Java中 BIO的代码示例和使用场景

BIO全称是Blocking I/O，即阻塞I/O，在Java中主要体现在`java.io`包下的`InputStream`、`OutputStream`等流处理上。当从网络或其他源读取数据时，如果数据不可立即获取，BIO会一直阻塞等待，直到数据可用。

下面是一个简单的BIO网络通信的代码示例，使用`Socket`和`PrintWriter`进行服务器端和客户端的连接：

**服务器端（Server.java）**:

import java.io.*;
import java.net.*;

public class Server {
    public static void main(String[] args) throws IOException {
        ServerSocket server = new ServerSocket(8000);
        Socket client = null;

        while ((client = server.accept()) != null) {
            Thread thread = new Thread(new ClientHandler(client));
            thread.start();
        }
    }
}

class ClientHandler implements Runnable {
    private final Socket socket;

    public ClientHandler(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        try (BufferedReader reader = new BufferedReader(
                new InputStreamReader(socket.getInputStream()));
             PrintWriter writer = new PrintWriter(socket.getOutputStream(), true)) {

            String requestLine;
            while ((requestLine = reader.readLine()) != null) {
                System.out.println("Received: " + requestLine);
                writer.println("Hello from Server!");
            }

        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

**客户端（Client.java）**:

import java.io.*;
import java.net.InetAddress;
import java.net.UnknownHostException;

public class Client {
    public static void main(String[] args) {
        try {
            String host = "localhost";
            int port = 8000;

            Socket socket = new Socket(InetAddress.getByName(host), port);

            try (BufferedWriter writer = new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream()));
                 BufferedReader reader = new BufferedReader(
                     new InputStreamReader(socket.getInputStream()))) {

                writer.write("GET / HTTP/1.1\n");
                writer.newLine();
                writer.write("Host: " + host + "\n");
                writer.newLine();
                writer.write("Connection: close\n");
                writer.newLine();

                System.out.println("Sent request to " + host + ":" + port);

                String responseLine;
                while ((responseLine = reader.readLine()) != null) {
                    System.out.println("Received: " + responseLine);
                }

            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                if (socket != null) {
                    try {
                        socket.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

BIO的主要应用场景是在并发不是特别高的场景下，因为其同步阻塞的特性，不适合大量并发连接的高性能网络服务，比如Web服务器。在现代应用中，非阻塞I/O（NIO）和反应式编程更受欢迎，它们提供了解决高并发的能力。