在Java中进行HTTP调用是一个常见的需求，尤其是在开发RESTful API、微服务架构或者集成第三方服务时。本文将详细介绍几种主流的Java HTTP调用组件，包括 HttpURLConnection、Apache HttpClient、OkHttp 和 Spring WebClient，并提供示例代码和使用场景。

一、HttpURLConnection

1. 简介

HttpURLConnection 是Java官方提供的一种轻量级HTTP调用方式，无需额外引入第三方库，适用于简单场景。

2. 使用示例

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

public class HttpURLConnectionExample {

    private static final String GET_URL = "http://example.com/api";

    public static void main(String[] args) {
        try {
            URL url = new URL(GET_URL);
            HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.setRequestMethod("GET");

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

            BufferedReader in = new BufferedReader(new InputStreamReader(httpURLConnection.getInputStream()));
            String inputLine;
            StringBuffer response = new StringBuffer();
            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();

            System.out.println("Response Body: " + response.toString());

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

二、Apache HttpClient

1. 简介

Apache HttpClient 是一个功能强大且广泛使用的HTTP客户端库，提供了丰富的功能和灵活的配置选项。

2. 引入依赖

Maven 依赖：

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

3. 使用示例

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.util.EntityUtils;

public class HttpClientExample {

    private static final String GET_URL = "http://example.com/api";

    public static void main(String[] args) {
        try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
            HttpGet request = new HttpGet(GET_URL);
            HttpResponse response = httpClient.execute(request);

            int statusCode = response.getStatusLine().getStatusCode();
            System.out.println("Response Code: " + statusCode);
            String responseBody = EntityUtils.toString(response.getEntity());
            System.out.println("Response Body: " + responseBody);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

三、OkHttp

1. 简介

OkHttp 是一个现代化的HTTP客户端库，具有高效、易用等特点，特别适合在Android应用中使用。

2. 引入依赖

Maven 依赖：

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>4.9.3</version>
</dependency>

3. 使用示例

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

import java.io.IOException;

public class OkHttpExample {

    private static final String GET_URL = "http://example.com/api";

    public static void main(String[] args) {
        OkHttpClient client = new OkHttpClient();

        Request request = new Request.Builder()
                .url(GET_URL)
                .build();

        try (Response response = client.newCall(request).execute()) {
            if (!response.isSuccessful()) {
                throw new IOException("Unexpected code: " + response);
            }

            System.out.println("Response Code: " + response.code());
            System.out.println("Response Body: " + response.body().string());

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

四、Spring WebClient

1. 简介

Spring WebClient 是Spring Framework 5中引入的非阻塞响应式HTTP客户端，功能强大且支持响应式编程模型。

2. 引入依赖

Maven 依赖：

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

3. 使用示例

import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Mono;

public class WebClientExample {

    private static final String GET_URL = "http://example.com/api";

    public static void main(String[] args) {
        WebClient webClient = WebClient.create();
        
        Mono<String> response = webClient.get()
                .uri(GET_URL)
                .retrieve()
                .bodyToMono(String.class);

        response.subscribe(System.out::println);
    }
}

使用场景比较

总结

选择合适的HTTP调用组件取决于实际的需求和使用场景。HttpURLConnection 适用于简单的HTTP调用，Apache HttpClient 功能强大，适合复杂需求，OkHttp 以简洁和高效著称，非常适合Android项目，而 Spring WebClient 则是非阻塞响应式HTTP调用的最佳选择。