单元测试问题：RestTemplate Mock始终返回Null

问题 #

我在为实现BankId认证功能的BankIdClient类编写单元测试时遇到了问题：测试中customRestTemplate.postForEntity(...)返回的authResponse始终为null，Mockito的模拟配置没有生效，已经排查3小时仍未解决。

测试代码如下：

package se.kt.client;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Mockito;
import org.mockito.junit.MockitoJUnitRunner;
import org.mockito.junit.jupiter.MockitoExtension;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.ResponseEntity;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.web.client.RestTemplate;
import org.springframework.http.HttpEntity;
import se.kt.common.vo.PublicApplicationForm;
import se.kt.models.BankIdAuthRequest;
import se.kt.models.BankIdAuthResponse;
import se.kt.models.BankIdCollectResponse;
import static org.mockito.ArgumentMatchers.any;
import static org.junit.Assert.assertTrue;
import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.ArgumentMatchers.eq;


@RunWith(MockitoJUnitRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class BankIdClientTest {

    private final RestTemplate restTemplate = Mockito.mock(RestTemplate.class);

    @InjectMocks
    private BankIdClient bankIdClient;

    @BeforeEach
    public void setUp() {
    }

    @Test
    public void testBankIdAuthentication_success() throws InterruptedException {
        PublicApplicationForm form = new PublicApplicationForm();
        form.setSsn("123456-7890");
        form.setIp_address("123.123.123.123");

        BankIdAuthRequest authRequest = bankIdClient.authRequestFromApplicationForm(form, "123");

        BankIdAuthResponse authResponse = new BankIdAuthResponse();
        authResponse.setOrderRef("123456");

        BankIdCollectResponse collectResponse = new BankIdCollectResponse();
        collectResponse.setStatus("completed");

        Mockito.when(restTemplate.postForEntity(anyString(), any(HttpEntity.class), any()))
                .thenReturn(ResponseEntity.ok(authResponse));
        Mockito.when(restTemplate.getForEntity(anyString(), any()))
                .thenReturn(ResponseEntity.ok(collectResponse));

        assertTrue(bankIdClient.bankIdAuthentication(authRequest));
    }
}

待测试的BankIdClient类代码：

package se.kt.client;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Component;
import org.springframework.web.client.RestTemplate;
import se.kt.common.domain.AbstractApplicationForm;
import se.kt.common.vo.PublicApplicationForm;
import se.kt.models.BankIdAuthRequest;
import se.kt.models.BankIdAuthResponse;
import se.kt.models.BankIdCollectResponse;

import javax.validation.constraints.AssertFalse;
import javax.validation.constraints.AssertTrue;
import java.util.Objects;

@Component
public class BankIdClient {

    private static final Logger log = LoggerFactory.getLogger(BankIdClient.class);
    private final RestTemplate customRestTemplate;
    private static final String CONTENT_TYPE = "Content-Type";

    @Value("${BankId.AuthUrl}")
    private String bankIdAuthUrl;

    @Value("${BankId.CollectUrl}")
    private String bankIdCollectUrl;

    @Value("${BankId.SecretKey}")
    private String bankIdSecretKey;

    public BankIdClient(RestTemplate customRestTemplate) {
        this.customRestTemplate = customRestTemplate;
    }

    public BankIdAuthRequest authRequestFromApplicationForm(PublicApplicationForm form, String jobId) {
        BankIdAuthRequest bankIdAuthRequest = new BankIdAuthRequest();
        bankIdAuthRequest.setPno(form.getSsn());
        bankIdAuthRequest.setIpAddress(form.getIp_address());
        bankIdAuthRequest.setRefID(jobId);
        bankIdAuthRequest.setSecretKey(bankIdSecretKey);
        bankIdAuthRequest.setAvsikt("Kt application");

        return bankIdAuthRequest;
    }

    public boolean bankIdAuthentication(BankIdAuthRequest bankIdAuthRequest) throws InterruptedException {
        //Setup header and body for request.
        HttpHeaders headers = new HttpHeaders();
        headers.add(CONTENT_TYPE, MediaType.APPLICATION_JSON.toString());

        ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter();

        try {
            String bankIdAuthFormJson = ow.writeValueAsString(bankIdAuthRequest);

            HttpEntity<String> httpEntity = new HttpEntity<>(bankIdAuthFormJson, headers);
            ResponseEntity<BankIdAuthResponse> authResponse = customRestTemplate.postForEntity(bankIdAuthUrl, httpEntity, BankIdAuthResponse.class);

            bankIdCollectUrl += Objects.requireNonNull(authResponse.getBody()).getOrderRef();

            ResponseEntity<BankIdCollectResponse> collectResponse;

            do {
                collectResponse = customRestTemplate.getForEntity(bankIdCollectUrl, BankIdCollectResponse.class);

                Thread.sleep(1500);

                if (Objects.requireNonNull(collectResponse.getBody()).getStatus().equals("completed"))
                    return true;
                if (Objects.requireNonNull(collectResponse.getBody()).getStatus().equals("failed"))
                    return false;
            } while (Objects.requireNonNull(collectResponse.getBody()).getStatus().equals("progress"));

        } catch (JsonProcessingException e) {
            log.info(e.getMessage());
        } catch (NullPointerException e) {
            log.info(e.toString());
            log.info("BankId API not responding correctly. Check server connection");
        }
        return false;
    }

    public void cancelBankIdAuthentication(@Value("${BankId.CancelUrl}") String bankIdCancelUrl) {
        customRestTemplate.postForEntity(bankIdCancelUrl, null, String.class);
    }
}

问题分析与修复方案 #

1. Spring容器干扰Mockito注入 #

测试类同时使用了@SpringBootTest和Mockito的@InjectMocks，Spring会自动创建BankIdClient的实例，覆盖Mockito的注入逻辑，导致你手动创建的restTemplate Mock对象根本没被用到。

修复：移除@SpringBootTest和JUnit4的@RunWith注解，改用JUnit5的MockitoExtension，让Mockito完全控制对象生命周期：

@ExtendWith(MockitoExtension.class)
class BankIdClientTest {
    // ... 其他代码
}

2. Mock对象未正确关联到目标类 #

测试中手动创建RestTemplate Mock对象，但没有通过@Mock注解标记，@InjectMocks无法识别并注入到BankIdClient的customRestTemplate字段。

修复：用@Mock注解创建Mock对象：

@Mock
private RestTemplate restTemplate;

@InjectMocks
private BankIdClient bankIdClient;

3. @Value属性未初始化 #

BankIdClient中的bankIdAuthUrl、bankIdSecretKey等属性依赖配置文件，单元测试中没有加载配置，这些属性为null，会导致postForEntity的URL参数为null，Mockito匹配器无法匹配，进而返回null。

修复：用ReflectionTestUtils手动设置私有属性值：

// 在测试方法开头添加
ReflectionTestUtils.setField(bankIdClient, "bankIdAuthUrl", "https://mock-auth-url.com");
ReflectionTestUtils.setField(bankIdClient, "bankIdCollectUrl", "https://mock-collect-url.com/");
ReflectionTestUtils.setField(bankIdClient, "bankIdSecretKey", "mock-secret-key");

4. Mock匹配器不够精确 #

原测试中使用any()匹配返回类型，可能导致匹配模糊。建议使用精确的类型匹配：

when(restTemplate.postForEntity(anyString(), any(), eq(BankIdAuthResponse.class)))
        .thenReturn(ResponseEntity.ok(authResponse));
when(restTemplate.getForEntity(anyString(), eq(BankIdCollectResponse.class)))
        .thenReturn(ResponseEntity.ok(collectResponse));

修改后的完整测试代码 #

package se.kt.client;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import org.springframework.http.ResponseEntity;
import org.springframework.test.util.ReflectionTestUtils;
import org.springframework.web.client.RestTemplate;
import se.kt.common.vo.PublicApplicationForm;
import se.kt.models.BankIdAuthRequest;
import se.kt.models.BankIdAuthResponse;
import se.kt.models.BankIdCollectResponse;

import static org.junit.jupiter.api.Assertions.assertTrue;
import static org.mockito.ArgumentMatchers.any;
import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.ArgumentMatchers.eq;
import static org.mockito.Mockito.when;

@ExtendWith(MockitoExtension.class)
class BankIdClientTest {

    @Mock
    private RestTemplate restTemplate;

    @InjectMocks
    private BankIdClient bankIdClient;

    @Test
    void testBankIdAuthentication_success() throws InterruptedException {
        // 初始化BankIdClient的配置属性
        ReflectionTestUtils.setField(bankIdClient, "bankIdAuthUrl", "https://mock-auth-url.com");
        ReflectionTestUtils.setField(bankIdClient, "bankIdCollectUrl", "https://mock-collect-url.com/");
        ReflectionTestUtils.setField(bankIdClient, "bankIdSecretKey", "mock-secret-key");

        PublicApplicationForm form = new PublicApplicationForm();
        form.setSsn("123456-7890");
        form.setIp_address("123.123.123.123");

        BankIdAuthRequest authRequest = bankIdClient.authRequestFromApplicationForm(form, "123");

        BankIdAuthResponse authResponse = new BankIdAuthResponse();
        authResponse.setOrderRef("123456");

        BankIdCollectResponse collectResponse = new BankIdCollectResponse();
        collectResponse.setStatus("completed");

        // 精确模拟post和get请求
        when(restTemplate.postForEntity(anyString(), any(), eq(BankIdAuthResponse.class)))
                .thenReturn(ResponseEntity.ok(authResponse));
        when(restTemplate.getForEntity(anyString(), eq(BankIdCollectResponse.class)))
                .thenReturn(ResponseEntity.ok(collectResponse));

        assertTrue(bankIdClient.bankIdAuthentication(authRequest));
    }
}

内容的提问来源于stack exchange，提问作者Mr.Gomer