Java 中 HTTP 客户端如何实现

在 Java 中，可以使用多种库来实现 HTTP 客户端。这里，我将向您展示如何使用 Java 的 java.net.HttpURLConnection 类以及第三方库 Apache HttpClient 和 OkHttp 来实现 HTTP 客户端。

1. 使用 HttpURLConnection（Java 标准库）

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

public class HttpClientExample {
    public static void main(String[] args) {
        try {
            String apiUrl = "https://api.example.com/data";
            URL url = new URL(apiUrl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("GET");
            connection.setRequestProperty("User-Agent", "Mozilla/5.0");
            connection.setRequestProperty("Accept", "application/json");

            if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
                System.out.println("GET request failed");
                return;
            }

            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());

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

2. 使用 Apache HttpClient（第三方库）

首先，您需要将 Apache HttpClient 库添加到项目的依赖中。如果您使用 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，您可以使用以下代码实现 HTTP 客户端：

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 HttpClientExample {
    public static void main(String[] args) {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        String apiUrl = "https://api.example.com/data";
        HttpGet httpGet = new HttpGet(apiUrl);
        httpGet.setHeader("User-Agent", "Mozilla/5.0");
        httpGet.setHeader("Accept", "application/json");

        try {
            HttpResponse httpResponse = httpClient.execute(httpGet);
            int statusCode = httpResponse.getStatusLine().getStatusCode();

            if (statusCode != 200) {
                System.out.println("GET request failed");
                return;
            }

            HttpEntity httpEntity = httpResponse.getEntity();
            String response = EntityUtils.toString(httpEntity);
            System.out.println("Response: " + response);

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

3. 使用 OkHttp（第三方库）

首先，您需要将 OkHttp 库添加到项目的依赖中。如果您使用 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，您可以使用以下代码实现 HTTP 客户端：

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

public class HttpClientExample {
    public static void main(String[] args) {
        OkHttpClient client = new OkHttpClient();
        String apiUrl = "https://api.example.com/data";
        Request request = new Request.Builder()
                .url(apiUrl)
                .header("User-Agent", "Mozilla/5.0")
                .header("Accept", "application/json")
                .build();

        try (Response response = client.newCall(request).execute()) {
            if (!response.isSuccessful()) {
                System.out.println("GET request failed");
                return;
            }

            String responseBody = response.body().string();
            System.out.println("Response: " + responseBody);

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

以上示例展示了如何使用 Java 标准库中的 HttpURLConnection 类以及第三方库 Apache HttpClient 和 OkHttp 来实现 HTTP 客户端。您可以根据项目需求和个人喜好选择合适的库。