如何用Mockito Stub HttpClient的send方法并返回预期响应？

核心思路 #

要Mock HttpClient.send() 方法并返回自定义JSON响应，需借助Mockito完成Mock对象的创建与Stub配置，同时优先通过依赖注入重构业务代码，让HttpClient可被替换为Mock实例（避免直接在方法内new HttpClient导致无法Mock）。

步骤1：重构业务代码（关键） #

将HttpClient通过构造方法注入到业务类中，而非在方法内直接实例化：

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.net.http.HttpRequest.BodyPublishers;
import java.net.http.HttpResponse.BodyHandlers;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class ApiService {
    private static final Logger LOGGER = LoggerFactory.getLogger(ApiService.class);
    private final HttpClient httpClient;

    // 构造方法注入HttpClient，方便测试时替换为Mock对象
    public ApiService(HttpClient httpClient) {
        this.httpClient = httpClient;
    }

    public String callExternalApi(String requestBody, String authorizationHeader) throws Exception {
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("http://localhost:8080/api"))
                .POST(BodyPublishers.ofString(requestBody))
                .header("Authorization", authorizationHeader)
                .header("Content-Type", "application/json")
                .build();

        HttpResponse<String> response = httpClient.send(request, BodyHandlers.ofString());
        LOGGER.debug(response.body());
        return response.body();
    }
}

步骤2：编写JUnit5测试用例（结合Mockito） #

通过Mockito创建Mock的HttpClient和HttpResponse，Stub send() 方法返回预期的JSON响应：

import static org.mockito.ArgumentMatchers.any;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
import static org.mockito.Mockito.verify;

import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import org.junit.jupiter.api.Test;

public class ApiServiceTest {

    @Test
    void testCallExternalApi_ReturnsExpectedJson() throws Exception {
        // 1. 创建Mock的HttpClient实例
        HttpClient mockHttpClient = mock(HttpClient.class);

        // 2. 准备预期的API响应JSON
        String expectedJsonResponse = "{\"id\": 1, \"name\": \"测试实体\", \"status\": \"success\"}";

        // 3. Mock HttpResponse对象，配置返回的body和状态码
        HttpResponse<String> mockResponse = mock(HttpResponse.class);
        when(mockResponse.body()).thenReturn(expectedJsonResponse);
        when(mockResponse.statusCode()).thenReturn(200); // 可选：模拟成功状态码

        // 4. Stub HttpClient的send方法，指定返回Mock的HttpResponse
        when(mockHttpClient.send(any(HttpRequest.class), any(HttpResponse.BodyHandler.class)))
                .thenReturn(mockResponse);

        // 5. 初始化业务类，传入Mock的HttpClient
        ApiService apiService = new ApiService(mockHttpClient);

        // 6. 执行测试方法
        String actualResponse = apiService.callExternalApi("{\"param\": \"test\"}", "Bearer auth-token");

        // 7. 验证结果是否符合预期
        assert actualResponse.equals(expectedJsonResponse);
        // 可选：验证send方法是否被正确调用
        verify(mockHttpClient).send(any(HttpRequest.class), any(HttpResponse.BodyHandler.class));
    }
}

关键说明 #

• 依赖注入的必要性：如果业务代码中直接使用HttpClient.newHttpClient()创建实例，会导致无法替换为Mock对象，因此必须重构为依赖注入形式（构造方法/Setter注入均可）。
• Stub的灵活性：可以根据测试场景，修改mockResponse的配置，比如返回404状态码、错误JSON等，模拟不同的API响应情况。
• 参数匹配：示例中使用any()匹配任意HttpRequest和BodyHandler，若需要更精确的匹配（比如验证请求URL、Header是否正确），可以替换为argThat()自定义匹配逻辑。

内容的提问来源于stack exchange，提问作者BSM