這篇文章來介紹一下另一個比較重要的概念----Selector。我們知道系統線程的切換是消耗系統資源的，如果我們每一個連接都用一個線程來管理，資源的開銷會非常大，這個時候就可以用Selector。通過Selector可以實現一個線程管理多個Channel，如下圖：
Selector使用
打開
使用之前獲得一個Selector對象
Selector selector = Selector.open();
註冊
要把Channel註冊到Selector上，Channel必需是非阻塞的。因此FileChannel是無法註冊到Selector的。如果註冊的時候不調用configureBlocking方法就會拋出IllegalBlockingModeException異常。
SelectionKey
SelectionKey共有四種
- OP_ACCEPT
- OP_CONNECT
- OP_WRITE
- OP_READ
ServerSocketChannel註冊
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
ServerSocketChannel的Operation Set只能是OP_ACCEPT，如果在註冊的時候添加了OP_CONNECT、OP_WRITE或OP_READ會報異常。例如按照以下寫法
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT | SelectionKey.OP_CONNECT);
就會拋出下面的異常
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)
ServerSocketChannel的validOps可以看到只有OP_ACCEPT是合法的
public final int validOps() {
return SelectionKey.OP_ACCEPT;
}
SocketChannel註冊
socketChannel.register(selector, SelectionKey.OP_CONNECT);
SocketChannel的Operation Set只能是OP_CONNECT、OP_WRITE和OP_READ，如果在註冊的時候添加了OP_ACCEPT同樣會報異常。
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);
}
註冊成功之後，我們通過一個demo實現，客戶端和服務端交互：
服務端：
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 如果不設置非阻塞，register的時候會報異常
// 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("接收來自客戶端的數據：" + new String(buffer.array()));
selectionKey.interestOps(SelectionKey.OP_WRITE);
} else if (selectionKey.isWritable()) {
System.err.println("Writable");
SocketChannel channel = (SocketChannel) selectionKey.channel();
String content = "向客戶端發送數據 : " + System.currentTimeMillis();
ByteBuffer buffer = ByteBuffer.wrap(content.getBytes());
channel.write(buffer);
selectionKey.interestOps(SelectionKey.OP_READ);
}
}
}
}
}
我們來看一下服務端的邏輯
1、服務端註冊到selector，然後interest set（ops）設置爲SelectionKey.OP_ACCEPT等待客戶端連接。
2、客戶端連接到達，調用到selectionKey.isAcceptable()方法，接收客戶端連接，然後獲得一個channel，並把
interest set設置爲SelectionKey.OP_READ等待從通道中讀數據。
3、當客戶端發送的數據到達，selectionKey.isReadable() 被觸發，接收客戶端的數據並打印，然後把selectionKey.interestOps 設置爲SelectionKey.OP_WRITE，向客戶端發送數據。
4、當可寫之後selectionKey.isWritable()被觸發，向客戶端發送數據，同時selectionKey.interestOps再次設置爲
SelectionKey.OP_READ等待客戶端數據到達。
客戶端:
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("收到服務端數據" + 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("向服務端發送數據" + new String(buffer.array()));
}
iterator.remove();
}
}
}
}
再來看一下服務端的邏輯
1、向服務端發起連接請求。
2、selectionKey.isConnectable()被觸發，連接成功之後，selectionKey.interestOps設置爲SelectionKey.OP_WRITE，準備向服務端發送數據。
3、channel可寫之後selectionKey.isWritable()被觸發，向服務端發送數據，之後selectionKey.interestOps設置爲SelectionKey.OP_READ，等待服務端過來的數據。
4、服務端數據發過來之後，selectionKey.isReadable()被觸發，讀取服務端數據之後selectionKey.interestOps設置爲SelectionKey.OP_WRITE向服務端寫數據。