Netty源码解析之Channel+Channelhandler

Channel

package io.netty.channel;

public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable {

    ChannelId id();

    EventLoop eventLoop();

    Channel parent();

    ChannelConfig config();

    boolean isOpen();

    boolean isRegistered();

    boolean isActive();

    ChannelMetadata metadata();

    SocketAddress localAddress();

    SocketAddress remoteAddress();

    ChannelFuture closeFuture();

    boolean isWritable();

    long bytesBeforeUnwritable();

    long bytesBeforeWritable();

    Unsafe unsafe();

    ChannelPipeline pipeline();

    ByteBufAllocator alloc();

    @Override
    Channel read();

    @Override
    Channel flush();

    interface Unsafe {

        RecvByteBufAllocator.Handle recvBufAllocHandle();

        SocketAddress localAddress();

        SocketAddress remoteAddress();

        void register(EventLoop eventLoop, ChannelPromise promise);

        void bind(SocketAddress localAddress, ChannelPromise promise);

        void connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise);

        void disconnect(ChannelPromise promise);

        void close(ChannelPromise promise);

        void closeForcibly();

        void deregister(ChannelPromise promise);

        void beginRead();

        void write(Object msg, ChannelPromise promise);

        void flush();

        ChannelPromise voidPromise();

        ChannelOutboundBuffer outboundBuffer();
    }
}

• 实现出站触发器，channel能够主动触发出站事件
• 提供Channel通用组件方法ChannelId、EventLoop、ChannelConfig、ChannelMetadata、ChannelPipeline、ByteBufAllocator
• 内部Unsafe接口，为了保证Channel代码简洁，实际干活的事都交给它

ThreadLocal VS FastThreadLocal

• ThreadLocal使用map保存数据，并且以当前ThreadLocal对象作为key
• FastThreadLocal使用数组保存数据，每个FastThreadLocal对象创建都会有一个数组索引
• Netty 的FastThreadLocalRunnable处理了线程池使用时内存泄漏风险
• JDK的ThreadLocal需要手动处理，使用线程池时忘记清理FastThreadLocal会内存泄漏
• 推荐文章：https://www.jianshu.com/p/3fc2fbac4bb7

ChannelOutboundBuffer

• 写消息时的缓冲区，不直接写到socket，而是先进入buffer，然后统一flush

• 推荐文章：https://www.jianshu.com/p/fbef615f1a24

AbstractChannel

• 成员变量包括Unsafe、DefaultChannelPipeline、EventLoop
• 出站事件委托给DefaultChannelPipeline
• IO事件委托给Unsafe
• AbstractUnsafe实现了register、bind、disconnect、close、deregister、write等方法的模版方法，实际实现doXXX由AbstractChannel提供抽象方法，交由具体子类实现

AbstractNioChannel

• NIO通道的抽象实现，对SocketChannel和ServerSocketChannel的抽象
• 关联Java NIO的Channel，使用SocketChannel和ServerSocketChannel的父类SelectableChannel
• 成员变量SelectableChannel、SelectionKey、readInterestOp
• 读标志位，在NIO中，服务端通道创建后监听的是SelectionKey.OP_ACCEPT、而客户端通道监听的是SelectionKey.OP_READ，这里统一抽象，由子类指定读标志位。本来NIO的服务端通道是没有读这个概念，这里把服务端接受连接和客户端的读都抽象成一个概念，服务端读的是Channel，客户端读的是Buffer。

AbstractNioByteChannel

• readInterestOp

    protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
        super(parent, ch, SelectionKey.OP_READ);
    }

• 实现ByteBuf的读写，以及事件的传递
• 但是底层的通道是不支持Netty的ByteBuf读写的，因此提供抽象方法由子类实现读写转换

NioSocketChannel

• 实现JDK SocketChannel语义，桥接JDK的SocketChannel和Netty的Channel
• 实现JDK SocketChannel底层调用,doXXX等底层相关方法实现

    @Override
    protected int doReadBytes(ByteBuf byteBuf) throws Exception {
        final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
        allocHandle.attemptedBytesRead(byteBuf.writableBytes());
        return byteBuf.writeBytes(javaChannel(), allocHandle.attemptedBytesRead());
    }

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

    @Override
    protected long doWriteFileRegion(FileRegion region) throws Exception {
        final long position = region.transferred();
        return region.transferTo(javaChannel(), position);
    }

AbstractNioMessageChannel

• 实现抽象的消息读写
• 读写操作对象的是Object

NioServerSocketChannel

• 实现JDK ServerSocketChannel语义
• readInterestOp

    public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }

• 读取的msg对象实际是NioSocketChannel

    protected int doReadMessages(List