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1. Overview

In Kotlin, all classes are final by default which, beyond its clear advantages, can be problematic in Spring applications. Simply put, some areas in Spring only work with non-final classes.

The natural solution is to manually open Kotlin classes using the open keyword or to use the kotlin-allopen plugin – which automatically opens all classes that are necessary for Spring to work.

2. Maven Dependencies

Let’s start by adding the Kotlin-Allopen dependency:

<dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-maven-allopen</artifactId>
    <version>1.1.4-3</version>
</dependency>

To enable the plugin, we need to configure the kotlin-allopen in the build section:

<build>
   ...
  <plugins>
        ...
        <plugin>
            <artifactId>kotlin-maven-plugin</artifactId>
            <groupId>org.jetbrains.kotlin</groupId>
            <version>1.1.4-3</version>
            <configuration>
                <compilerPlugins>
                    <plugin>spring</plugin>
                </compilerPlugins>
                <jvmTarget>1.8</jvmTarget>
            </configuration>
            <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>
            <dependencies>
                <dependency>
                    <groupId>org.jetbrains.kotlin</groupId>
                    <artifactId>kotlin-maven-allopen</artifactId>
                    <version>1.1.4-3</version>
                </dependency>
            </dependencies>
        </plugin>
    </plugins>
</build>

3. Setup

Now let’s consider SimpleConfiguration.kt, a simple configuration class:

@Configuration
class SimpleConfiguration {
}

4. Without Kotlin-Allopen

If we build our project without the plugin, we’ll see get the following error message:

org.springframework.beans.factory.parsing.BeanDefinitionParsingException: 
  Configuration problem: @Configuration class 'SimpleConfiguration' may not be final. 
  Remove the final modifier to continue.

The only way to solve it is to open it manually:

@Configuration
open class SimpleConfiguration {
}

5. Including Kotlin-Allopen

Opening all classes for Spring is not very handy. If we use the plugin, all necessary classes will be open.

We can clearly see that if we look at the compiled class:

@Configuration
public open class SimpleConfiguration public constructor() {
}

6. Conclusion

In this quick article, we’ve seen how to solve the “class may not be final” problem in Spring and Kotlin.

Source code for this article can be found over on GitHub.

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