前面两篇文章介绍了NIO中的Buffer和Channel,有了之前的基础,这篇文章来介绍一下另一个比较重要的概念----Selector。我们知道系统线程的切换是消耗系统资源的,如果我们每一个连接都用一个线程来管理,资源的开销会非常大,这个时候就可以用Selector。通过Selector可以实现一个线程管理多个Channel,如下图:
使用之前获得一个Selector对象
Selector selector = Selector.open(); 复制代码
要把Channel注册到Selector上,Channel必需是非阻塞的。因此FileChannel是无法注册到Selector的。如果注册的时候不调用 configureBlocking
方法就会抛出 IllegalBlockingModeException
异常。
SelectionKey共有四种
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT); 复制代码
ServerSocketChannel的Operation Set只能是OP_ACCEPT,如果在注册的时候添加了OP_CONNECT、OP_WRITE或OP_READ会报异常。例如按照以下写法
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT | SelectionKey.OP_CONNECT); 复制代码
就会抛出下面的异常
Exception in thread "main" java.lang.IllegalArgumentException at java.nio.channels.spi.AbstractSelectableChannel.register(AbstractSelectableChannel.java:199) at java.nio.channels.SelectableChannel.register(SelectableChannel.java:280) at com.nio.sample.selector.SelectorServerSocketChannelSample.main(SelectorServerSocketChannelSample.java:27) 复制代码
ServerSocketChannel的 validOps
可以看到只有OP_ACCEPT是合法的
public final int validOps() { return SelectionKey.OP_ACCEPT; } 复制代码
socketChannel.register(selector, SelectionKey.OP_CONNECT); 复制代码
SocketChannel的Operation Set只能是OP_CONNECT、OP_WRITE和OP_READ,如果在注册的时候添加了OP_ACCEPT同样会报异常。
SocketChannel的 validOps
可以看到只有OP_READ、OP_WRITE、OP_CONNECT是合法的
public final int validOps() { return (SelectionKey.OP_READ | SelectionKey.OP_WRITE | SelectionKey.OP_CONNECT); } 复制代码
注册成功之后,我们通过一个demo实现,客户端和服务端交互:
服务端:
public static void main(String[] args) throws Exception { ServerSocketChannel serverSocketChannel = ServerSocketChannel.open(); serverSocketChannel.socket().bind(new InetSocketAddress(9000)); serverSocketChannel.configureBlocking(false); Selector selector = Selector.open(); // configureBlocking 如果不设置非阻塞,register的时候会报异常 // java.nio.channels.IllegalBlockingModeException serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT); while (true) { int selected = selector.select(); if (selected > 0) { Iterator<SelectionKey> iterator = selector.selectedKeys().iterator(); while (iterator.hasNext()) { SelectionKey selectionKey = iterator.next(); iterator.remove(); if (selectionKey.isAcceptable()) { System.err.println("Acceptable"); SocketChannel socketChannel = serverSocketChannel.accept(); socketChannel.configureBlocking(false); socketChannel.register(selector, SelectionKey.OP_READ); } else if (selectionKey.isReadable()) { System.err.println("Readable"); SocketChannel socketChannel = (SocketChannel) selectionKey.channel(); ByteBuffer buffer = ByteBuffer.allocate(128); socketChannel.read(buffer); System.out.println("接收来自客户端的数据:" + new String(buffer.array())); selectionKey.interestOps(SelectionKey.OP_WRITE); } else if (selectionKey.isWritable()) { System.err.println("Writable"); SocketChannel channel = (SocketChannel) selectionKey.channel(); String content = "向客户端发送数据 : " + System.currentTimeMillis(); ByteBuffer buffer = ByteBuffer.wrap(content.getBytes()); channel.write(buffer); selectionKey.interestOps(SelectionKey.OP_READ); } } } } } 复制代码
我们来看一下服务端的逻辑
1、服务端注册到selector,然后interest set(ops)设置为SelectionKey.OP_ACCEPT等待客户端连接。
2、客户端连接到达,调用到selectionKey.isAcceptable()方法,接收客户端连接,然后获得一个channel,并把
interest set设置为SelectionKey.OP_READ等待从通道中读数据。
3、当客户端发送的数据到达,selectionKey.isReadable() 被触发,接收客户端的数据并打印,然后把selectionKey.interestOps 设置为SelectionKey.OP_WRITE,向客户端发送数据。
4、当可写之后selectionKey.isWritable()被触发,向客户端发送数据,同时selectionKey.interestOps再次设置为
SelectionKey.OP_READ等待客户端数据到达。
客户端:
public static void main(String[] args) throws IOException { SocketChannel socketChannel = SocketChannel.open(); socketChannel.configureBlocking(false); Selector selector = Selector.open(); socketChannel.register(selector, SelectionKey.OP_CONNECT); socketChannel.connect(new InetSocketAddress("127.0.0.1", 9000)); while (true) { int select = selector.select(); if (select > 0) { Iterator<SelectionKey> iterator = selector.selectedKeys().iterator(); while (iterator.hasNext()) { SelectionKey selectionKey = iterator.next(); if (selectionKey.isConnectable()) { System.err.println("Connectable"); SocketChannel clientChannel = (SocketChannel) selectionKey.channel(); clientChannel.finishConnect(); selectionKey.interestOps(SelectionKey.OP_WRITE); } else if (selectionKey.isReadable()) { System.out.println("Readable"); SocketChannel channel = (SocketChannel) selectionKey.channel(); ByteBuffer buffer = ByteBuffer.allocate(128); channel.read(buffer); selectionKey.interestOps(SelectionKey.OP_WRITE); System.out.println("收到服务端数据" + new String(buffer.array())); } else if (selectionKey.isWritable()) { SocketChannel clientChannel = (SocketChannel) selectionKey.channel(); String str = "qiwoo mobile"; ByteBuffer buffer = ByteBuffer.wrap(str.getBytes()); clientChannel.write(buffer); selectionKey.interestOps(SelectionKey.OP_READ); System.out.println("向服务端发送数据" + new String(buffer.array())); } iterator.remove(); } } } } 复制代码
再来看一下服务端的逻辑
1、向服务端发起连接请求。
2、selectionKey.isConnectable()被触发,连接成功之后,selectionKey.interestOps设置为SelectionKey.OP_WRITE,准备向服务端发送数据。
3、channel可写之后selectionKey.isWritable()被触发,向服务端发送数据,之后selectionKey.interestOps设置为SelectionKey.OP_READ,等待服务端过来的数据。
4、服务端数据发过来之后,selectionKey.isReadable()被触发,读取服务端数据之后selectionKey.interestOps设置为SelectionKey.OP_WRITE向服务端写数据。