1. 阻塞IO (Blocking IO)
在传统的阻塞IO模型中，示例中的 serverSocket.accept()，这是一个阻塞调用，意味着调用线程将被挂起直到一个连接请求到达。这是典型的阻塞行为。
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;public class BlockingIOServer {public static void main(String[] args) throws IOException {ServerSocket serverSocket = new ServerSocket(8080);System.out.println("Server started on port 8080");while (true) {Socket clientSocket = serverSocket.accept();// 注意：accept() 是一个阻塞调用new Thread(() -> {try {byte[] buffer = new byte[1024];int bytesRead = clientSocket.getInputStream().read(buffer);// read() 也是一个阻塞调用if (bytesRead > 0) {String message = new String(buffer, 0, bytesRead);System.out.println("Received: " + message);}} catch (IOException e) {e.printStackTrace();} finally {try {clientSocket.close();} catch (IOException e) {e.printStackTrace();}}}).start();}}
}2. 非阻塞IO (Non-Blocking IO)
在Java中，你可以使用 NIO（Non-blocking IO）库来实现非阻塞IO。例如，你使用 Selector 和 SelectableChannel 来监听多个 SocketChannel 的可读状态。当没有数据可读时，你的程序不会被阻塞，而是可以立即返回并处理其他任务，只有当数据真正到达时，你才会被通知去读取。
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;public class NonBlockingIOServer {public static void main(String[] args) throws IOException {Selector selector = Selector.open();ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();serverSocketChannel.configureBlocking(false);serverSocketChannel.socket().bind(new InetSocketAddress(8080));serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);System.out.println("Server started on port 8080");while (true) {if (selector.select() == 0) {//这里会阻塞,等待Channel事件发生continue; // no events ready for processing}Set<SelectionKey> keys = selector.selectedKeys();Iterator<SelectionKey> iterator = keys.iterator();while (iterator.hasNext()) {SelectionKey key = iterator.next();iterator.remove();if (key.isAcceptable()) {ServerSocketChannel ssc = (ServerSocketChannel) key.channel();SocketChannel sc = ssc.accept();//这里是非阻塞sc.configureBlocking(false);sc.register(selector, SelectionKey.OP_READ);} else if (key.isReadable()) {SocketChannel sc = (SocketChannel) key.channel();ByteBuffer buffer = ByteBuffer.allocate(1024);int bytesRead = sc.read(buffer);//这里也是非阻塞if (bytesRead > 0) {buffer.flip();byte[] data = new byte[buffer.remaining()];buffer.get(data);System.out.println("Received: " + new String(data));}}}}}
}在这个示例中，selector.select() 和 serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT) 结合使用。serverSocketChannel 已经配置为非阻塞模式，这意味着 serverSocketChannel.accept() 本身不会阻塞当前线程。然而，selector.select() 方法是一个阻塞调用，它会阻塞直到至少一个通道上的事件准备好被处理。当调用 selector.select() 时，线程将等待至少一个注册的通道有事件发生（例如，一个连接请求到达或数据可读）。
3. 异步IO (Asynchronous IO)
异步IO是最先进的IO模型，它不仅不会阻塞线程，而且还会在IO操作完成后主动通知应用程序。也就是说，你只需要告诉操作系统你想做什么，然后继续执行其他任务，当IO操作完成时，操作系统会通过回调函数、事件通知等方式告知你。
示例：
在这个示例中，serverChannel.accept() 是异步调用，它使用了 CompletionHandler。这意味着调用线程不会被阻塞，而是会立即返回。当一个连接请求到达时，CompletionHandler 的 completed 方法将在异步线程池中的某个线程上被调用。因此，serverChannel.accept(null, new CompletionHandler<...>) 在调用时不会阻塞当前线程。
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousChannelGroup;
import java.nio.channels.AsynchronousServerSocketChannel;
import java.nio.channels.AsynchronousSocketChannel;
import java.nio.channels.CompletionHandler;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class AsyncIOServer {public static void main(String[] args) throws IOException {ExecutorService threadPool = Executors.newFixedThreadPool(5);AsynchronousChannelGroup group = AsynchronousChannelGroup.withThreadPool(threadPool);AsynchronousServerSocketChannel serverChannel = AsynchronousServerSocketChannel.open(group).bind(new java.net.InetSocketAddress(8080));System.out.println("Server started on port 8080");serverChannel.accept(null, new CompletionHandler<AsynchronousSocketChannel, Void>() {@Overridepublic void completed(AsynchronousSocketChannel result, Void attachment) {ByteBuffer buffer = ByteBuffer.allocate(1024);result.read(buffer, buffer, new CompletionHandler<Integer, ByteBuffer>() {@Overridepublic void completed(Integer result, ByteBuffer attachment) {if (result > 0) {attachment.flip();byte[] data = new byte[attachment.remaining()];attachment.get(data);System.out.println("Received: " + new String(data));attachment.clear();result.read(attachment, attachment, this);} else {result.close();}}@Overridepublic void failed(Throwable exc, ByteBuffer attachment) {exc.printStackTrace();}});serverChannel.accept(null, this);}@Overridepublic void failed(Throwable exc, Void attachment) {exc.printStackTrace();}});}
}总结一下，阻塞IO会阻止线程直到IO操作完成；非阻塞IO允许线程在没有数据可处理时立即返回；而异步IO则完全不需要线程等待，而是通过回调或事件通知来处理完成的IO操作。
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