        }    }

再次看一下findContextOutbound的方法

写方法搜寻管道

   private AbstractChannelHandlerContext findContextOutbound() {        AbstractChannelHandlerContext ctx = this;        do {            ctx = ctx.prev;        } while (!ctx.outbound);        return ctx;    }

读方法搜寻管道

 private AbstractChannelHandlerContext findContextInbound() {        AbstractChannelHandlerContext ctx = this;        do {            ctx = ctx.next;        } while (!ctx.inbound);        return ctx;    }

和读数据的顺序完全相反，是从队尾开始读取管道,如果管道是写管道就返回该管道，即编码器必须继承ChannelOutboundHandlerAdapter这个父类。

继续走总路线， EventExecutor executor = next.executor()返回的就是NioEventLoop的类，第一篇第二篇都有介绍，executor.inEventLoop()判断当前线程是不是任务线程，注册启动的时候会启动任务线程，如果是直接发消息，如果不是则加入队列。

 public void execute(Runnable task) {        if (task == null) {            throw new NullPointerException("task");        }        boolean inEventLoop = inEventLoop();        if (inEventLoop) {            addTask(task);        } else {            startThread();            addTask(task);            if (isShutdown() && removeTask(task)) {                reject();            }        }        if (!addTaskWakesUp && wakesUpForTask(task)) {            wakeup(inEventLoop);        }    }

将写任务加入队列，任务线程一直在读取队列执行任务。

接下来我们直接跟进任务，执行逻辑肯定会在实现Runnable接口中实现，即WriteAndFlushTask类中实现，找到run方法

  public final void run() {            try {            //从NioSocketChannelUnsafe中获取  ChannelOutboundBuffer outboundBuffer = new ChannelOutboundBuffer(AbstractChannel.this);                ChannelOutboundBuffer buffer = ctx.channel().unsafe().outboundBuffer();                // Check for null as it may be set to null if the channel is closed already                //记录消息的大小                if (ESTIMATE_TASK_SIZE_ON_SUBMIT && buffer != null) {                    buffer.decrementPendingOutboundBytes(size);                }                //在子线程中写                write(ctx, msg, promise);            } finally {                // Set to null so the GC can collect them directly                ctx = null;                msg = null;                promise = null;                handle.recycle(this);            }        }

接着进入write(ctx, msg, promise);

 protected void write(AbstractChannelHandlerContext ctx, Object msg, ChannelPromise promise) {            ctx.invokeWrite(msg, promise);        }

look最终的写还是调用了我们的管道封装类去write，重新进入AbstractChannelHandlerContext 

    private void invokeWrite(Object msg, ChannelPromise promise) {        if (invokeHandler()) {            invokeWrite0(msg, promise);        } else {            write(msg, promise);        }    }

和上一篇的读取一样，先判断是否进入管道方法执行，如果是进入管道执行，如果不是，那么继续下一个管道写入处理，直到没有满足的管道为止。

  private void invokeWrite0(Object msg, ChannelPromise promise) {        try {            ((ChannelOutboundHandler) handler()).write(this, msg, promise);        } catch (Throwable t) {            notifyOutboundHandlerException(t, promise);        }    }

这个方法真正实现了管道的写方法，如自定义一个编码器：

public class Encoder extends ChannelOutboundHandlerAdapter {@Overridepublic void write(ChannelHandlerContext ctx, Object msg,ChannelPromise promise) {if (msg instanceof ByteBuf) {ByteBuf bytebuffer = (ByteBuf) msg;byte[] data = bytebuffer.array();ctx.writeAndFlush(Unpooled.copiedBuffer(byteMerger(data.length, data)));}}

我们在这个方法里又调用了ctx.writeAndFlush(Unpooled.copiedBuffer(byteMerger(data.length, data)));，那么现在刚好形成一个队列管道的小循环。

不知道看客吗明白没有，好详细的介绍一下invokeHandler()

  private boolean invokeHandler() {        // Store in local variable to reduce volatile reads.        int handlerState = this.handlerState;        return handlerState == ADD_COMPLETE || (!ordered && handlerState == ADD_PENDING);    }

handlerState是什么鬼，初始值是多少，看样子是枚举啊，首先我找到了

  private volatile int handlerState = INIT;

so，一个初始值有木有，INIT=0，

那么第一次handler（）这个判断就会返回false，而导致程序走 invokeWrite0(msg, promise);即走客户端自己调用的编码器，然后将信息再写回父类，继又开始重新调用

  public ChannelFuture writeAndFlush(Object msg, ChannelPromise promise) {    //判断输入的消息不为空        if (msg == null) {            throw new NullPointerException("msg");        }         //检查ChannelPromise是否执行完或者取消        if (isNotValidPromise(promise, true)) {            ReferenceCountUtil.release(msg);            // cancelled            return promise;        }       //开始写消息        write(msg, true, promise);        return promise;    }

这个方法，这就形成了一个微循环，首先我将消息发送给netty框架，然后netty去找我设置的编码器，把我发送的数据换成netty控制器要传的数据，即字节，最终通过socket通道将消息发送给服务端。

由于管道的头为HeadContext，并且  

HeadContext(DefaultChannelPipeline pipeline) {            super(pipeline, null, HEAD_NAME, false, true);            unsafe = pipeline.channel().unsafe();            setAddComplete();        }

代码outbound属性为true，所以最终会调用HeadContext，并且它实现了setAddComplete方法，所以中的数据是由HeadContext类负责写到通道中，继续跟到代码

覆盖了父类的方法，最后又让NioSocketChannelUnsafe去真正的写入数据

public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {            unsafe. public final void write(Object msg, ChannelPromise promise) {            assertEventLoop();            ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;            if (outboundBuffer == null) {                // If the outboundBuffer is null we know the channel was closed and so                // need to fail the future right away. If it is not null the handling of the rest                // will be done in flush0()                // See https://github.com/netty/netty/issues/2362                safeSetFailure(promise, WRITE_CLOSED_CHANNEL_EXCEPTION);                // release message now to prevent resource-leak                ReferenceCountUtil.release(msg);                return;            }            int size;            try {                msg = filterOutboundMessage(msg);                size = pipeline.estimatorHandle().size(msg);                if (size < 0) {                    size = 0;                }            } catch (Throwable t) {                safeSetFailure(promise, t);                ReferenceCountUtil.release(msg);                return;            }            outboundBuffer.addMessage(msg, size, promise);        }(msg, promise); }

其中filterOutboundMessage方法判断数据是否合法

 protected final Object filterOutboundMessage(Object msg) {    //发送的数据最终必须被转化为ByteBuf或FileRegion，否则抛异常        if (msg instanceof ByteBuf) {            ByteBuf buf = (ByteBuf) msg;            if (buf.isDirect()) {                return msg;            }            return newDirectBuffer(buf);        }        if (msg instanceof FileRegion) {            return msg;        }        throw new UnsupportedOperationException(                "unsupported message type: " + StringUtil.simpleClassName(msg) + EXPECTED_TYPES);    }

然后判断消息实体是有效的，即长度大于0，最后将消息加入outboundBuffer.addMessage(msg,size, promise);，ChannelOutboundBuffer维护了一个消息队列：

public void addMessage(Object msg, int size, ChannelPromise promise) {        Entry entry = Entry.newInstance(msg, size, total(msg), promise);        if (tailEntry == null) {            flushedEntry = null;            tailEntry = entry;        } else {            Entry tail = tailEntry;            tail.next = entry;            tailEntry = entry;        }        if (unflushedEntry == null) {            unflushedEntry = entry;        }        // increment pending bytes after adding message to the unflushed arrays.        // See https://github.com/netty/netty/issues/1619        incrementPendingOutboundBytes(entry.pendingSize, false);    }

既然消息先被保存到队列中，那么最后执行写操作的一定是flush了，跟进

protected void flush0() {            if (inFlush0) {                // Avoid re-entrance                return;            }            final ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;            if (outboundBuffer == null || outboundBuffer.isEmpty()) {                return;            }            inFlush0 = true;            // Mark all pending write requests as failure if the channel is inactive.            if (!isActive()) {                try {                    if (isOpen()) {                        outboundBuffer.failFlushed(FLUSH0_NOT_YET_CONNECTED_EXCEPTION, true);                    } else {                        // Do not trigger channelWritabilityChanged because the channel is closed already.                        outboundBuffer.failFlushed(FLUSH0_CLOSED_CHANNEL_EXCEPTION, false);                    }                } finally {                    inFlush0 = false;                }                return;            }            try {            //真正实现写方法                doWrite(outboundBuffer);            } catch (Throwable t) {                if (t instanceof IOException && config().isAutoClose()) {                    /**                     * Just call {@link #close(ChannelPromise, Throwable, boolean)} here which will take care of                     * failing all flushed messages and also ensure the actual close of the underlying transport                     * will happen before the promises are notified.                     *                     * This is needed as otherwise {@link #isActive()} , {@link #isOpen()} and {@link #isWritable()}                     * may still return {@code true} even if the channel should be closed as result of the exception.                     */                    close(voidPromise(), t, FLUSH0_CLOSED_CHANNEL_EXCEPTION, false);                } else {                    outboundBuffer.failFlushed(t, true);                }            } finally {                inFlush0 = false;            }        }

进入注释的那个方法doWrite

 int writeSpinCount = -1;        boolean setOpWrite = false;        for (;;) {            Object msg = in.current();            if (msg == null) {                // Wrote all messages.                clearOpWrite();                // Directly return here so incompleteWrite(...) is not called.                return;            }            if (msg instanceof ByteBuf) {                ByteBuf buf = (ByteBuf) msg;                int readableBytes = buf.readableBytes();                if (readableBytes == 0) {                    in.remove();                    continue;                }                boolean done = false;                long flushedAmount = 0;                if (writeSpinCount == -1) {                    writeSpinCount = config().getWriteSpinCount();                }                for (int i = writeSpinCount - 1; i >= 0; i --) {                    int localFlushedAmount = doWriteBytes(buf);                    if (localFlushedAmount == 0) {                        setOpWrite = true;                        break;                    }                    flushedAmount += localFlushedAmount;                    if (!buf.isReadable()) {                        done = true;                        break;                    }                }                in.progress(flushedAmount);                if (done) {                    in.remove();                } else {                    // Break the loop and so incompleteWrite(...) is called.                    break;                }            } else if (msg instanceof FileRegion) {                FileRegion region = (FileRegion) msg;                boolean done = region.transferred() >= region.count();                if (!done) {                    long flushedAmount = 0;                    if (writeSpinCount == -1) {                        writeSpinCount = config().getWriteSpinCount();                    }                    for (int i = writeSpinCount - 1; i >= 0; i--) {                        long localFlushedAmount = doWriteFileRegion(region);                        if (localFlushedAmount == 0) {                            setOpWrite = true;                            break;                        }                        flushedAmount += localFlushedAmount;                        if (region.transferred() >= region.count()) {                            done = true;                            break;                        }                    }                    in.progress(flushedAmount);                }                if (done) {                    in.remove();                } else {                    // Break the loop and so incompleteWrite(...) is called.                    break;                }            } else {                // Should not reach here.                throw new Error();            }        }        incompleteWrite(setOpWrite);    }

循环获取当前的消息，为空退出循环，不为空开始写消息

 protected int doWriteBytes(ByteBuf buf) throws Exception {        final int expectedWrittenBytes = buf.readableBytes();        return buf.readBytes(javaChannel(), expectedWrittenBytes);    }

最后调用这个方法，将消息通过socket写入给服务器，到此写流程就完结了。