使用RestTemplate exchange方法时抛出InvalidUseOfMatchersException问题

问题重现 #

运行以下Mockito测试用例时抛出异常：

测试代码 #

import org.mockito.junit.MockitoJunitRunner;
import static org.mockito.Mockito.when;
import static org.mockito.ArgumentMatchers.*;

@RunWith(MockitoJunitRunner.class)
public class APITest{

private RestTemplate restTemplate;

private HttpHeaders httpHeaders;

private APIService apiService;

private String url = "http://sample.com";

private String APIRequestJson = "{ \"text\":\"Greetings\" }";

private String APIResponse;

@Before
public void setup(){
    restTemplate = mock(RestTemplate.class);
    httpHeaders = mock(HttpHeaders.class);
    apiService = new APIService(restTemplate,httpHeaders);
    APIResponse = "{ \"result\":\"ok\" }";
}

@Test
public void validate_msg_bad_request() {
    ResponseEntity<?> responseEntity = new ResponseEntity<>(APIResponse, HttpStatus.BAD_REQUEST);

    when(restTemplate.exchange(
       anyString(), eq(HttpMethod.POST), any(HttpEntity.class), eq(String.class))
    ).thenReturn((ResponseEntity<String>) responseEntity);

    msgResponse = apiService.validateMsg(APIRequestJson, url);

    Assert.assertequals("400",msgResponse.getStatusCode());
}

}

异常信息 #

Method threw 'org.mockito.exceptions.misusing.InvalidUseOfMatchersException' exception.Cannot evaluate org.springframework.web.client.RestTemplate$MockitoMock$935390488.toString()

org.mockito.exceptions.misusing.WrongTypeOfReturnValue:
ResponseEntity cannot be returned by toString()
toString() should return String

原因分析 #

1. 不安全的泛型类型转换
   创建ResponseEntity<?>实例后强制转为ResponseEntity<String>，这种转换会丢失泛型类型信息，Mockito在处理存根返回值时，无法正确识别返回值的实际类型，进而错误地将ResponseEntity实例关联到某个toString()方法的返回逻辑上（泛型擦除导致内部类型混淆），触发WrongTypeOfReturnValue错误。

2. 测试代码的语法问题

• msgResponse未声明具体类型，编译器无法正确推断类型，加剧了Mockito的类型识别混乱；
• 断言方法名拼写错误（Assert.assertequals应为Assert.assertEquals），且断言参数类型不匹配（getStatusCode()返回HttpStatus枚举，直接与字符串"400"比较会导致类型不兼容）。

修复方案 #

1. 修正ResponseEntity的创建方式 #

直接创建匹配泛型类型的ResponseEntity<String>实例，避免强制转换：

ResponseEntity<String> responseEntity = new ResponseEntity<>(APIResponse, HttpStatus.BAD_REQUEST);

2. 修正存根语句 #

使用正确类型的返回值，无需强制转换：

when(restTemplate.exchange(
    anyString(), eq(HttpMethod.POST), any(HttpEntity.class), eq(String.class))
).thenReturn(responseEntity);

3. 修正测试代码的其他问题 #

• 声明msgResponse的具体类型；
• 修正断言方法名及参数匹配逻辑：

@Test
public void validate_msg_bad_request() {
    ResponseEntity<String> responseEntity = new ResponseEntity<>(APIResponse, HttpStatus.BAD_REQUEST);

    when(restTemplate.exchange(
       anyString(), eq(HttpMethod.POST), any(HttpEntity.class), eq(String.class))
    ).thenReturn(responseEntity);

    ResponseEntity<String> msgResponse = apiService.validateMsg(APIRequestJson, url);

    Assert.assertEquals(HttpStatus.BAD_REQUEST, msgResponse.getStatusCode());
    // 或按字符串比较
    // Assert.assertEquals("400", msgResponse.getStatusCode().toString());
}

内容的提问来源于stack exchange，提问作者CodeLearner