Netty4.x 源码实战系列(三):NioServerSocketChannel全剖析

浏览: 205 发布日期: 2018-01-01 分类: java

根据上一篇《Netty4.x 源码实战系列(二):服务端bind流程详解》所述,在进行服务端开发时,必须通过ServerBootstrap引导类的channel方法来指定channel类型, channel方法的调用其实就是实例化了一个用于生成此channel类型对象的工厂对象。 并且在bind调用后,会调用此工厂对象来生成一个新channel。

本篇将通过NioServerSocketChannel实例化过程,来深入剖析NioServerSocketChannel。

在开始代码分析之前,我们先看一下NioServerSocketChannel的类继承结构图:

调用工厂完成NioServerSocketChannel实例的创建

在绑定侦听端口过程中,我们调用了AbstractBootstrap的initAndRegister方法来完成channel的创建与初始化,channel实例化代码如下:

channelFactory.newChannel()

而channelFactory对象是我们通过ServerBootstrap.channel方法的调用生成的

 public B channel(Class<? extends C> channelClass) {
    if (channelClass == null) {
        throw new NullPointerException("channelClass");
    }
    return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}

通过代码可知,此工厂对象是ReflectiveChannelFactory实例

public class ReflectiveChannelFactory<T extends Channel> implements ChannelFactory<T> {

    private final Class<? extends T> clazz;

    public ReflectiveChannelFactory(Class<? extends T> clazz) {
        if (clazz == null) {
            throw new NullPointerException("clazz");
        }
        this.clazz = clazz;
    }

    @Override
    public T newChannel() {
        try {
            return clazz.getConstructor().newInstance();
        } catch (Throwable t) {
            throw new ChannelException("Unable to create Channel from class " + clazz, t);
        }
    }
}

所以 channelFactory.newChannel() 实例化其实就是NioServerSocketChannel无参构造方法反射而成。

NioServerSocketChannel实例化过程分析

我们先看一下NioServerSocketChannel的无参构造代码

public NioServerSocketChannel() {
    this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}

无参构造方法中有两个关键点:
1、使用默认的多路复用器辅助类 DEFAULT_SELECTOR_PROVIDER

 private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();

2、通过newSocket创建ServerSocketChannel

private static ServerSocketChannel newSocket(SelectorProvider provider) {
    try {
       
        return provider.openServerSocketChannel();
    } catch (IOException e) {
        throw new ChannelException(
                "Failed to open a server socket.", e);
    }
}

我们将newSocket生成的ServerSocketChannel对象继续传递给本类中的NioServerSocketChannel(ServerSocketChannel channel)构造方法

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

在其内部,我们会调用父类AbstractNioMessageChannel的构造方法:

protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    super(parent, ch, readInterestOp);
}

因为是服务端新生成的channel,第一个参数指定为null,表示没有父channel,第二个参数指定为ServerSocketChannel,第三个参数指定ServerSocketChannel关心的事件类型为SelectionKey.OP_ACCEPT。

在AbstractNioMessageChannel内部会继续调用父类AbstractNioChannel的构造方法:

protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    // 继续调用父类构造方法
    super(parent);
    // 将ServerSocketChannel对象保存
    this.ch = ch;
    // 设置关心的事件
    this.readInterestOp = readInterestOp;
    try {
        // 设置当前通道为非阻塞的
        ch.configureBlocking(false);
    } catch (IOException e) {
        try {
            ch.close();
        } catch (IOException e2) {
            if (logger.isWarnEnabled()) {
                logger.warn(
                        "Failed to close a partially initialized socket.", e2);
            }
        }

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

在AbstractNioChannel中做了下面几件事:
1、继续调用父类AbstractChannel(Channel parent)构造方法;
2、通过this.ch = ch 保存ServerSocketChannel, 因为NioServerSocketChannel是Netty封装的对象,而ServerSocketChannel是有前面默认selector_provider生成的,是java nio的, 其实“this.ch = ch”可以被认为是绑定java nio服务端通道至netty对象中;
3、设置ServerSocketChannel关心的事件类型;
4、设置ServerSocketChannel为非阻塞的(熟悉Java NIO的都知道如果不设置为false,启动多路复用器会报异常)

我们再看一下AbstractChannel(Channel parent)的内部代码细节

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

此构造方法中,主要做了三件事:
1、给channel生成一个新的id
2、通过newUnsafe初始化channel的unsafe属性
3、newChannelPipeline初始化channel的pipeline属性

id的生成我们就不细究了,我们主要看看newUnsafe 及 newChannelPipeline是如何创建unsafe对象及pipeline对象的。

newUnsafe()方法调用
在AbstractChannel类中,newUnsafe()是一个抽象方法

protected abstract AbstractUnsafe newUnsafe();

通过上面的类继承结构图,我们找到AbstractNioMessageChannel类中有newUnsafe()的实现

@Override
protected AbstractNioUnsafe newUnsafe() {
    return new NioMessageUnsafe();
}

此方法返回一个NioMessageUnsafe实例对象,而NioMessageUnsafe是AbstractNioMessageChannel的内部类

private final class NioMessageUnsafe extends AbstractNioUnsafe {

    private final List<Object> readBuf = new ArrayList<Object>();

    @Override
    public void read() {
        assert eventLoop().inEventLoop();
        final ChannelConfig config = config();
        final ChannelPipeline pipeline = pipeline();
        final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
        allocHandle.reset(config);

        boolean closed = false;
        Throwable exception = null;
        try {
            try {
                do {
                    int localRead = doReadMessages(readBuf);
                    if (localRead == 0) {
                        break;
                    }
                    if (localRead < 0) {
                        closed = true;
                        break;
                    }

                    allocHandle.incMessagesRead(localRead);
                } while (allocHandle.continueReading());
            } catch (Throwable t) {
                exception = t;
            }

            int size = readBuf.size();
            for (int i = 0; i < size; i ++) {
                readPending = false;
                pipeline.fireChannelRead(readBuf.get(i));
            }
            readBuf.clear();
            allocHandle.readComplete();
            pipeline.fireChannelReadComplete();

            if (exception != null) {
                closed = closeOnReadError(exception);

                pipeline.fireExceptionCaught(exception);
            }

            if (closed) {
                inputShutdown = true;
                if (isOpen()) {
                    close(voidPromise());
                }
            }
        } finally {
            if (!readPending && !config.isAutoRead()) {
                removeReadOp();
            }
        }
    }
}

NioMessageUnsafe 只覆盖了 父类AbstractNioUnsafe中的read方法,通过NioMessageUnsafe 及其父类的代码便可以知道, 其实unsafe对象是真正的负责底层channel的连接/读/写等操作的,unsafe就好比一个底层channel操作的代理对象。

newChannelPipeline()方法调用
newChannelPipeline直接在AbstractChannel内实现

protected DefaultChannelPipeline newChannelPipeline() {
    return new DefaultChannelPipeline(this);
}

该方法返回了创建了一个DefaultChannelPipeline对象

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.next = tail;
    tail.prev = head;
}

此DefaultChannelPipeline对象会绑定NioServerSocketChannel对象,并初始化了HeadContext及TailContext对象。

tail = new TailContext(this);
head = new HeadContext(this);

head及tail初始化完成后,它们会相互连接。

通过上面的代码可以得出,pipeline就是一个双向链表。关于Pipeline的更多细节,此处不做赘述,欢迎大家关注下一篇文章。

我们在回到NioServerSocketChannel的构造方法 NioServerSocketChannel(ServerSocketChannel channel)

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

父类构造方法调用完成后,NioServerSocketChannel还要初始化一下自己的配置对象

config = new NioServerSocketChannelConfig(this, javaChannel().socket());

NioServerSocketChannelConfig是NioServerSocketChannel的内部类

private final class NioServerSocketChannelConfig extends DefaultServerSocketChannelConfig {
    private NioServerSocketChannelConfig(NioServerSocketChannel channel, ServerSocket javaSocket) {
        super(channel, javaSocket);
    }

    @Override
    protected void autoReadCleared() {
        clearReadPending();
    }
}

而NioServerSocketChannelConfig 又是继承自DefaultServerSocketChannelConfig,通过代码分析,此config对象就是就会对底层ServerSocket一些配置设置行为的封装。

至此NioServerSocketChannel对象应该创建完成了~

总结:

1、NioServerSocketChannel对象内部绑定了Java NIO创建的ServerSocketChannel对象;

2、Netty中,每个channel都有一个unsafe对象,此对象封装了Java NIO底层channel的操作细节;

3、Netty中,每个channel都有一个pipeline对象,此对象就是一个双向链表;

返回顶部