Netty 源码 Channel（二）核心类

时间 2019-11-08

原文原文链接

Netty 源码 Channel（二）核心类

Netty 系列目录(http://www.javashuo.com/article/p-hskusway-em.html)html

1、Channel 类图

2、AbstractChannel

2.1 几个重要属性

// SocketChannel 的 parent 是 ServerSocketChannel
private final Channel parent;
// 惟一标识
private final ChannelId id;
// Netty 内部使用
private final Unsafe unsafe;
// pipeline
private final DefaultChannelPipeline pipeline;
// 绑定的线程
private volatile EventLoop eventLoop;

protected AbstractChannel(Channel parent, ChannelId id) {
    this.parent = parent;
    this.id = id;
    unsafe = newUnsafe();
    pipeline = newChannelPipeline();
}

2.2 核心 API

read、write、connect、bind 都委托给了 pipeline 处理。java

3、AbstractNioChannel

3.1 几个重要属性

// NIO 底层 Channel
private final SelectableChannel ch;
// 感兴趣的事件
protected final int readInterestOp;
// 绑定的 SelectionKey，当 selectionKey 修改后其它线程能够感知
volatile SelectionKey selectionKey;

3.2 核心 API

(1) doRegisterios

将 channel 注册到 eventLoop 线程上，此时统一注册的感兴趣的事件类型为 0。api

@Override
protected void doRegister() throws Exception {
    boolean selected = false;
    for (;;) {
        try {
            // 1. 将 channel 注册到 eventLoop 线程上
            selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
            return;
        } catch (CancelledKeyException e) {
            if (!selected) {
                // 2. 对注册失败的 channel，JDK 将在下次 select 将其删除
                //    然而此时尚未调用 select，固然也能够调用 selectNow 强删
                eventLoop().selectNow();
                selected = true;
            } else {
                // 3. JDK API 描述不会有异常，实际上...
                throw e;
            }
        }
    }
}

(2) doBeginRead网络

doBeginRead 只作了一件事就是注册 channel 感兴趣的事件。此至就能够监听网络事件了。app

@Override
protected void doBeginRead() throws Exception {
    // Channel.read() or ChannelHandlerContext.read() was called
    final SelectionKey selectionKey = this.selectionKey;
    if (!selectionKey.isValid()) {
        return;
    }

    readPending = true;
    final int interestOps = selectionKey.interestOps();
    if ((interestOps & readInterestOp) == 0) {
        selectionKey.interestOps(interestOps | readInterestOp);
    }
}

4、AbstractNioByteChannel

AbstractNioByteChannel 中最重要的方法是 doWrite，咱们一块儿来看一下：socket

@Override
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
    // 1. spin 是自旋的意思，也就是最多循环的次数
    int writeSpinCount = config().getWriteSpinCount();
    do {
        // 2. 从 ChannelOutboundBuffer 弹出一条消息
        Object msg = in.current();
        if (msg == null) {
            // 3. 写完了就要清除半包标记
            clearOpWrite();
            // 4. 直接返回，不调用 incompleteWrite 方法
            return;
        }
        // 5. 正确处理了一条 msg 消息，循环次数就减 1
        writeSpinCount -= doWriteInternal(in, msg);
    } while (writeSpinCount > 0);
    // 6. writeSpinCount < 0 认为有半包须要继续处理
    incompleteWrite(writeSpinCount < 0);
}

为何要设置最大自旋次数，一次把 ChannelOutboundBuffer 中的全部 msg 处理完了不是更好吗？若是不设置的话，线程会一直尝试进行网络 IO 写操做，此时线程没法处理其它网络 IO 事件，可能致使线程假死。ide

下面咱们看一下 msg 消息是如何处理的，这里以 ByteBuf 消息为例：oop

private int doWriteInternal(ChannelOutboundBuffer in, Object msg) throws Exception {
    if (msg instanceof ByteBuf) {
        ByteBuf buf = (ByteBuf) msg;
        // 1. 不可读则丢弃这条消息，继续处理下一条消息
        if (!buf.isReadable()) {
            in.remove();
            return 0;
        }
        
        // 2. 由具体的子类重写 doWriteBytes 方法，返回处理了多少字节
        final int localFlushedAmount = doWriteBytes(buf);
        if (localFlushedAmount > 0) {
            // 3. 更新进度
            in.progress(localFlushedAmount);
            if (!buf.isReadable()) {
                in.remove();
            }
            return 1;
        }
    // 文件处理，这里略过，相似 ByteBuf
    } else if (msg instanceof FileRegion) {
        // 省略 ...
    } else {
        throw new Error();
    }
    return WRITE_STATUS_SNDBUF_FULL;    // WRITE_STATUS_SNDBUF_FULL=Integer.MAX_VALUE
}

doWriteBytes 进行消息发送，它是一个抽象方法，由具体的子类实现。若是本次发送的字节数为 0，说明发送的 TCP 缓冲区已满，发生了 ZERO_WINDOW。此时再次发送可能还是 0，空循环会占用 CPU 资源。所以返回 Integer.MAX_VALUE。直接退出循环，设置半包标识，下次继续处理。this

// 没有写完，有两种状况：
// 一是 TCP 缓冲区已满，doWriteBytes 定入 0 个字节，致使 doWriteInternal 返回 Integer.MAX_VALUE，
//     这时设置了半包标识，会自动轮询写事件
// 二是自旋的次数已到，将线程交给其它任务执行，未写完的数据经过 flushTask 继续写
protected final void incompleteWrite(boolean setOpWrite) {
    // Did not write completely.
    if (setOpWrite) {
        setOpWrite();
    } else {
        // Schedule flush again later so other tasks can be picked up in the meantime
        Runnable flushTask = this.flushTask;
        if (flushTask == null) {
            flushTask = this.flushTask = new Runnable() {
                @Override
                public void run() {
                    flush();
                }
            };
        }
        eventLoop().execute(flushTask);
    }
}

最后咱们来看一下半包是如何处理的，能够看到所谓的半包标记其实就是是否取 OP_WRITE 事件。

protected final void clearOpWrite() {
    final SelectionKey key = selectionKey();
    final int interestOps = key.interestOps();
    if ((interestOps & SelectionKey.OP_WRITE) != 0) {
        key.interestOps(interestOps & ~SelectionKey.OP_WRITE);
    }
}

protected final void setOpWrite() {
    final SelectionKey key = selectionKey();
    final int interestOps = key.interestOps();
    if ((interestOps & SelectionKey.OP_WRITE) == 0) {
        key.interestOps(interestOps | SelectionKey.OP_WRITE);
    }
}

5、AbstractNioMessageChannel

AbstractNioMessageChannel#doWrite 方法和 AbstractNioByteChannel#doWrite 相似，前者能够写 POJO 对象，后者只能写 ByteBuf 和 FileRegion。

6、NioServerSocketChannel

NioServerSocketChannel 经过 doReadMessages 接收客户端的链接请求：

@Override
protected int doReadMessages(List<Object> buf) throws Exception {
    SocketChannel ch = SocketUtils.accept(javaChannel());
    if (ch != null) {
        buf.add(new NioSocketChannel(this, ch));
        return 1;
    }
    return 0;
}

7、NioSocketChannel

(1) doConnect

protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
    if (localAddress != null) {
        doBind0(localAddress);
    }

    boolean success = false;
    try {
        boolean connected = SocketUtils.connect(javaChannel(), remoteAddress);
        if (!connected) {
            selectionKey().interestOps(SelectionKey.OP_CONNECT);
        }
        success = true;
        return connected;
    } finally {
        if (!success) {
            doClose();
        }
    }
}

链接时有三种状况：

直接就链接成功，返回 true
若是没有链接成功，就注册 OP_CONNECT 事件进行监听，返回 false
发生异常

(2) doWriteBytes

向 ServerSocket 中写入数据。

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

(3) doReadBytes

从 ServerSocket 中读取数据。

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

最底层仍是调用 Channel 的 read 方法。

// AbstractByteBuf#writeBytes
public int writeBytes(ScatteringByteChannel in, int length) throws IOException {
    ensureWritable(length);
    int writtenBytes = setBytes(writerIndex, in, length);
    if (writtenBytes > 0) {
        writerIndex += writtenBytes;
    }
    return writtenBytes;
}
// UnpooledHeapByteBuf#setBytes
public int setBytes(int index, ScatteringByteChannel in, int length) throws IOException {
    ensureAccessible();
    try {
        return in.read((ByteBuffer) internalNioBuffer().clear().position(index).limit(index + length));
    } catch (ClosedChannelException ignored) {
        return -1;
    }
}

天天用心记录一点点。内容也许不重要，但习惯很重要！