HTTP 协议在 Java 中的运用

HTTP（超文本传输协议）是一种用于在网络上传输数据的协议，它基于 TCP/IP 协议。在 Java 中，我们可以使用多种库来处理 HTTP 请求和响应。最常用的库是 Java 的内置库 java.net.HttpURLConnection 和第三方库 Apache HttpClient 和 OkHttp。

1. 使用 HttpURLConnection：

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

public class Main {
    public static void main(String[] args) {
        try {
            String apiUrl = "https://api.example.com/data"; // 替换为你要请求的 API 地址
            URL url = new URL(apiUrl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("GET");
            connection.setRequestProperty("User-Agent", "Mozilla/5.0");
            connection.setConnectTimeout(5000);
            connection.setReadTimeout(5000);

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

            if (responseCode == HttpURLConnection.HTTP_OK) {
                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();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

2. 使用 Apache HttpClient：

首先，需要添加 Apache HttpClient 库的依赖。如果你使用 Maven，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，可以使用以下代码发送 HTTP 请求：

import org.apache.http.HttpEntity;
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 Main {
    public static void main(String[] args) {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        String apiUrl = "https://api.example.com/data"; // 替换为你要请求的 API 地址
        HttpGet httpGet = new HttpGet(apiUrl);
        httpGet.setHeader("User-Agent", "Mozilla/5.0");

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

            if (responseCode == 200) {
                HttpEntity httpEntity = httpResponse.getEntity();
                String response = EntityUtils.toString(httpEntity);
                System.out.println("Response: " + response);
            } else {
                System.out.println("GET request failed");
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                httpClient.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

3. 使用 OkHttp：

首先，需要添加 OkHttp 库的依赖。如果你使用 Maven，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，可以使用以下代码发送 HTTP 请求：

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

public class Main {
    public static void main(String[] args) {
        OkHttpClient client = new OkHttpClient();
        String apiUrl = "https://api.example.com/data"; // 替换为你要请求的 API 地址
        Request request = new Request.Builder()
                .url(apiUrl)
                .header("User-Agent", "Mozilla/5.0")
                .build();

        try (Response response = client.newCall(request).execute()) {
            int responseCode = response.code();
            System.out.println("Response Code: " + responseCode);

            if (responseCode == 200) {
                String response = response.body().string();
                System.out.println("Response: " + response);
            } else {
                System.out.println("GET request failed");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

以上示例展示了如何在 Java 中使用 HttpURLConnection、Apache HttpClient 和 OkHttp 库发送 HTTP GET 请求。你可以根据需要修改这些示例以适应不同的 API 和请求类型（如 POST、PUT、DELETE 等）。