如何用Spock模拟Spring RestTemplate的getForEntity方法解决空指针问题

业务代码 #

Product getProducts(ProductSearch request) {
  Map<String, Object> queryParam = new HashMap<>();
  queryParam.put("id", request.getProductId());
  queryParam.put("category", request.getCategory());

  UriComponentsBuilder uriComponentsBuilder = UriComponentsBuilder.fromHttpUrl(url);
  queryParam.forEach(uriComponentsBuilder::queryParam);

  String url = uriComponentsBuilder.toUriString();
  return restTemplate.getForEntity(url, Product.class).getBody();
}

问题场景 #

尝试用Spock模拟RestTemplate的getForEntity方法，但测试代码运行时抛出NullPointerException，测试代码如下：

class ProductClientSpec extends Specification {
  RestTemplate restTemplate = Mock()
  ProductClient legacyOpenSlotsClient = new ProductClient(restTemplate)

  def setup() {}

  def 'should return product'() {
    given:
    ProductSearch request = createProductRequest()
    Product product = createProductResponse()

    when:
    ProductClient.getProducts(request)

    then:
    1 * restTemplate.getForEntity("http://localhost:8080/products", Product.class).getBody() >> product
  }
}

问题原因 #

1. 链式打桩逻辑错误：Spock中未被显式模拟的方法默认返回null，restTemplate.getForEntity(...)返回null后，调用getBody()自然触发空指针。
2. 方法调用错误：业务代码中getProducts是实例方法，但测试里用了静态调用ProductClient.getProducts(request)，不符合代码逻辑。

正确解决方案 #

方案1：Mock ResponseEntity对象 #

先模拟getForEntity返回一个Mock的ResponseEntity，再对该对象的getBody()方法打桩：

class ProductClientSpec extends Specification {
  RestTemplate restTemplate = Mock()
  ProductClient productClient = new ProductClient(restTemplate)

  def 'should return product correctly'() {
    given:
    ProductSearch request = createProductRequest()
    Product product = createProductResponse()
    ResponseEntity<Product> mockResponseEntity = Mock()

    when:
    Product result = productClient.getProducts(request)

    then:
    // 匹配任意URL，也可根据业务逻辑写具体带参数的URL
    1 * restTemplate.getForEntity(_ as String, Product.class) >> mockResponseEntity
    1 * mockResponseEntity.getBody() >> product
    result == product
  }
}

方案2：直接返回真实ResponseEntity实例 #

如果不需要Mock ResponseEntity的其他方法，直接返回真实的ResponseEntity对象更简洁：

class ProductClientSpec extends Specification {
  RestTemplate restTemplate = Mock()
  ProductClient productClient = new ProductClient(restTemplate)

  def 'should return product correctly'() {
    given:
    ProductSearch request = createProductRequest()
    Product product = createProductResponse()

    when:
    Product result = productClient.getProducts(request)

    then:
    1 * restTemplate.getForEntity(_ as String, Product.class) >> new ResponseEntity<>(product, HttpStatus.OK)
    result == product
  }
}

额外注意事项 #

• 必须调用实例的getProducts方法，而非静态调用。
• 不要写死URL字符串，业务代码会拼接查询参数，可用_匹配任意URL，或用Spock匹配器（如contains("id=123")）匹配具体参数，避免因参数变化导致测试失败。

内容的提问来源于stack exchange，提问作者BreenDeen