【java】BIO,NIO,多路IO复用，AIO

在Java中，处理I/O操作的模型主要有四种：阻塞I/O (BIO), 非阻塞I/O (NIO), 异步I/O (AIO), 以及IO多路复用。下面详细介绍这四种I/O模型的工作原理和应用场景。

1. 阻塞I/O (BIO)

工作原理

阻塞I/O是最传统的I/O模型。在这种模型中，当一个线程发起一个I/O请求（如读写操作）时，该线程会被阻塞，直到I/O操作完成。这意味着线程必须等待I/O操作完成才能继续执行。

代码示例

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;public class BioServer {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(); // 阻塞等待客户端连接new Thread(() -> {try (BufferedReader reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()))) {String line;while ((line = reader.readLine()) != null) {System.out.println("Received: " + line);}} catch (IOException e) {e.printStackTrace();}}).start();}}
}

优点

实现简单。

缺点

每个连接都需要一个线程来处理，当并发连接数增加时，线程的数量也会增加，可能导致系统资源耗尽。

2. 非阻塞I/O (NIO)

工作原理

非阻塞I/O模型允许线程在发起I/O请求时不会被阻塞，如果数据不可用或设备忙，则立即返回一个错误或特殊值。线程可以选择立即再次尝试I/O操作或去做其他事情，从而提高了CPU的利用率。

代码示例

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 NioServer {public static void main(String[] args) throws IOException {Selector selector = Selector.open();ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();serverSocketChannel.socket().bind(new InetSocketAddress(8080));serverSocketChannel.configureBlocking(false);serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);while (true) {selector.select();Set<SelectionKey> selectedKeys = selector.selectedKeys();Iterator<SelectionKey> keyIterator = selectedKeys.iterator();while (keyIterator.hasNext()) {SelectionKey key = keyIterator.next();if (key.isAcceptable()) {ServerSocketChannel ssc = (ServerSocketChannel) key.channel();SocketChannel sc = ssc.accept();sc.configureBlocking(false);sc.register(selector, SelectionKey.OP_READ);} else if (key.isReadable()) {SocketChannel sc = (SocketChannel) key.channel();ByteBuffer buffer = ByteBuffer.allocate(1024);int readBytes = sc.read(buffer);if (readBytes > 0) {buffer.flip();byte[] data = new byte[buffer.remaining()];buffer.get(data);System.out.println("Received: " + new String(data));}}keyIterator.remove();}}}
}

优点

提高了单个线程处理多个连接的能力，降低了系统资源消耗。
可以处理大量并发连接。

缺点

实现相对复杂。
需要手动管理缓冲区、选择器等。

3. IO多路复用

工作原理

IO多路复用允许一个进程同时监听多个文件描述符（例如socket），并只在某个描述符准备好进行读写操作时才进行处理。常用的多路复用机制有select、poll和epoll。这种模型非常适合处理大量并发连接的场景。

代码示例

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;public class SelectServer {public static void main(String[] args) throws IOException {Selector selector = Selector.open();ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();serverSocketChannel.socket().bind(new InetSocketAddress(8080));serverSocketChannel.configureBlocking(false);serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);while (true) {selector.select();for (SelectionKey key : selector.selectedKeys()) {if (key.isAcceptable()) {ServerSocketChannel ssc = (ServerSocketChannel) key.channel();SocketChannel sc = ssc.accept();sc.configureBlocking(false);sc.register(selector, SelectionKey.OP_READ);} else if (key.isReadable()) {SocketChannel sc = (SocketChannel) key.channel();// 读取数据...}}selector.selectedKeys().clear();}}
}

优点

可以同时监听多个文件描述符，提高处理大量并发连接的能力。
提高了资源利用率。

缺点

在Java中，select和poll的性能不如epoll，后者仅在Linux系统中可用。

4. 异步I/O (AIO)

工作原理

异步I/O是真正的异步操作模型，进程发起I/O请求后可以立即返回并继续执行其他任务，而无需等待I/O操作完成。当I/O操作完成后，操作系统会通知进程结果。

代码示例

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousServerSocketChannel;
import java.nio.channels.AsynchronousSocketChannel;
import java.nio.channels.CompletionHandler;
import java.util.concurrent.CountDownLatch;public class AioServer {private static final CountDownLatch latch = new CountDownLatch(1);public static void main(String[] args) throws IOException, InterruptedException {AsynchronousServerSocketChannel server = AsynchronousServerSocketChannel.open().bind(new java.net.InetSocketAddress(8080));server.accept(null, new AcceptHandler(server));latch.await();}static class AcceptHandler implements CompletionHandler<AsynchronousSocketChannel, Object> {private AsynchronousServerSocketChannel server;public AcceptHandler(AsynchronousServerSocketChannel server) {this.server = server;}@Overridepublic void completed(AsynchronousSocketChannel result, Object attachment) {result.read(ByteBuffer.allocate(1024), null, new ReadHandler(result));server.accept(null, this);}@Overridepublic void failed(Throwable exc, Object attachment) {exc.printStackTrace();latch.countDown();}}static class ReadHandler implements CompletionHandler<Integer, Object> {private AsynchronousSocketChannel channel;public ReadHandler(AsynchronousSocketChannel channel) {this.channel = channel;}@Overridepublic void completed(Integer result, Object attachment) {ByteBuffer buffer = (ByteBuffer) attachment;buffer.flip();byte[] data = new byte[buffer.remaining()];buffer.get(data);System.out.println("Received: " + new String(data));channel.close();}@Overridepublic void failed(Throwable exc, Object attachment) {exc.printStackTrace();try {((AsynchronousSocketChannel) attachment).close();} catch (IOException e) {e.printStackTrace();}}}
}