java如何发送http请求

使用 HttpURLConnection

Java 原生提供了 HttpURLConnection 类用于发送 HTTP 请求。以下是一个简单的 GET 请求示例：

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

public class HttpUrlConnectionExample {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://example.com/api");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");

        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();

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

使用 Apache HttpClient

Apache HttpClient 是一个更强大的 HTTP 客户端库。以下是发送 POST 请求的示例：

import org.apache.http.HttpResponse;
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 HttpClientExample {
    public static void main(String[] args) throws Exception {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost("https://example.com/api");

        String json = "{\"key\":\"value\"}";
        StringEntity entity = new StringEntity(json);
        httpPost.setEntity(entity);
        httpPost.setHeader("Content-type", "application/json");

        HttpResponse response = httpClient.execute(httpPost);
        String responseString = EntityUtils.toString(response.getEntity(), "UTF-8");

        System.out.println("Response: " + responseString);
        httpClient.close();
    }
}

使用 OkHttp

OkHttp 是一个现代高效的 HTTP 客户端。以下是发送异步 GET 请求的示例：

import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import java.io.IOException;

public class OkHttpExample {
    public static void main(String[] args) {
        OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder()
                .url("https://example.com/api")
                .build();

        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                e.printStackTrace();
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                System.out.println("Response: " + response.body().string());
            }
        });
    }
}

使用 Spring RestTemplate

Spring 框架提供了 RestTemplate 用于简化 HTTP 请求：

import org.springframework.web.client.RestTemplate;

public class RestTemplateExample {
    public static void main(String[] args) {
        RestTemplate restTemplate = new RestTemplate();
        String result = restTemplate.getForObject("https://example.com/api", String.class);
        System.out.println("Response: " + result);
    }
}

使用 Java 11+ HttpClient

Java 11 引入了新的 HttpClient API：

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class JavaHttpClientExample {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://example.com/api"))
                .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println("Response: " + response.body());
    }
}

每种方法各有优缺点，HttpURLConnection 是 Java 原生支持，Apache HttpClient 功能强大但较重量级，OkHttp 性能优异，RestTemplate 适合 Spring 项目，Java 11+ HttpClient 是最新的标准 API。根据项目需求和 Java 版本选择合适的方法。