My mixed Scala/Java Maven project doesn't compile

Question

I have a mixed Scala/Java project that doesn't compile well.

The problem arises when Java code is trying to call Scala code in the same package.

Of course, I have the standard layout:

• src/main/java
• src/main/scala
• src/test/java
• src/test/scala

I've looked at other similar Stackoverflow questions but this question is a little outdated. This other question doesn't help either.

I have also followed the scala-maven-plugin documentation page.

<build>
    <pluginManagement>
        <plugins>
            <plugin>
                <groupId>net.alchim31.maven</groupId>
                <artifactId>scala-maven-plugin</artifactId>
                <version>3.1.6</version>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.0.2</version>
            </plugin>
        </plugins>
    </pluginManagement>
    <plugins>
        <plugin>
            <groupId>net.alchim31.maven</groupId>
            <artifactId>scala-maven-plugin</artifactId>
            <executions>
                <execution>
                    <id>scala-compile-first</id>
                    <phase>process-resources</phase>
                    <goals>
                        <goal>add-source</goal>
                        <goal>compile</goal>
                    </goals>
                </execution>
                <execution>
                    <id>scala-test-compile</id>
                    <phase>process-test-resources</phase>
                    <goals>
                        <goal>testCompile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <executions>
                <execution>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

I have tried unsuccessfully to follow this blog post.

IDEA project with Scala plugin imported from the pom.xml can compile and run my project successfully.

What is the right way of doing it? Does the Java code gets compiled twice? First by the Scala plugin and the by the Java plugin?.

Solution 1

Here is a working example of pom.xml :

<?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>stackoverflow</groupId>
<artifactId>q24448582</artifactId>
<version>1.0-SNAPSHOT</version>

<properties>
    <scala.version>2.10.3</scala.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.scala-lang</groupId>
        <artifactId>scala-library</artifactId>
        <version>${scala.version}</version>
    </dependency>

</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.scala-tools</groupId>
            <artifactId>maven-scala-plugin</artifactId>
            <version>2.15.2</version>
            <executions>

                <execution>
                    <id>compile</id>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                    <phase>compile</phase>
                </execution>
                <execution>
                    <id>test-compile</id>
                    <goals>
                        <goal>testCompile</goal>
                    </goals>
                    <phase>test-compile</phase>
                </execution>
                <execution>
                    <phase>process-resources</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

</project>

Solution 2

Problem 1

At first I thought the problem was due to using a modern version of the maven-compiler-plugin. I was using version 3.0 instead of 2.0.2.

The solution has been: - Remove the maven-compiler-plugin - Add this setting: incremental

Finally my pom.xml is this:

<plugin>
    <groupId>net.alchim31.maven</groupId>
    <artifactId>scala-maven-plugin</artifactId>
    <version>3.1.6</version>
    <configuration>
        <recompileMode>incremental</recompileMode>
        <args>
            <arg>-deprecation</arg>
            <arg>-explaintypes</arg>
            <arg>-target:jvm-1.7</arg>
        </args>
    </configuration>
    <executions>
        <execution>
            <id>scala-compile-first</id>
            <phase>process-resources</phase>
            <goals>
                <goal>add-source</goal>
                <goal>compile</goal>
            </goals>
        </execution>
        <execution>
            <id>scala-test-compile</id>
            <phase>process-test-resources</phase>
            <goals>
                <goal>add-source</goal>
                <goal>testCompile</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Problem 2

Another problem I was experiencing is this:

<dependencies>
    <dependency>
        <groupId>org.json4s</groupId>
        <artifactId>json4s-native_${scala.version}</artifactId>
        <version>3.2.9</version>
    </dependency>
    <dependency>
        <groupId>org.scala-lang</groupId>
        <artifactId>scala-library</artifactId>
        <version>${scala.version}.4</version>
    </dependency>
</dependencies>

<properties>
    <scala.version>2.10</scala.version>
</properties>

This make this solved problem arise: https://issues.scala-lang.org/browse/SI-5733

Probably it was using an old version of the Scala compiler.

The solution has been to rename scala.version to scala.major, as this property has a special meaning.

Solution 3

In order to get the scala-maven-plugin to respect Java 1.8, but still allow mixed compilation of java and scala, we use this:

...
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.5</version>
    <configuration>
        <source>1.8</source>
        <target>1.8</target>
    </configuration>
    <executions>
        <execution>
            <id>default-compile</id>
            <phase>none</phase>
        </execution>
    </executions>
</plugin>
<plugin>
    <groupId>net.alchim31.maven</groupId>
    <artifactId>scala-maven-plugin</artifactId>
    <version>3.2.2</version>
    <configuration>
        <recompileMode>incremental</recompileMode>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>compile</goal>
                <goal>testCompile</goal>
            </goals>
        </execution>
    </executions>
</plugin>
...