Maven compile with multiple src directories

You can add a new source directory with build-helper:

<build>
    <plugins>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>build-helper-maven-plugin</artifactId>
            <version>3.2.0</version>
            <executions>
                <execution>
                    <phase>generate-sources</phase>
                    <goals>
                        <goal>add-source</goal>
                    </goals>
                    <configuration>
                        <sources>
                            <source>src/main/generated</source>
                        </sources>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

I naively do it this way :

<build>
  <finalName>osmwse</finalName>
  <sourceDirectory>src/main/java, src/interfaces, src/services</sourceDirectory>
</build>

This worked for me

<build>
    <sourceDirectory>.</sourceDirectory>
    <plugins>
        <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
        <includes>
            <include>src/main/java/**/*.java</include>
            <include>src/main2/java/**/*.java</include>
        </includes>
        </configuration>
        </plugin>
    </plugins>
</build>

To make it work in intelliJ, you can also add generatedSourcesDirectory to the compiler plugin this way:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.1</version>
    <configuration>
        <generatedSourcesDirectory>src/main/generated</generatedSourcesDirectory>
    </configuration>
</plugin>

This also works with maven by defining the resources tag. You can name your src folder names whatever you like.

    <resources>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.java</include>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
        </resource>

        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.java</include>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
        </resource>

        <resource>
            <directory>src/main/generated</directory>
            <includes>
                <include>**/*.java</include>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
        </resource>
    </resources>

This worked for with maven 3.5.4 and now Intellij Idea see this code as source:

<plugin>
     <groupId>org.apache.maven.plugins</groupId>
     <artifactId>maven-compiler-plugin</artifactId>
     <version>3.3</version>
     <configuration>
         <generatedSourcesDirectory>src/main/generated</generatedSourcesDirectory>                    
     </configuration>
</plugin>

While the answer from evokk is basically correct, it is missing test classes. You must add test classes with goal add-test-source:

<execution>
    <phase>generate-sources</phase>
    <goals>
        <goal>add-test-source</goal>
    </goals>
    <configuration>
       <sources>
            <source>target/generated/some-test-classes</source>
        </sources>
    </configuration>
</execution>

Used the build-helper-maven-plugin from the post - and update src/main/generated. And mvn clean compile works on my ../common/src/main/java, or on ../common, so kept the latter. Then yes, confirming that IntelliJ IDEA (ver 10.5.2) level of the compilation failed as David Phillips mentioned. The issue was that IDEA did not add another source root to the project. Adding it manually solved the issue. It's not nice as editing anything in the project should come from maven and not from direct editing of IDEA's project options. Yet I will be able to live with it until they support build-helper-maven-plugin directly such that it will auto add the sources.

Then needed another workaround to make this work though. Since each time IDEA re-imported maven settings after a pom change me newly added source was kept on module, yet it lost it's Source Folders selections and was useless. So for IDEA - need to set these once:

Select - Project Settings / Maven / Importing / keep source and test folders on reimport.

Add - Project Structure / Project Settings / Modules / {Module} / Sources / Add Content Root.

Now keeping those folders on import is not the best practice in the world either, ..., but giving it a try.

This can be done in two steps:

For each source directory you should create own module.

In all modules you should specify the same build directory: ${build.directory}

If you work with started Jetty (jetty:run), then recompilation of any class in any module (with Maven, IDEA or Eclipse) will lead to Jetty's restart. The same behavior you'll get for modified resources.

In the configuration, you can use <compileSourceRoots>.

oal:          org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-cli)
[DEBUG] Style:         Regular
[DEBUG] Configuration: <?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <basedir default-value="${basedir}"/>
  <buildDirectory default-value="${project.build.directory}"/>
  <compilePath default-value="${project.compileClasspathElements}"/>
  <compileSourceRoots default-value="${project.compileSourceRoots}"/>
  <compilerId default-value="javac">${maven.compiler.compilerId}</compilerId>
  <compilerReuseStrategy default-value="${reuseCreated}">${maven.compiler.compilerReuseStrategy}</compilerReuseStrategy>
  <compilerVersion>${maven.compiler.compilerVersion}</compilerVersion>
  <debug default-value="true">${maven.compiler.debug}</debug>
  <debuglevel>${maven.compiler.debuglevel}</debuglevel>
  <encoding default-value="${project.build.sourceEncoding}">${encoding}</encoding>
  <executable>${maven.compiler.executable}</executable>
  <failOnError default-value="true">${maven.compiler.failOnError}</failOnError>
  <failOnWarning default-value="false">${maven.compiler.failOnWarning}</failOnWarning>
  <forceJavacCompilerUse default-value="false">${maven.compiler.forceJavacCompilerUse}</forceJavacCompilerUse>
  <fork default-value="false">${maven.compiler.fork}</fork>
  <generatedSourcesDirectory default-value="${project.build.directory}/generated-sources/annotations"/>
  <maxmem>${maven.compiler.maxmem}</maxmem>
  <meminitial>${maven.compiler.meminitial}</meminitial>
  <mojoExecution default-value="${mojoExecution}"/>
  <optimize default-value="false">${maven.compiler.optimize}</optimize>
  <outputDirectory default-value="${project.build.outputDirectory}"/>
  <parameters default-value="false">${maven.compiler.parameters}</parameters>
  <project default-value="${project}"/>
  <projectArtifact default-value="${project.artifact}"/>
  <release>${maven.compiler.release}</release>
  <session default-value="${session}"/>
  <showDeprecation default-value="false">${maven.compiler.showDeprecation}</showDeprecation>
  <showWarnings default-value="false">${maven.compiler.showWarnings}</showWarnings>
  <skipMain>${maven.main.skip}</skipMain>
  <skipMultiThreadWarning default-value="false">${maven.compiler.skipMultiThreadWarning}</skipMultiThreadWarning>
  <source default-value="1.6">${maven.compiler.source}</source>
  <staleMillis default-value="0">${lastModGranularityMs}</staleMillis>
  <target default-value="1.6">${maven.compiler.target}</target>
  <useIncrementalCompilation default-value="true">${maven.compiler.useIncrementalCompilation}</useIncrementalCompilation>
  <verbose default-value="false">${maven.compiler.verbose}</verbose>
</configuration>

these are all the configurations available for 3.8.1 version of compiler plugin. Different versions have different configurations which you can find by running your code with -X after the general mvn command. Like

mvn clean install -X
mvn compiler:compile -X

and search with id or goal or plugin name This may help with other plugins too. Eclipse, intelliJ may not show all configurations as suggestions.