Ambiguous Mapping Error in Spring

In Spring (specifically Spring MVC), an "ambiguous mapping" error typically arises when two or more controller methods are mapped to the same URI (URL pattern) in the application. This confuses the dispatcher servlet, as it doesn't know which method to invoke for a given request. 

Causes of Ambiguous Mapping Error

Multiple Methods with Same Mapping: Two or more controller methods are annotated with the same URL pattern using @RequestMapping or its shortcut annotations like @GetMapping, @PostMapping, etc. 

Controller & Method with Same Mapping: A class-level @RequestMapping on the controller combined with a method-level @RequestMapping results in the same URL pattern for multiple methods.

Inheritance Issues: If a controller inherits from another class, and both parent and child have methods mapped to the same URL pattern, it can cause an ambiguous mapping. 

How to Resolve Ambiguous Mapping Error

Review Mappings: Start by checking all your controller methods. Ensure that no two methods are mapped to the same URL pattern. Tools or IDE features that provide an overview of all routes can be handy here. 

Consider HTTP Verbs: If two methods are mapped to the same URL but are supposed to handle different HTTP verbs, ensure you've specified the verbs correctly using annotations like @GetMapping vs. @PostMapping or by setting the method attribute of @RequestMapping. 

Review Inherited Mappings: If you're inheriting controllers or using interface-based mappings, review the parent class or interface to ensure there are no clashing mappings. 

Namespace Controller Mappings: To avoid clashes, use a class-level @RequestMapping to namespace or group-related endpoints. For instance, all user-related endpoints can be grouped under /users. 

Use @RestController over @Controller: If you're building a RESTful service, use @RestController instead of the generic @Controller annotation. This won't solve the ambiguous mapping by itself but can help clarify intent. 

Check Third-party Libraries: If you've included third-party libraries or dependencies that define their own Spring MVC controllers, they might have routes that clash with yours. Check their documentation and see if you can override or exclude specific routes. 

Example: Consider the following scenario which causes an ambiguous mapping error:

@RestController
@RequestMapping("/api")
public class MyController {

    @GetMapping("/endpoint")
    public String method1() {
        return "Method 1";
    }

    @GetMapping("/endpoint")
    public String method2() {
        return "Method 2";
    }
}

In the above code, both method1 and method2 are mapped to the same URL pattern /api/endpoint, causing an ambiguous mapping error. To resolve it, you should change the mapping of one of the methods or merge the two methods if the duplication was unintentional. 

In conclusion, always ensure clear and unambiguous URL patterns for your controller methods. When adding new routes or refactoring existing ones, be aware of potential clashes, and test your changes to confirm that all routes work as expected.