Selector、SelectionKey和Channel 源码联合解析

Selector、SelectionKey和Channel 这三个组件构成了Java nio包的核心，也是Reactor模型在代码层面的体现。Selector能让单线程同时处理多个客户端Channel，非常适用于高并发，传输数据量较小的场景。要使用Selector，首先要将对应的Channel及IO事件（读、写、连接）注册到Selector，注册后会产生一个SelectionKey对象，用于关联Selector和Channel，及后续的IO事件处理。这三者的关系如下图所示。

对nio编程不熟的同学可以搜索一些简单的demo跑一下，下面 我们直接进入源码，窥探一些nio的奥秘。

Selector

其实，不管是 Selector 还是 SelectionKey 的源码，其具体实现类都是依赖于底层操作系统的，这里我们只看一下抽象类 Selector 的源码，日后有事件，再找一个具体的实现类深入分析一下。

public abstract class Selector implements Closeable {
    protected Selector() {
     }/*** 获取一个 Selector对象，具体实现依赖于底层操作系统*/public static Selector open() throws IOException {
    return SelectorProvider.provider().openSelector();}/*** 判断该 Selector 是否已开启*/public abstract boolean isOpen();/*** 当前所有向Selector注册的Channel 所对应的SelectionKey的集合*/public abstract Set<SelectionKey> keys();/*** 相关事件已经被 Selector 捕获的 SelectionKey的集合*/public abstract Set<SelectionKey> selectedKeys();/*** 阻塞到至少有一个通道在你注册的事件上就绪了*/public abstract int select() throws IOException;/*** 和select()一样，除了最长会阻塞timeout毫秒*/public abstract int select(long timeout) throws IOException;/*** 此方法执行非阻塞的选择操作，如果自从上一次选择操作后，* 没有通道变成可选择的，则此方法直接返回 0*/public abstract int selectNow() throws IOException;/*** 用完Selector后调用其close()方法会关闭该Selector，且使注册到该Selector上的所有SelectionKey实例无效* 通道本身并不会关闭*/public abstract void close() throws IOException;
}

SelectionKey

表示 SelectableChannel 在 Selector 中的注册的标记 / 句柄。

public abstract class SelectionKey {
    protected SelectionKey() {
     }// -- Channel and selector operations --/*** 获取该 SelectionKey 对应的Channel，Channel注册到Selector时会产生该 SelectionKey对象*/public abstract SelectableChannel channel();/*** 获取该 SelectionKey 对应的 Selector*/public abstract Selector selector();/*** 该 SelectionKey 是否是有效的*/public abstract boolean isValid();// ------ Operation-set accessors ------/*** 获取该 SelectionKey 的兴趣事件 (既 SelectionKey 的4个 事件静态常量)*/public abstract int interestOps();/*** 设置该 SelectionKey 的兴趣事件*/public abstract SelectionKey interestOps(int ops);/*** 获取该 SelectionKey 的已操作集*/public abstract int readyOps();// ------ Operation bits and bit-testing convenience methods ------/*** channel中的数据是否已经可以读取*/public static final int OP_READ = 1 << 0;/*** channel是否可以开始写入数据*/public static final int OP_WRITE = 1 << 2;/*** channel是否已经建立连接*/public static final int OP_CONNECT = 1 << 3;/*** ServerSocketChannel 是否可以与客户端建立连接*/public static final int OP_ACCEPT = 1 << 4;/*** channel是否可读*/public final boolean isReadable() {
    return (readyOps() & OP_READ) != 0;}/*** channel是否可写*/public final boolean isWritable() {
    return (readyOps() & OP_WRITE) != 0;}/*** channel是否建立连接*/public final boolean isConnectable() {
    return (readyOps() & OP_CONNECT) != 0;}/*** ServerSocketChannel是否可与客户端channel建立连接*/public final boolean isAcceptable() {
    return (readyOps() & OP_ACCEPT) != 0;}
}

Channel组件

平时编码用的比较多的就是 SocketChannel 和 ServerSocketChannel，而将 Channel 与 Selecor 关联到一起的核心API则定义在它们的公共父类SelectableChannel中，整个Channel组件的核心类图如下所示。

SelectableChannel

public abstract class SelectableChannel extends AbstractInterruptibleChannel implements Channel {
    protected SelectableChannel() {
     }/*** 当前channel是否注册到了某个selector上，新创建的channel都是未注册状态*/public abstract boolean isRegistered();/*** 根据给定的 Selector，获取本channel注册上去的 SelectionKey*/public abstract SelectionKey keyFor(Selector sel);/*** 将当前channel及关注的事件，注册到Selector上，返回一个 SelectionKey*/public final SelectionKey register(Selector sel, int ops) throws ClosedChannelException {
    return register(sel, ops, null);}public abstract SelectionKey register(Selector sel, int ops, Object att) throws ClosedChannelException;/*** 设置该channel的阻塞模式，默认为 true阻塞*/public abstract SelectableChannel configureBlocking(boolean block) throws IOException;/*** 是否为阻塞IO模式*/public abstract boolean isBlocking();
}

ServerSocketChannel

相当于 BIO 中的 ServerSocket，主要用于服务端与客户端建立连接通信的channel。

public abstract class ServerSocketChannel extends AbstractSelectableChannel implements NetworkChannel {
    protected ServerSocketChannel(SelectorProvider provider) {
    super(provider);}/*** 获取一个 ServerSocketChannel实例，具体实现依赖底层操作系统*/public static ServerSocketChannel open() throws IOException {
    return SelectorProvider.provider().openServerSocketChannel();}// -- ServerSocket-specific operations --/*** 绑定ip地址及要监听的端口*/public final ServerSocketChannel bind(SocketAddress local) throws IOException {
    return bind(local, 0);}public abstract ServerSocketChannel bind(SocketAddress local, int backlog) throws IOException;/*** 与一个客户端channel建立连接，返回该客户端的存根 SocketChannel*/public abstract SocketChannel accept() throws IOException;
}

SocketChannel

相当于 BIO 中的 Socket，主要用于通信双方的读写操作。

public abstract class SocketChannel extends AbstractSelectableChannelimplements ByteChannel, ScatteringByteChannel, GatheringByteChannel, NetworkChannel {
    protected SocketChannel(SelectorProvider provider) {
    super(provider);}/*** 根据 SocketAddress 获取一个 SocketChannel，具体实现依赖底层操作系统*/public static SocketChannel open(SocketAddress remote) throws IOException {
    SocketChannel sc = open();try {
    sc.connect(remote);} catch (Throwable x) {
    try {
    sc.close();} catch (Throwable suppressed) {
    x.addSuppressed(suppressed);}throw x;}assert sc.isConnected();return sc;}public static SocketChannel open() throws IOException {
    return SelectorProvider.provider().openSocketChannel();}// -- Socket-specific operations --/*** 绑定要连接的远程服务的ip及端口*/@Overridepublic abstract SocketChannel bind(SocketAddress local) throws IOException;/*** 该channel与服务端是否已连接*/public abstract boolean isConnected();// -- ByteChannel operations --/*** 将 channel 中的数据读到 ByteBuffer*/public abstract int read(ByteBuffer dst) throws IOException;public final long read(ByteBuffer[] dsts) throws IOException {
    return read(dsts, 0, dsts.length);}public abstract long read(ByteBuffer[] dsts, int offset, int length) throws IOException;/*** 将 ByteBuffer 中的数据写到 channel*/public abstract int write(ByteBuffer src) throws IOException;public final long write(ByteBuffer[] srcs) throws IOException {
    return write(srcs, 0, srcs.length);}public abstract long write(ByteBuffer[] srcs, int offset, int length) throws IOException;
}