Java连接HTTP服务器是网络编程中的常见需求,广泛应用于数据交互、API调用、文件传输等场景，Java提供了多种方式实现HTTP连接，包括传统的HttpURLConnection、第三方库如Apache HttpClient和OkHttp，以及Java 11+引入的HttpClient，本文将详细介绍这些方法的核心原理、实现步骤及注意事项，帮助开发者根据实际需求选择合适的方案。

使用HttpURLConnection实现HTTP连接

HttpURLConnection是Java标准库中内置的HTTP客户端类,无需额外依赖，适合简单的HTTP请求场景，其核心步骤包括创建连接、设置请求参数、发送请求、处理响应等，以下以GET和POST请求为例说明实现方法。

GET请求示例

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
public class HttpGetExample {
    public static void main(String[] args) throws Exception {
        String urlString = "https://example.com/api/data";
        URL url = new URL(urlString);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        // 设置请求方法
        connection.setRequestMethod("GET");
        // 设置请求头（可选）
        connection.setRequestProperty("User-Agent", "Java/1.8");
        // 获取响应码
        int responseCode = connection.getResponseCode();
        System.out.println("Response Code: " + responseCode);
        if (responseCode == HttpURLConnection.HTTP_OK) {
            // 读取响应数据
            BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String inputLine;
            StringBuilder response = new StringBuilder();
            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();
            System.out.println("Response: " + response.toString());
        } else {
            System.out.println("GET request failed");
        }
        connection.disconnect();
    }
}

POST请求示例

POST请求需处理请求体数据,通常用于提交表单或JSON数据，以下以提交JSON数据为例：

import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
public class HttpPostExample {
    public static void main(String[] args) throws Exception {
        String urlString = "https://example.com/api/submit";
        String jsonInputString = "{\"name\":\"John\", \"age\":30}";
        URL url = new URL(urlString);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "application/json");
        connection.setDoOutput(true);
        try (OutputStream os = connection.getOutputStream()) {
            byte[] input = jsonInputString.getBytes("utf-8");
            os.write(input, 0, input.length);
        }
        int responseCode = connection.getResponseCode();
        System.out.println("Response Code: " + responseCode);
        connection.disconnect();
    }
}

HttpURLConnection的优缺点

优点：

• 无需第三方依赖,Java标准库支持
• 适合简单的HTTP请求场景
• 支持基本HTTP方法（GET、POST、PUT等）和自定义请求头

缺点：

• 功能相对基础,缺乏高级特性（如连接池、异步请求）
• 处理复杂逻辑（如文件上传、多部分表单）代码较为繁琐
• 性能较低,不适合高并发场景

使用Apache HttpClient实现HTTP连接

Apache HttpClient是功能强大的开源HTTP客户端库，支持连接池、重试机制、异步请求等高级特性，适合企业级应用，需添加依赖（Maven）：

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

基本GET请求示例

import org.apache.http.client.methods.CloseableHttpResponse;
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.util.EntityUtils;
public class HttpClientGetExample {
    public static void main(String[] args) throws Exception {
        try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
            HttpGet request = new HttpGet("https://example.com/api/data");
            request.addHeader("User-Agent", "Java/HttpClient");
            try (CloseableHttpResponse response = httpClient.execute(request)) {
                String result = EntityUtils.toString(response.getEntity());
                System.out.println("Response: " + result);
            }
        }
    }
}

POST请求示例（JSON数据）

import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;
public class HttpClientPostExample {
    public static void main(String[] args) throws Exception {
        try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
            HttpPost post = new HttpPost("https://example.com/api/submit");
            post.setHeader("Content-Type", "application/json");
            post.setEntity(new StringEntity("{\"name\":\"John\", \"age\":30}"));
            try (CloseableHttpResponse response = httpClient.execute(post)) {
                String result = EntityUtils.toString(response.getEntity());
                System.out.println("Response: " + result);
            }
        }
    }
}

HttpClient的核心优势

• 连接池管理：通过PoolingHttpClientConnectionManager复用连接，提升性能
• 重试机制：支持自动重试失败的请求
• 异步请求：通过Future或回调实现非阻塞IO
• 灵活的请求配置：支持自定义Cookie、认证、拦截器等

使用Java 11+ HttpClient实现HTTP连接

Java 11引入了标准化的HttpClient，取代了旧的HttpURLConnection，支持异步请求和WebSocket，是现代Java应用的首选。

同步GET请求示例

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
public class Java11HttpClientExample {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://example.com/api/data"))
                .header("User-Agent", "Java/11")
                .build();
        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println("Response: " + response.body());
    }
}

异步POST请求示例

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.util.concurrent.CompletableFuture;
public class AsyncHttpClientExample {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://example.com/api/submit"))
                .header("Content-Type", "application/json")
                .POST(HttpRequest.BodyPublishers.ofString("{\"name\":\"John\"}"))
                .build();
        CompletableFuture<HttpResponse<String>> future = client.sendAsync(request, HttpResponse.BodyHandlers.ofString());
        future.thenAccept(response -> System.out.println("Response: " + response.body()));
        // 等待异步请求完成
        future.join();
    }
}

Java 11 HttpClient的特性

• 简洁的API：链式调用设计，代码更易读
• 原生支持异步：通过CompletableFuture实现非阻塞IO
• 响应式编程支持：可与Flow API结合
• WebSocket支持：内置WebSocket客户端实现

HTTP连接的常见问题及解决方案

相关问答FAQs

Q1: 如何处理HTTP请求中的HTTPS证书问题？
A: 对于自签名证书的HTTPS服务器，可以通过自定义SSL上下文禁用证书验证（仅建议测试环境使用）：

TrustManager[] trustAllCerts = new TrustManager[] {
    new X509TrustManager() {
        public java.security.cert.X509Certificate[] getAcceptedIssuers() { return null; }
        public void checkClientTrusted(X509Certificate[] certs, String authType) {}
        public void checkServerTrusted(X509Certificate[] certs, String authType) {}
    }
};
SSLContext sc = SSLContext.getInstance("SSL");
sc.init(null, trustAllCerts, new java.security.SecureRandom());
HttpsURLConnection.setDefaultSSLSocketFactory(sc.getSocketFactory());

Q2: 如何实现HTTP连接的自动重试机制？
A: 可以结合RetryPolicy实现自定义重试逻辑，以下是Apache HttpClient的重试示例：

HttpRequestRetryHandler retryHandler = (exception, executionCount, context) -> {
    if (executionCount >= 3) {
        return false; // 超过最大重试次数
    }
    if (exception instanceof NoHttpResponseException) {
        return true; // 服务器无响应时重试
    }
    return false;
};
CloseableHttpClient httpClient = HttpClients.custom()
        .setRetryHandler(retryHandler)
        .build();