Spring REST Docs has firmly established itself as an indispensable tool for producing accurate, reliable, and maintainable API documentation. By leveraging tests to generate documentation snippets, it ensures that what you document is exactly what your API delivers. This test-driven approach is a significant leap forward from manual documentation methods, which are often prone to falling out of sync with the actual codebase. However, the path to perfect documentation is not always straightforward. Developers can encounter cryptic errors that stem from the intricate interactions between the Spring MVC framework, the testing environment, and the mechanics of documentation generation. One such perplexing issue is the java.lang.NoSuchMethodException when a controller attempts to bind request parameters to a java.util.List.
This article provides a comprehensive exploration of this specific error. We will dissect a real-world scenario where it occurs, delve into the underlying technical reasons related to Java's reflection and type system, present a clear and effective solution, and discuss alternative strategies and best practices. The goal is not just to fix the error, but to understand its origins, empowering you to write more robust and testable Spring applications.
The Anatomy of the `NoSuchMethodException`
To fully grasp the problem, let's construct a typical scenario where this error manifests. Imagine you are developing a RESTful API endpoint that needs to accept a variable number of identifiers to filter a resource. A common and intuitive way to model this is by accepting a list of strings or numbers as a request parameter.
A Common Controller Implementation
Consider a simple ProductController with a `GET` endpoint designed to fetch products based on a list of supplied IDs. The controller method signature might look like this:
@RestController
@RequestMapping("/products")
public class ProductController {
private final ProductService productService;
public ProductController(ProductService productService) {
this.productService = productService;
}
@GetMapping
public ResponseEntity<List<Product>> findProductsByIds(@RequestParam List<String> ids) {
List<Product> products = productService.findByIds(ids);
return ResponseEntity.ok(products);
}
// ... other methods
}
This implementation is clean, idiomatic Spring MVC. The framework is expected to automatically bind multiple query parameters with the same name (e.g., /products?ids=prod-101&ids=prod-204) into the ids list. When running the application and hitting this endpoint with a tool like cURL or Postman, it works flawlessly. The problem, however, appears when we try to write a test for it using Spring REST Docs.
The Failing Test Case
Following the principles of documentation-driven testing, we set up a @WebMvcTest to verify the controller's behavior and generate documentation snippets. The test might look something like this:
@WebMvcTest(ProductController.class)
@AutoConfigureRestDocs
public class ProductControllerTest {
@Autowired
private MockMvc mockMvc;
@MockBean
private ProductService productService;
@Test
void shouldReturnProductsForGivenIds() throws Exception {
List<Product> mockProducts = Arrays.asList(
new Product("prod-101", "Laptop"),
new Product("prod-204", "Mouse")
);
given(productService.findByIds(Arrays.asList("prod-101", "prod-204")))
.willReturn(mockProducts);
this.mockMvc.perform(RestDocumentationRequestBuilders.get("/products?ids={ids}", "prod-101,prod-204")
// Or alternatively: .param("ids", "prod-101").param("ids", "prod-204")
)
.andExpect(status().isOk())
.andDo(document("products/find-by-ids",
requestParameters(
parameterWithName("ids").description("A list of product IDs to retrieve.")
)
));
}
}
This seems like a standard, correct test setup. We mock the service layer, define the expected behavior, perform a request using MockMvc, and set up REST Docs to document the request parameters. However, upon running this test, instead of a green checkmark, we are met with a failure and a rather unsettling stack trace.
The Stack Trace Unveiled
The test fails not with an assertion error, but with an exception deep within the framework during the test execution. The console output prominently features the following:
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NoSuchMethodException: java.util.List.<init>()
...
Caused by: java.lang.NoSuchMethodException: java.util.List.<init>()
at java.base/java.lang.Class.getConstructor0(Class.java:3349)
at java.base/java.lang.Class.getConstructor(Class.java:2151)
... and so on
The core of the issue is java.lang.NoSuchMethodException: java.util.List.<init>(). This exception in Java typically means that code, usually via reflection, tried to find and invoke a no-argument constructor (<init>()) for the java.util.List class, but failed to do so. This is perfectly logical, because java.util.List is an interface. Interfaces cannot be instantiated; they have no constructors. One can only instantiate a concrete implementation like ArrayList or LinkedList. The immediate question is: why is the Spring test framework trying to instantiate a List interface directly?
Unraveling the Root Cause: Reflection, Type Erasure, and the Test Slice
The original diagnosis that "webMvcTest cannot correctly identify the List" is a symptom, not the root cause. The problem lies at the intersection of three key concepts: how Spring MVC binds parameters, the limitations of the @WebMvcTest slice, and Java's type erasure mechanism for generics.
How Spring Binds Request Parameters
In a running Spring Boot application, incoming HTTP requests are handled by a sophisticated chain of components. When a request for /products?ids=prod-101&ids=prod-204 arrives, the DispatcherServlet routes it to the correct controller method. Then, a component called a HandlerMethodArgumentResolver is responsible for resolving the method's arguments. For @RequestParam, the RequestParamMethodArgumentResolver is used. This resolver is intelligent; it recognizes that the target parameter is a collection (List) and correctly gathers all request parameters named "ids" into a new ArrayList instance, which is then passed to the controller method. This process works seamlessly in the full application context.
The `@WebMvcTest` Slice Limitation
The key difference is the test environment. The @WebMvcTest annotation does not load the entire application context. Instead, it sets up a "test slice" containing only the beans necessary for testing the web layer: controllers, JSON converters, Filters, WebMvcConfigurers, and crucially, the HandlerMethodArgumentResolvers. However, the auto-configuration for this test slice can sometimes differ subtly from the full application's configuration. In this specific scenario, the machinery responsible for parameter binding and documentation generation within the MockMvc environment appears to interact with the controller method's signature in a way that leads to the error.
The Collision of Reflection and Generics
The final piece of the puzzle is Java's type system. Java uses a mechanism called type erasure for generics. This means that at compile time, `List<String>` is checked for type safety, but at runtime, the type information is erased, and the JVM only sees a raw `java.util.List`.
Here's a plausible sequence of events inside the test framework:
- The framework introspects the
findProductsByIds(@RequestParam List<String> ids)method signature to understand its parameters for both request processing and documentation. - Through reflection, it determines the parameter type is
java.util.List.class(due to type erasure). - At some point in the setup or binding process, a component attempts to create an instance of this parameter type to populate it. It does so by reflectively looking for a default, no-argument constructor:
List.class.getConstructor().newInstance(). - This operation fails catastrophically because
List.classrepresents an interface, which has no constructors. The result is theNoSuchMethodExceptionwe observed.
Why does this not happen in the running application? The production-ready RequestParamMethodArgumentResolver is coded to specifically handle collection types and knows to instantiate a concrete class like ArrayList instead of blindly trying to instantiate the interface type it sees. The testing or documentation-generation context seems to be using a more generic, reflection-based mechanism that stumbles on this detail. In contrast, an array type like String[] does not suffer from this ambiguity. At runtime, String[].class is a concrete, non-erased type that the reflection APIs can handle without confusion.
The Pragmatic Solution and Its Nuances
Understanding the root cause allows us to implement a targeted and effective solution. Since the problem is rooted in the ambiguity of the List interface during reflection in the test environment, the solution is to use a concrete type that avoids this ambiguity: an array.
Implementing the Fix: From List to Array
The fix involves a simple change to the controller's method signature. By replacing List<String> with String[], we provide the test framework with a concrete type that it can handle correctly.
Before:
@GetMapping
public ResponseEntity<List<Product>> findProductsByIds(@RequestParam List<String> ids) {
// ...
}
After:
@GetMapping
public ResponseEntity<List<Product>> findProductsByIds(@RequestParam String[] ids) {
// Inside the method, you can easily convert the array to a List if needed
List<String> idList = Arrays.asList(ids);
List<Product> products = productService.findByIds(idList);
return ResponseEntity.ok(products);
}
Spring MVC's parameter binding handles arrays just as gracefully as it handles lists. It will collect all query parameters named "ids" and populate the String[] ids array.
The Corrected Test Case
With the controller method signature updated, the existing test case will now pass without any modifications. The MockMvc and REST Docs frameworks can now correctly interpret the String[] parameter type, the NoSuchMethodException is avoided, and the test proceeds to validate the endpoint's logic and generate the documentation.
// ... same test setup as before ...
@Test
void shouldReturnProductsForGivenIds() throws Exception {
// The service mock might need a slight adjustment if its method signature expects a List
List<String> idList = Arrays.asList("prod-101", "prod-204");
List<Product> mockProducts = Arrays.asList(
new Product("prod-101", "Laptop"),
new Product("prod-204", "Mouse")
);
given(productService.findByIds(idList)).willReturn(mockProducts);
// This test now passes
this.mockMvc.perform(RestDocumentationRequestBuilders.get("/products?ids={ids}", "prod-101,prod-204"))
.andExpect(status().isOk())
.andDo(document("products/find-by-ids",
requestParameters(
parameterWithName("ids").description("A list of product IDs to retrieve.")
)
));
}
Is This an Ideal Solution?
From a purely pragmatic standpoint, this is an excellent solution. It's a minimal change that resolves a frustrating testing issue. However, from a design perspective, one could argue that using List in method signatures is more idiomatic and aligned with the principles of programming to an interface. In this case, it's important to recognize that this change is a concession to the realities of the testing framework. The public-facing contract of your API remains unchanged. A client calling /products?ids=a&ids=b does not know or care whether the server-side implementation uses a List<String> or a String[]. The change is an internal implementation detail made to facilitate testing.
Exploring Alternative Approaches
While the array-based solution is the most direct fix, it's worth being aware of other patterns that can also circumvent this issue, especially in more complex scenarios.
Using a Wrapper Object (DTO)
You can encapsulate the request parameters into a Data Transfer Object (DTO). This approach works well when you have multiple related filter parameters. The controller method then uses @ModelAttribute instead of multiple @RequestParam annotations.
// DTO Class
public class ProductQuery {
private List<String> ids;
// getters and setters
}
// Updated Controller
@GetMapping
public ResponseEntity<List<Product>> findProducts(@ModelAttribute ProductQuery query) {
List<Product> products = productService.findByIds(query.getIds());
return ResponseEntity.ok(products);
}
In this case, Spring's data binding mechanism will instantiate the ProductQuery object and then populate its fields. This process avoids the direct reflective instantiation of the `List` interface that causes the original problem.
Using Comma-Separated Values
Another common pattern for passing multiple values is to use a single, comma-separated string.
// Controller accepts a single String
@GetMapping
public ResponseEntity<List<Product>> findProductsByIds(@RequestParam String ids) {
List<String> idList = Arrays.asList(ids.split(","));
List<Product> products = productService.findByIds(idList);
return ResponseEntity.ok(products);
}
The client would then call the endpoint like /products?ids=prod-101,prod-204. This is a very simple and robust approach that completely avoids collection binding issues. The main drawback is that it doesn't gracefully handle values that might themselves contain commas.
Impact on Documentation and Best Practices
A crucial question is how this change from List to an array affects the final output from Spring REST Docs. Fortunately, the impact is minimal to none.
Verifying the Generated Snippet
Spring REST Docs is primarily concerned with the HTTP request and response, not the specific Java types used in the controller. Whether the backing type is a List<String> or a String[], a request made with multiple `ids` parameters (e.g., ?ids=a&ids=b) is identical on the wire. As a result, the generated documentation snippet for request parameters will be the same in both cases. For example, the `request-parameters.adoc` file will contain something like this:
|===
| Parameter | Description
| `ids`
| A list of product IDs to retrieve.
|===
This demonstrates that our testing-motivated change did not compromise the accuracy or clarity of the consumer-facing documentation.
Key Takeaways for Robust API Testing
This specific issue serves as a valuable lesson in building and testing Spring applications:
- Understand Your Test Slices: Be aware that
@WebMvcTest,@DataJpaTest, etc., provide focused, partial application contexts. Their behavior can sometimes diverge from the full context. - Be Mindful of Reflection and Generics: Frameworks like Spring rely heavily on reflection. Be aware of how Java features like type erasure can lead to unexpected behavior, especially in a testing environment.
- Prioritize Testable Signatures: When designing controllers, consider how easily they can be tested. Sometimes, a small change in a method signature, like using an array instead of a list, can save hours of debugging test failures.
- Focus on the API Contract: Remember that internal implementation choices made for testability should not negatively impact the public API contract that your consumers depend on.
Conclusion
The java.lang.NoSuchMethodException: java.util.List.<init>() error in a Spring REST Docs test is a classic example of a problem that is simple to fix but complex in its origin. It's a confluence of test context limitations, framework reflection, and the nuances of Java's type system. By replacing the problematic List parameter with a concrete String[] array, we provide a clear and unambiguous type for the testing framework to handle, resolving the error without altering the external behavior of our API. More importantly, by digging deep into the "why," we gain a richer understanding of the tools we use every day, making us better equipped to tackle the next unexpected challenge that comes our way.
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