Why is my Spring RESTful API with MongoDB throwing a \'Field userService required a bean of type that could not be found\' error?

'Field required a bean of type that could not be found' Error in Spring RESTful API Using MongoDB

When developing RESTful web services with Spring and MongoDB, you may encounter an error like the following:

<code class="text">APPLICATION FAILED TO START
...
Field userService in main.java.rest.UsersController required a bean of
type 'main.java.service.UserService' that could not be found.</code>

This issue typically arises when Spring cannot locate the necessary bean within the application context. In this case, the error message indicates that Spring is unable to find an instance of the UserService bean.

Possible Solution 1: Configuring Package Scanning

By default, Spring scans for beans within packages annotated with @SpringBootApplication. If the service class (e.g., UserService) is located outside the scanned package, you can explicitly specify the base packages to scan using @SpringBootApplication(scanBasePackages={"...", "..."}).

Possible Solution 2: Restructuring Project Packages

Alternatively, you can restructure your project's package structure to ensure that all bean-defining classes are within the scanned packages. For example, you could move the service classes to a package underneath the main package where Application.java resides.

Once you have addressed the bean configuration issue, the error should be resolved, and your application should be able to run successfully.

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