Using Kotlin class in Java: Cannot find symbol
JavaMavenBuildInteropKotlinJava Problem Overview
I have found this similar question regarding Android, but I am using plain Java with Maven as build tool. I think it is better to post a new question.
I have created a Kotlin class to which I am trying to refer from Java class as MyKotlinClass.class
. The Maven build fails, whereas compilation in IntelliJ Idea works fine. I have already added Kotlin plugin to maven:
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
<version>${kotlin.version}</version>
<executions>
<execution>
<id>compile</id>
<phase>compile</phase>
<goals>
<goal>compile</goal>
</goals>
</execution>
<execution>
<id>test-compile</id>
<phase>test-compile</phase>
<goals>
<goal>test-compile</goal>
</goals>
</execution>
</executions>
</plugin>
However that does not help:
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.3.2:compile (default-compile) on project app: Compilation failure
[ERROR] MyClassLinkingKotlin.java:[100,40] error: cannot find symbol
The line/column exactly refers to symbol MyKotlinClass.class
. It will fail even when using like this:
System.err.println(MyKotlinClass.class)
Java Solutions
Solution 1 - Java
Your Maven configuration adds the Kotlin compiler plugin, but doesn't adjust the Java compiler plugin execution so that the Java compiler runs after the Kotlin compiler. Therefore, the Java compiler runs before Kotlin, and doesn't see Kotlin-compiled classes.
Here's a snippet showing the correct configuration for a mixed-language project (taken from the documentation):
<build>
<plugins>
<plugin>
<artifactId>kotlin-maven-plugin</artifactId>
<groupId>org.jetbrains.kotlin</groupId>
<version>${kotlin.version}</version>
<executions>
<execution>
<id>compile</id>
<goals> <goal>compile</goal> </goals>
<configuration>
<sourceDirs>
<sourceDir>${project.basedir}/src/main/kotlin</sourceDir>
<sourceDir>${project.basedir}/src/main/java</sourceDir>
</sourceDirs>
</configuration>
</execution>
<execution>
<id>test-compile</id>
<goals> <goal>test-compile</goal> </goals>
<configuration>
<sourceDirs>
<sourceDir>${project.basedir}/src/test/kotlin</sourceDir>
<sourceDir>${project.basedir}/src/test/java</sourceDir>
</sourceDirs>
</configuration>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.5.1</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>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>
</plugins>
</build>
Solution 2 - Java
If you are using Android Studio and have already added the Kotlin plugins via Settings -> Plugins -> Kotlin, then it could be that you haven't setup the rest of Gradle to use Kotlin. Here's a snippet from https://medium.com/@elye.project/setup-kotlin-for-android-studio-1bffdf1362e8:
Step 1: Setup the Kotlin Plugin in Android Studio
> Android Studio → Preferences… →Plugins → Browse Repository → type > “Kotlin” in search box → install
Step 2: Add Kotlin classpath to project Build.Gradle
buildscript {
ext.kotlin_version = "1.1.1"
ext.supportLibVersion = "25.3.0"
repositories {
jcenter()
}
dependencies {
classpath 'com.android.tools.build:gradle:2.3.0'
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
classpath "org.jetbrains.kotlin:kotlin-android-extensions:$kotlin_version"
// NOTE: Do not place your application dependencies here; they belong
// in the individual module build.gradle files
}
}
Step 3: Add Kotlin library and apply Kotlin Plugins in your module Build.gradle
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
android {
// ... various gradle setup
}
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
testCompile 'junit:junit:4.12'
compile "com.android.support:appcompat-v7:$supportLibVersion"
compile "com.android.support:recyclerview-v7:$supportLibVersion"
compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}
Solution 3 - Java
I ran into this same error, but had the correct pom setup. My issue was that I had just converted a Java class to a Kotlin class with Intellij, which left that Kotlin file in src/main/java
.
The solution for me was to create a src/main/kotlin
and move my Kotlin class there, and leave my Java files in src/main/java
. But you do definitely need the maven setup that @yole's answer shows.
Solution 4 - Java
If anyone is using Plain Java project with Gradle
instead of Maven
then change your module gradle file as follows:
Say you have following plugin in your module:
apply plugin: 'java-library'
If you wanna have kotlin plugin then add kotlin plugin before java plugin:
apply plugin: 'kotlin'
apply plugin: 'java-library'
Reasson is
> Your configuration adds the Kotlin compiler plugin, but doesn't adjust > the Java compiler plugin execution so that the Java compiler runs > after the Kotlin compiler. Therefore, the Java compiler runs before > Kotlin, and doesn't see Kotlin-compiled classes.
as explained by @yole in answers