[FIXED] Classes annotated with @Configuration could be implicitly subclassed and must not be final

Issue

I created a new project on Kotlin to generate a report. Knowing Kotlin can reduce number of lines of code and is safer than Java, I used Spring-jpa and Kotlin to get this done. All my @Configuration classes had an error:

Classes annotated with @Configuration could be implicitly subclassed and must not be final

FYI, I am using a maven project with Kotlin 1.3.50. From my knowledge, I knew that spring does subclass for injecting values.

How do I make spring happy but not keep writing open in each of my class where spring complains?

Solution

As you might already know, by default kotlin classes can not be extended. In order to support inheritance, you got to add open keyword. Also, we know for a fact that spring annotation processing needs classes marked as @Configuration can be subclassed.

The alternative of writing open in each of the classes is to add a plugin : kotlin-allopen

ex :

<plugin>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-maven-plugin</artifactId>
    <version>${kotlin.version}</version>
    <configuration>
        <compilerPlugins>
            <plugin>spring</plugin>
        </compilerPlugins>
    </configuration>
    <dependencies>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-allopen</artifactId>
            <version>${kotlin.version}</version>
        </dependency>
    </dependencies>
    <executions>
        <execution>
            <id>compile</id>
            <phase>compile</phase>
            <goals>
                <goal>compile</goal>
            </goals>
        </execution>
        <execution>
            <id>test-compile</id>
            <phase>test-compile</phase>
            <goals>
                <goal>test-compile</goal>
            </goals>
        </execution>
    </executions>
</plugin>




<compilerPlugins>
    <plugin>spring</plugin>
</compilerPlugins>

The above does the trick of adding open to the following annotations: @Component, @Async, @Transactional, @Cacheable and @SpringBootTest. Thanks to meta-annotations support classes annotated with @Configuration, @Controller, @RestController, @Service or @Repository are automatically opened since these annotations are meta-annotated with @Component.

You can find the documentation here: https://kotlinlang.org/docs/reference/compiler-plugins.html

Answered By - NewUser
Answer Checked By - David Goodson (JavaFixing Volunteer)