如何实现Java HTTP长连接

在Java中实现HTTP长连接，通常是指使用HTTP Keep-Alive特性。Keep-Alive允许在一个TCP连接上发送多个HTTP请求和响应，而不是为每个请求/响应对建立一个新的连接。这样可以减少连接建立和关闭的开销，提高性能。

要在Java中实现HTTP长连接，可以使用以下方法：

1. 使用HttpURLConnection：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class HttpLongConnectionExample {
    public static void main(String[] args) throws Exception {
        URL url = new URL("http://example.com");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        connection.setKeepAlive(true); // 启用Keep-Alive
        connection.setReadTimeout(5000); // 设置读取超时时间

        int responseCode = connection.getResponseCode();
        System.out.println("Response Code: " + responseCode);

        BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
        String inputLine;
        while ((inputLine = in.readLine()) != null) {
            System.out.println(inputLine);
        }
        in.close();

        connection.disconnect(); // 关闭连接
    }
}

2. 使用Apache HttpClient：

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;

public class HttpClientLongConnectionExample {
    public static void main(String[] args) {
        PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager();
        connectionManager.setMaxTotal(10); // 设置最大连接数
        connectionManager.setDefaultMaxPerRoute(5); // 设置每个路由的最大连接数

        HttpClient httpClient = HttpClients.custom()
                .setConnectionManager(connectionManager)
                .build();

        HttpGet httpGet = new HttpGet("http://example.com");

        try {
            HttpResponse response = httpClient.execute(httpGet);
            System.out.println("Response Code: " + response.getStatusLine().getStatusCode());

            // 处理响应内容...

            httpClient.getConnectionManager().shutdown(); // 关闭连接池
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

注意：在使用长连接时，需要确保服务器端也支持Keep-Alive。否则，连接可能会在服务器端被关闭。同时，要注意合理管理连接池，避免资源耗尽。