Introduce Kotlin to your existing Java Maven Project

Introduction

Introducing Kotlin into a Maven project written in Java is easy. Simply rewrite pom.xml to add the Kotlin source code and build with the Java code. As expected * 100% interoperable with Java ™ *! (\ *'E` \ *)

• Kotlin: Using Maven

If I modify pom.xml according to the official document, it works somehow, but I didn't understand why Maven works in the first place, so I investigated it.

It's almost Maven rather than Kotlin

Before / After of pom.xml

Let's take a simple pom.xml as an example. By rewriting pom.xml, you can add the code of Kotlin to the Maven Project that originally worked only with * .java.

<?xml version="1.0" encoding="UTF-8"?>
<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>megmogmog1965</groupId>
    <artifactId>JavaJarToKotlin</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
+       <!-- Kotlin -->
+       <kotlin.version>1.1.2-5</kotlin.version>
    </properties>

    <dependencies>
+       <!-- Kotlin -->
+       <dependency>
+           <groupId>org.jetbrains.kotlin</groupId>
+           <artifactId>kotlin-stdlib-jre8</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>
        <plugins>
+           <!-- Kotlin -->
+           <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>
+           </plugin>

            <!-- for .java -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.6.1</version>
+               <executions>
+                   <!-- Replacing default-compile as it is treated specially by maven -->
+                   <execution>
+                       <id>default-compile</id>
+                       <phase>none</phase>
+                   </execution>
+                   <!-- Replacing default-testCompile as it is treated specially by maven -->
+                   <execution>
+                       <id>default-testCompile</id>
+                       <phase>none</phase>
+                   </execution>
+                   <execution>
+                       <id>compile</id>
+                       <phase>compile</phase>
+                       <goals>
+                           <goal>compile</goal>
+                       </goals>
+                   </execution>
+                   <execution>
+                       <id>testCompile</id>
+                       <phase>test-compile</phase>
+                       <goals>
+                           <goal>testCompile</goal>
+                       </goals>
+                   </execution>
+               </executions>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

Commentary

First of all, I don't understand the meaning of Phase or Goal, so I'll try to make a diagram. Below is the Build Lifecycle diagram of the above pom.xml.

In short, I want to build the Kotlin code first and * .java later. Otherwise, the reference from * .java to Kotlin cannot be resolved.

properties

<properties>
    <!-- Kotlin -->
    <kotlin.version>1.1.2-5</kotlin.version>
</properties>

• Specify the version of Kotlin

dependencies

<dependencies>
    <!-- Kotlin -->
    <dependency>
        <groupId>org.jetbrains.kotlin</groupId>
        <artifactId>kotlin-stdlib-jre8</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>

• Add kotlin-stdlib to Project
• Contains println, listOf, etc.

plugins --> kotlin-maven-plugin

<plugins>
    <!-- Kotlin -->
    <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>
    </plugin>
    ...

• Add compile Goal Execution of kotlin-maven-plugin to compile Phase of Build Lifecycle

Same as test-compile

plugins --> maven-compiler-plugin

<plugins>
    ...
    
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.6.1</version>
        <executions>
            <!-- Replacing default-compile as it is treated specially by maven -->
            <execution>
                <id>default-compile</id>
                <phase>none</phase>
            </execution>
            <!-- Replacing default-testCompile as it is treated specially by maven -->
            <execution>
                <id>default-testCompile</id>
                <phase>none</phase>
            </execution>
            <execution>
                <id>compile</id>
                <phase>compile</phase>
                <goals>
                    <goal>compile</goal>
                </goals>
            </execution>
            <execution>
                <id>testCompile</id>
                <phase>test-compile</phase>
                <goals>
                    <goal>testCompile</goal>
                </goals>
            </execution>
        </executions>
        ...

• Remove Execution with default-compile ID from compile Phase in Build Lifecycle
• Add compile Goal Execution (= Plugin execution) of maven-compiler-plugin to compile Phase of Build Lifecycle

In other words, in order to execute kotlin-maven-plugin: compile first, I deleted it and added it again.

Same as test-compile