How do you compile Java+Kotlin project using Maven?

Question

I'm trying to compile maven project which has Kotlin classes referencing Java classes. Here's a part of my parent POM:

...

<dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-stdlib</artifactId>
    <version>${kotlin.version}</version>
</dependency>

...

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>${compiler-plugin-version}</version>
    <configuration>
        <source>${java-version}</source>
        <target>${java-version}</target>
        <encoding>${project.build.sourceEncoding}</encoding>
    </configuration>
</plugin>

<plugin>
    <artifactId>kotlin-maven-plugin</artifactId>
    <groupId>org.jetbrains.kotlin</groupId>
    <version>${kotlin.plugin.version}</version>

    <executions>
        <execution>
            <id>compile</id>
            <phase>process-sources</phase>
            <goals>
                <goal>compile</goal>
            </goals>
        </execution>

        <execution>
            <id>test-compile</id>
            <phase>process-test-sources</phase>
            <goals>
                <goal>test-compile</goal>
            </goals>
        </execution>
    </executions>

    <configuration>
        <scanForAnnotations>false</scanForAnnotations>
    </configuration>
</plugin>

And related parts of the child POM:

<dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-stdlib</artifactId>
</dependency>

...

<plugin>
    <artifactId>kotlin-maven-plugin</artifactId>
    <groupId>org.jetbrains.kotlin</groupId>
    <configuration>
        <sourceDirs>
            <source>${project.basedir}/src/main/kotlin</source>
        </sourceDirs>
    </configuration>
</plugin>

And the Kotlin class:

Stateless
open class DummyServiceImpl : DummyService {

    PersistenceContext(unitName = Consts.UNIT_NAME)
    private val em: EntityManager? = null

    override fun get(id: Long?): Dummy {
        return em!!.find<Dummy>(javaClass<Dummy>(), id)
    }

    override fun sayHi(): String {
        return "Dummy service says \"Hi!\"."
    }
}

DummyService and Consts classes are Java classes residing in the same module as DummyServiceImpl. So when I compile the module containing DummyServiceImpl with Maven it goes like this:

[error] C:\somepath\service\DummyServiceImpl.kt: (14, 31) Unresolved reference: DummyService
[error] C:\somepath\service\DummyServiceImpl.kt: (16, 35) Unresolved reference: Consts

If I switch Kotlin plugin execution phase to compile then it predictably fails if there're references from Java to Kotlin classes:

[ERROR] /C:/somepath/service/impl/DummyServiceClientImpl.java:[5,27] cannot find symbol
[ERROR] symbol:   class DummyServiceImpl

So, what's to be done about this? Note that building with IDEA's make goes perfectly fine.

Answer 1

Make sure you have this declaration in <build> of your pom.xml

    <plugins>         <plugin>             <groupId>org.jetbrains.kotlin</groupId>             <artifactId>kotlin-maven-plugin</artifactId>             <version>${kotlin.version}</version>             <executions>                 <execution>                     <id>compile</id>                     <phase>process-sources</phase>                     <goals>                         <goal>compile</goal>                     </goals>                     <configuration>                         <sourceDirs>                             <source>src/main/java</source>                             <source>src/main/kotlin</source>                             <source>src/main/resources</source>                         </sourceDirs>                     </configuration>                 </execution>                 <execution>                     <id>test-compile</id>                     <phase>process-test-sources</phase>                     <goals>                         <goal>test-compile</goal>                     </goals>                     <configuration>                         <sourceDirs>                             <source>src/test/java</source>                             <source>src/test/kotlin</source>                             <source>src/test/resources</source>                         </sourceDirs>                     </configuration>                 </execution>             </executions>         </plugin>     </plugins> 

Make sure that all folders (3x in src/main & 3x in src/test) mentioned in the configuration actually exist, even if they don’t contain any classes/resources. You can still fine-tune the configuration once it works for you.

Also pay attention to use exactly the same order I mentioned above to let the compiler compile the Java code first.

Answer 2

Referred from this Kotlin + java demo apps

You have to execute mvn kotlin:compile before mvn package. mvn kotlin:compile will compile the Kotlin files to class files.

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">     <modelVersion>4.0.0</modelVersion>     <groupId>com.wffweb</groupId>     <artifactId>kotlinminimalproductionsample</artifactId>     <version>0.0.1</version>     <packaging>war</packaging>      <!--     To package as war:- mvn clean kotlin:compile package     -->      <properties>         <maven.build.timestamp.format>yyyyMMddHHmmss</maven.build.timestamp.format>         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>         <maven.compiler.source>1.8</maven.compiler.source>         <maven.compiler.target>1.8</maven.compiler.target>         <kotlin.version>1.2.21</kotlin.version>     </properties>      <dependencies>         <dependency>             <groupId>javax.servlet</groupId>             <artifactId>javax.servlet-api</artifactId>             <version>3.1.0</version>             <scope>provided</scope>         </dependency>         <dependency>             <groupId>javax.websocket</groupId>             <artifactId>javax.websocket-api</artifactId>             <version>1.1</version>             <scope>provided</scope>         </dependency>         <dependency>             <groupId>com.webfirmframework</groupId>             <artifactId>wffweb</artifactId>             <version>RELEASE</version>         </dependency>         <dependency>             <groupId>org.jetbrains.kotlin</groupId>             <artifactId>kotlin-stdlib-jdk8</artifactId>             <version>${kotlin.version}</version>         </dependency>         <dependency>             <groupId>org.jetbrains.kotlin</groupId>             <artifactId>kotlin-test</artifactId>             <version>${kotlin.version}</version>             <scope>test</scope>         </dependency>     </dependencies>      <build>         <resources>             <resource>                 <directory>src/main/java</directory>                 <includes>                     <include>**/*.properties</include>                 </includes>             </resource>         </resources>         <plugins>             <plugin>                 <groupId>org.codehaus.mojo</groupId>                 <artifactId>exec-maven-plugin</artifactId>                 <version>1.4.0</version>             </plugin>             <plugin>                 <groupId>org.apache.maven.plugins</groupId>                 <artifactId>maven-compiler-plugin</artifactId>                 <version>3.3</version>                 <configuration>                     <source>1.8</source>                     <target>1.8</target>                 </configuration>             </plugin>             <plugin>                 <groupId>org.jetbrains.kotlin</groupId>                 <artifactId>kotlin-maven-plugin</artifactId>                 <version>${kotlin.version}</version>                 <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>                 <configuration>                     <jvmTarget>1.8</jvmTarget>                 </configuration>             </plugin>         </plugins>     </build>  </project> 

Answer 3

I ended using the following configuration to mix Java and Kotlin source code:

<build>
    <sourceDirectory>${project.basedir}/src/main/java</sourceDirectory>
    <testSourceDirectory>${project.basedir}/src/test/java</testSourceDirectory>
    <plugins>
        <plugin>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-plugin</artifactId>
            <version>${kotlin.version}</version>
            <executions>
                <execution>
                    <id>compile</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                    <configuration>
                        <sourceDirs>
                            <source>src/main/kotlin</source>
                        </sourceDirs>
                    </configuration>
                </execution>
                <execution>
                    <id>test-compile</id>
                    <phase>test-compile</phase>
                    <goals>
                        <goal>test-compile</goal>
                    </goals>
                    <configuration>
                        <sourceDirs>
                            <source>src/test/kotlin</source>
                        </sourceDirs>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Answer 4

Make sure to NOT use the build options: sourceDirectory and testSourceDirectory

All configurations goes at kotlin-maven-plugin, for more accurate information see: https://kotlinlang.org/docs/maven.html#compile-kotlin-and-java-sources