How to get rid of duplicate class errors in Intellij for a Mavenized project using Lombok

Question

I have a Maven managed Lombok project and I use Intellij. After building, I always get lots of errors in Intellij about duplicate classes because of the generated sources in target/generated-sources/delombok. Is there something I can do to git rid of these errors? Right now I just delete the target folder, but this is really irritating to have to do.

I have the standard configuration in Maven and the Lombok source code in is in src/main/lombok:

       <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.5.1</version>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
                <encoding>UTF-8</encoding>
            </configuration>
        </plugin>

        <plugin>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok-maven-plugin</artifactId>
            <version>1.16.8.0</version>
            <executions>
                <execution>
                    <phase>generate-sources</phase>
                    <goals>
                        <goal>delombok</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

  <profiles>
    <profile>
        <id>lombok-needs-tools-jar</id>
        <activation>
            <file>
                <exists>${java.home}/../lib/tools.jar</exists>
            </file>
        </activation>
        <build>
            <plugins>
                <plugin>
                    <groupId>org.projectlombok</groupId>
                    <artifactId>lombok-maven-plugin</artifactId>
                    <version>1.16.8.0</version>
                    <dependencies>
                        <dependency>
                            <groupId>sun.jdk</groupId>
                            <artifactId>tools</artifactId>
                            <version>1.8</version>
                            <scope>system</scope>
                            <systemPath>${java.home}/../lib/tools.jar</systemPath>
                        </dependency>
                    </dependencies>
                </plugin>
            </plugins>
        </build>
    </profile>
</profiles>

Answer 1

According to delombok goal documentation: the default output directory is:

${project.build.directory}/generated-sources/delombok

I have found an JetBrains Team member comment stating that:

IDEA automatically excludes the build 'target' folder, providing that there are no generated sources under it, otherwise it excludes all sub-folders but the generated.

If you have some generated code or build artifacts that you want being excluded, you may put it under the 'target' folder.

This means that /generated-sources directory is by default not excluded and if you intend on excluding some files you should place them under parent /target directory and NOT under /generated-sources.

To achieve this you should configure the plugin and provide non-default <outputDirectory>:

<plugin>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok-maven-plugin</artifactId>
    <version>1.16.18.0</version>
    <executions>
        <execution>
            <phase>generate-sources</phase>
            <goals>
                <goal>delombok</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <sourceDirectory>src/main/java</sourceDirectory>
        <outputDirectory>${project.build.directory}/delombok</outputDirectory>
        <addOutputDirectory>false</addOutputDirectory>
        <encoding>UTF-8</encoding>
    </configuration>
</plugin>

This will stop IDEA from yielding Duplicate class found in (...) warnings.