Netty源码分析之处理新连接

Netty服务端处理新连接的流程：

1.检测新连接

2.基于NioServerSocketChannel创建客户端的NioSocketChannel

3.分配客户端channel的线程，注册线程所对应的selector

4.向selector注册读事件

新连接检测

　　服务端在创建完服务端的NioServerSocketChannel之后，绑定完端口号之后，会注册accept事件。当有新连接进入的时候，会触发accpet事件。之前博客有分析过EventLoop的thread的run方法会循环select检测是否有新的IO事件，如果检测到有IO事件，就通过processSelectedKey来处理对应的IO事件，这里的IO事件是accept,就会调用channel内部聚合的UnSafe类的read()方法。

　　这里循环调用doReadMessage()方法的条件是是否自动读，读取的连接数是否小于最大连接数，服务端channel默认一次最多读取16个新连接。

　　当没有超过最大连接数，并且是自动读的状态时候，就会循环调用doReadMessage，直到没有读到新连接，跳出while循环，

public void read() {
    assert AbstractNioMessageChannel.this.eventLoop().inEventLoop();

    ChannelConfig config = AbstractNioMessageChannel.this.config();
    if(!config.isAutoRead() && !AbstractNioMessageChannel.this.isReadPending()) {
        this.removeReadOp();
    } else {
        int maxMessagesPerRead = config.getMaxMessagesPerRead();
        ChannelPipeline pipeline = AbstractNioMessageChannel.this.pipeline();
        boolean closed = false;
        Throwable exception = null;

        try {
            int size;
            try {
                do {
                    size = AbstractNioMessageChannel.this.doReadMessages(this.readBuf);
                    if(size == 0) {
                        break;
                    }

                    if(size < 0) {
                        closed = true;
                        break;
                    }
                } while(config.isAutoRead() && this.readBuf.size() < maxMessagesPerRead);
            } catch (Throwable var11) {
                exception = var11;
            }

            AbstractNioMessageChannel.this.setReadPending(false);
            size = this.readBuf.size();
            int i = 0;

            while(true) {
                if(i >= size) {
                    this.readBuf.clear();
                    pipeline.fireChannelReadComplete();
                    if(exception != null) {
                        if(exception instanceof IOException && !(exception instanceof PortUnreachableException)) {
                            closed = !(AbstractNioMessageChannel.this instanceof ServerChannel);
                        }

                        pipeline.fireExceptionCaught(exception);
                    }

                    if(closed && AbstractNioMessageChannel.this.isOpen()) {
                        this.close(this.voidPromise());
                    }
                    break;
                }

                pipeline.fireChannelRead(this.readBuf.get(i));
                ++i;
            }
        } finally {
            if(!config.isAutoRead() && !AbstractNioMessageChannel.this.isReadPending()) {
                this.removeReadOp();
            }

        }

    }
}

　　

创建NioSocketChannel

　　这里read()方法是通过循环调用NioServerSocket的doReadMessage(byteBuf)方法进行实现channel的读取新连接。而doReadMessage是通过java nio的channel的accept获取当前新连接的channel，这里获取的channel也是java nio中的channel，然后将这个channel封装成NioSocketChannel，将NioServerSocketChannel和javaChannel都作为参数构造NioSocketChannel，放到buf中去，返回1，表示已经读取一条连接。

protected int doReadMessages(List<Object> buf) throws Exception {
    SocketChannel ch = this.javaChannel().accept();

    try {
        if(ch != null) {
            buf.add(new NioSocketChannel(this, ch));
            return 1;
        }
    } catch (Throwable var6) {
        logger.warn("Failed to create a new channel from an accepted socket.", var6);

        try {
            ch.close();
        } catch (Throwable var5) {
            logger.warn("Failed to close a socket.", var5);
        }
    }

    return 0;
}

　　NioSocketChannel的构造函数。

//配置Config类
public NioSocketChannel(Channel parent, java.nio.channels.SocketChannel socket) {
    super(parent, socket);
    this.config = new NioSocketChannel.NioSocketChannelConfig(this, socket.socket(), null);
}

protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
    super(parent, ch, 1);
}

//保存channel感兴趣的读事件，并将channel设置为非阻塞的
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    super(parent);
    this.ch = ch;
    this.readInterestOp = readInterestOp;

    try {
        ch.configureBlocking(false);
    } catch (IOException var7) {
        try {
            ch.close();
        } catch (IOException var6) {
            if(logger.isWarnEnabled()) {
                logger.warn("Failed to close a partially initialized socket.", var6);
            }
        }

        throw new ChannelException("Failed to enter non-blocking mode.", var7);
    }
}

　　这里配置channel的Config类使用了setTcpNoDelay(true)，这里禁止了Nagle算法，Nagle算法的目的是让小的数据包尽量集合成大的数据包发送出去，Netty为了使数据能够及时发出去，禁止了Nagle算法。

public DefaultSocketChannelConfig(io.netty.channel.socket.SocketChannel channel, Socket javaSocket) {
    super(channel);
    if(javaSocket == null) {
        throw new NullPointerException("javaSocket");
    } else {
        this.javaSocket = javaSocket;
        if(PlatformDependent.canEnableTcpNoDelayByDefault()) {
            try {
                this.setTcpNoDelay(true);
            } catch (Exception var4) {
                ;
            }
        }

    }
}

public SocketChannelConfig setTcpNoDelay(boolean tcpNoDelay) {
    try {
        this.javaSocket.setTcpNoDelay(tcpNoDelay);
        return this;
    } catch (SocketException var3) {
        throw new ChannelException(var3);
    }
}

　　

新连接NioEventLoop的分配和selector的注册

　　在读取完新连接之后，会调用fireChannelRead方法，而服务端的NioServerSocketChannel在初始化阶段，在上面的pipeline添加了连接处理器ServerBootstrap.ServerBootstrapAcceptor，read事件会从head传送到serverBootstrapAcceptor，serverBootstrapAcceptor也是一个ChannelHandler，它会对新连接进行处理。

处理流程：

　　1.设置客户端channel的childHandler

　　添加channelHandler，这里的channelHandler一般是一个ChannelInitializer，他可以获取channel的pipeline，并且在上面添加一系列的Handler，最后再将ChannelInitializer这个Handler删除。

　　2.设置options和attrs

　　options是底层tcp读写的相关参数，attrs可以在客户端channel上面绑定一些属性。这里的options和attrs都是用户通过代码设置的。比如

 bootstrap.childOption(ChannelOption.SO_KEEPALIVE, true)

　　设置的这些都会保存到ServerBootstrap这个类，然后在initChannel的时候会将这些参数都传入，构造一个ServerBootstrapAcceptor，这样当连接器接受到新的连接之后，新建子channel，就会带有这些属性。

　　3.选择NioEventLoop，并且注册selector

public void channelRead(ChannelHandlerContext ctx, Object msg) {
    final Channel child = (Channel)msg;
    //添加ChannelHandler
　　 child.pipeline().addLast(new ChannelHandler[]{this.childHandler});
    Map.Entry[] t = this.childOptions;
    int len$ = t.length;

    int i$;
    Map.Entry e;
    for(i$ = 0; i$ < len$; ++i$) {
        e = t[i$];

        try {
            if(!child.config().setOption((ChannelOption)e.getKey(), e.getValue())) {
                ServerBootstrap.logger.warn("Unknown channel option: " + e);
            }
        } catch (Throwable var10) {
            ServerBootstrap.logger.warn("Failed to set a channel option: " + child, var10);
        }
    }

    t = this.childAttrs;
    len$ = t.length;

    for(i$ = 0; i$ < len$; ++i$) {
        e = t[i$];
        child.attr((AttributeKey)e.getKey()).set(e.getValue());
    }

    try {
        this.childGroup.register(child).addListener(new ChannelFutureListener() {
            public void operationComplete(ChannelFuture future) throws Exception {
                if(!future.isSuccess()) {
                    ServerBootstrap.ServerBootstrapAcceptor.forceClose(child, future.cause());
                }

            }
        });
    } catch (Throwable var9) {
        forceClose(child, var9);
    }

}

 　　这里注册是使用用户传进来的workerGroup线程池，使用register方法完成注册。

public ChannelFuture register(Channel channel) {
    return this.next().register(channel);
}

　　这里的next()函数返回一个NioEventLoop,相当于从线程池里面挑选一个线程与这个channel进行绑定。最后通过层层调用，还是调用了java nio中channel的register方法，这时注册的时候，不关心任何事件。

public ChannelFuture register(Channel channel, ChannelPromise promise) {
    if(channel == null) {
        throw new NullPointerException("channel");
    } else if(promise == null) {
        throw new NullPointerException("promise");
    } else {
        channel.unsafe().register(this, promise);
        return promise;
    }
}

//AbstractChannel
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
    if(eventLoop == null) {
        throw new NullPointerException("eventLoop");
    } else if(AbstractChannel.this.isRegistered()) {
        promise.setFailure(new IllegalStateException("registered to an event loop already"));
    } else if(!AbstractChannel.this.isCompatible(eventLoop)) {
        promise.setFailure(new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
    } else {
        AbstractChannel.this.eventLoop = eventLoop;
        if(eventLoop.inEventLoop()) {
            this.register0(promise);
        } else {
            try {
                eventLoop.execute(new OneTimeTask() {
                    public void run() {
                        AbstractUnsafe.this.register0(promise);
                    }
                });
            } catch (Throwable var4) {
                AbstractChannel.logger.warn("Force-closing a channel whose registration task was not accepted by an event loop: {}", AbstractChannel.this, var4);
                this.closeForcibly();
                AbstractChannel.this.closeFuture.setClosed();
                this.safeSetFailure(promise, var4);
            }
        }

    }
}

private void register0(ChannelPromise promise) {
    try {
        if(!promise.setUncancellable() || !this.ensureOpen(promise)) {
            return;
        }

        AbstractChannel.this.doRegister();
        AbstractChannel.this.registered = true;
        this.safeSetSuccess(promise);
        AbstractChannel.this.pipeline.fireChannelRegistered();
        if(AbstractChannel.this.isActive()) {
            AbstractChannel.this.pipeline.fireChannelActive();
        }
    } catch (Throwable var3) {
        this.closeForcibly();
        AbstractChannel.this.closeFuture.setClosed();
        this.safeSetFailure(promise, var3);
    }

}

protected void doRegister() throws Exception {
    boolean selected = false;

    while(true) {
        try {
            this.selectionKey = this.javaChannel().register(this.eventLoop().selector, 0, this);
            return;
        } catch (CancelledKeyException var3) {
            if(selected) {
                throw var3;
            }

            this.eventLoop().selectNow();
            selected = true;
        }
    }
}

　　

NioSocketChannel读事件的注册

　　通过传播channelActive方法，最终会调用channel的read()方法，channel在创建的时候都是默认自动读的。

public ChannelPipeline fireChannelActive() {
    this.head.fireChannelActive();
    if(this.channel.config().isAutoRead()) {
        this.channel.read();
    }

    return this;
}

　　会将channel的Active状态在pipeline上面传播，调用read方法，最后会调用doBeginRead,去注册感兴趣的事件，NioSocketChannel感兴趣的事件是读事件，而NioServerSocketChannel感兴趣的事件则是Accept事件。

public ChannelHandlerContext read() {
    final AbstractChannelHandlerContext next = this.findContextOutbound();
    EventExecutor executor = next.executor();
    if(executor.inEventLoop()) {
        next.invokeRead();
    } else {
        Runnable task = next.invokeReadTask;
        if(task == null) {
            next.invokeReadTask = task = new Runnable() {
                public void run() {
                    next.invokeRead();
                }
            };
        }

        executor.execute(task);
    }

    return this;
}

protected void doBeginRead() throws Exception {
    if(!this.inputShutdown) {
        SelectionKey selectionKey = this.selectionKey;
        if(selectionKey.isValid()) {
            this.readPending = true;
            int interestOps = selectionKey.interestOps();
            if((interestOps & this.readInterestOp) == 0) {
                selectionKey.interestOps(interestOps | this.readInterestOp);
            }

        }
    }
}