write异步与流量控制 4.0.29.Final

AbstractChannelHandlerContext.class

private void write(Object msg, boolean flush, ChannelPromise promise) {
        AbstractChannelHandlerContext next = this.findContextOutbound();
        EventExecutor executor = next.executor();
        if(executor.inEventLoop()) {
            next.invokeWrite(msg, promise);
            if(flush) {
                next.invokeFlush();
            }
        } else {
            int size = this.channel.estimatorHandle().size(msg);
            if(size > 0) {
                ChannelOutboundBuffer task = this.channel.unsafe().outboundBuffer();
                if(task != null) {
                    task.incrementPendingOutboundBytes((long)size);
                }
            }

            Object task1;
            if(flush) {
                task1 = AbstractChannelHandlerContext.WriteAndFlushTask.newInstance(next, msg, size, promise);
            } else {
                task1 = AbstractChannelHandlerContext.WriteTask.newInstance(next, msg, size, promise);
            }

            safeExecute(executor, (Runnable)task1, promise, msg);
        }

    }

 对于write操作，会先判断下一个outboundhandler的执行线程是否是当前线程

public abstract class AbstractEventExecutor extends AbstractExecutorService implements EventExecutor {
    public AbstractEventExecutor() {
    }

    public EventExecutor next() {
        return this;
    }

    public boolean inEventLoop() {
        return this.inEventLoop(Thread.currentThread());
    }

 如果是，则执行invokewrite

private void invokeWrite(Object msg, ChannelPromise promise) {
        try {
            ((ChannelOutboundHandler)this.handler()).write(this, msg, promise);
        } catch (Throwable var4) {
            notifyOutboundHandlerException(var4, promise);
        }

    }

 如果不是则让handler所在线程执行

private static void safeExecute(EventExecutor executor, Runnable runnable, ChannelPromise promise, Object msg) {
        try {
            executor.execute(runnable);
        } catch (Throwable var9) {
            Throwable cause = var9;

            try {
                promise.setFailure(cause);
            } finally {
                if(msg != null) {
                    ReferenceCountUtil.release(msg);
                }

            }
        }

    }

 execute一个任务

abstract static class AbstractWriteTask extends RecyclableMpscLinkedQueueNode<Runnable> implements Runnable {
。。。。。。

        public final void run() {
            try {
                if(this.size > 0) {
                    ChannelOutboundBuffer buffer = this.ctx.channel.unsafe().outboundBuffer();
                    if(buffer != null) {
                        buffer.decrementPendingOutboundBytes((long)this.size);
                    }
                }

                this.write(this.ctx, this.msg, this.promise);
            } finally {
                this.ctx = null;
                this.msg = null;
                this.promise = null;
            }

        }

对于业务线程write，当时先增加水位，run时减少水位

到底层

DefaultChannelPipeline.HeadHandler

public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
            this.unsafe.write(msg, promise);
        }

        public void flush(ChannelHandlerContext ctx) throws Exception {
            this.unsafe.flush();
        }

AbstractChannel.class

public final void write(Object msg, ChannelPromise promise) {
            ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
            if(outboundBuffer == null) {
                this.safeSetFailure(promise, AbstractChannel.CLOSED_CHANNEL_EXCEPTION);
                ReferenceCountUtil.release(msg);
            } else {
                int size;
                try {
                    msg = AbstractChannel.this.filterOutboundMessage(msg);
                    size = AbstractChannel.this.estimatorHandle().size(msg);
                    if(size < 0) {
                        size = 0;
                    }
                } catch (Throwable var6) {
                    this.safeSetFailure(promise, var6);
                    ReferenceCountUtil.release(msg);
                    return;
                }

                outboundBuffer.addMessage(msg, size, promise);
            }
        }

可以看到，最终调用ChannelOutboundBuffer.addMessage

public void addMessage(Object msg, int size, ChannelPromise promise) {
        ChannelOutboundBuffer.Entry entry = ChannelOutboundBuffer.Entry.newInstance(msg, size, total(msg), promise);
        if(this.tailEntry == null) {
            this.flushedEntry = null;
            this.tailEntry = entry;
        } else {
            ChannelOutboundBuffer.Entry tail = this.tailEntry;
            tail.next = entry;
            this.tailEntry = entry;
        }

        if(this.unflushedEntry == null) {
            this.unflushedEntry = entry;
        }

        this.incrementPendingOutboundBytes((long)size, false);
    }

又增加水位，所以业务线程write会经历增加-减少-增加 3次，而在io线程write只会增加1次