Kotlin maven plugin order of compilation

Question

I have a question regarding the kotlin-maven plugin: How is the order of compilation determined? I mean: Must the kotlin compiler come first? Does it understand java code? The java compiler doesn't understand kotlin code, so I guess it checks against the byte code, but for this, the bytecode for kotlin must be present, so someone has to determine that the kotline compiler must compile its code before javac.

Example: I have java Class A that depends on Kotlin Class B and B also depends on A.

Do I have to do something manually in the maven plugin by declare it before the java maven compiler?

Answer 1

Kotlin compiler can parse java sources, so you could reference java classes in your kotlin code, even if those classes have not been compiled yet.

After Kotlin compiler had compiled kotlin code, Java compiler could compile those java classes taking compiled kotlin classes as a dependency.

So, answering your question, yes — Kotlin compiler plugin must be executed before java compiler plugin in a project that mixes kotlin and java sources. This is achieved by scheduling the execution on the earlier phase of build, namely process-sources.

Here is the example of kotlin maven plugin configuration for mixed Kotlin-Java maven module:

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

Answer 2

In order to compile a project that contains both, Kotlin and Java code, you have to make sure that the Kotlin compiler runs before the Java compiler. I configured the compile plugins as described in the official Kotlin documentation and it worked like a charm:

        <plugin>
            <artifactId>kotlin-maven-plugin</artifactId>
            <groupId>org.jetbrains.kotlin</groupId>
            <version>${kotlin.version}</version>
            <executions>
                <execution>
                    <id>compile</id>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
                <execution>
                    <id>test-compile</id>
                    <goals>
                        <goal>test-compile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <groupId>org.apache.maven.plugins</groupId>
            <version>3.5.1</version>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
            </configuration>
            <executions>
                <execution>
                    <id>default-compile</id>
                    <phase>none</phase>
                </execution>
                <execution>
                    <id>default-testCompile</id>
                    <phase>none</phase>
                </execution>
                <execution>
                    <id>java-compile</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
                <execution>
                    <id>java-test-compile</id>
                    <phase>test-compile</phase>
                    <goals>
                        <goal>testCompile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>