Java如何实现HTTP长连接

在Java中，可以使用HttpURLConnection或者第三方库（如Apache HttpClient、OkHttp等）来实现HTTP长连接。下面分别介绍这几种方法。

1. 使用HttpURLConnection实现HTTP长连接：

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

public class HttpLongConnection {
    public static void main(String[] args) throws IOException {
        URL url = new URL("http://example.com");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        connection.setReadTimeout(5000);
        connection.setConnectTimeout(5000);
        connection.setUseCaches(true);
        connection.setDoOutput(true);
        connection.setDoInput(true);
        connection.setRequestProperty("Connection", "keep-alive");

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

        BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
        String inputLine;
        StringBuilder response = new StringBuilder();

        while ((inputLine = in.readLine()) != null) {
            response.append(inputLine);
        }

        in.close();
        connection.disconnect();
    }
}

2. 使用Apache HttpClient实现HTTP长连接：

首先，需要添加Apache HttpClient的依赖。在Maven项目中，可以在pom.xml文件中添加以下依赖：

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.13</version>
</dependency>

然后，使用PoolingHttpClientConnectionManager创建一个连接池，并使用CloseableHttpClient发送请求：

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

public class HttpClientLongConnection {
    public static void main(String[] args) {
        PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager();
        connectionManager.setMaxTotal(10);
        connectionManager.setDefaultMaxPerRoute(5);

        try (CloseableHttpClient httpClient = HttpClients.custom()
                .setConnectionManager(connectionManager)
                .build()) {

            HttpGet httpGet = new HttpGet("http://example.com");
            HttpResponse response = httpClient.execute(httpGet);

            System.out.println("Response Status: " + response.getStatusLine());

            httpClient.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3. 使用OkHttp实现HTTP长连接：

首先，需要添加OkHttp的依赖。在Maven项目中，可以在pom.xml文件中添加以下依赖：

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>4.9.1</version>
</dependency>

然后，使用OkHttpClient发送请求：

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class OkHttpLongConnection {
    public static void main(String[] args) {
        OkHttpClient client = new OkHttpClient.Builder()
                .connectionPool(new okhttp3.ConnectionPool(10, 5, java.util.concurrent.TimeUnit.MINUTES))
                .build();

        Request request = new Request.Builder()
                .url("http://example.com")
                .build();

        try (Response response = client.newCall(request).execute()) {
            System.out.println("Response Status: " + response.code());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

以上示例展示了如何使用Java实现HTTP长连接。注意，长连接的使用需要服务器端的支持，如果服务器不支持长连接，那么客户端的长连接请求可能会被拒绝。