fatal error compiling invalid flag --module-path

Question

I have a project. Originally it was a single module project with structure like this

java-cloud-sample\
    src\
        main\
            java
    pom.xml

I decided to make it into a multi-module structure - I use java 9 anyway. So I separated it like this

java-cloud-sample\
    java-cloud-rest-api\
        src\
        pom.xml
    pom.xml

Where root pom.xml looks like this

<?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>com.lapots.breed.platform.cloud</groupId>
    <artifactId>java-cloud-sample</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>pom</packaging>

    <modules>
        <module>java-cloud-rest-api</module>
    </modules>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.6.RELEASE</version>
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <maven.compiler.source>1.9</maven.compiler.source>
        <maven.compiler.target>1.9</maven.compiler.target>
    </properties>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.6.2</version>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.0.0</version>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <version>3.0.1</version>
                <executions>
                    <execution>
                        <id>copy-dependencies</id>
                        <phase>package</phase>
                        <goals><goal>copy-dependencies</goal></goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>


</project>

And module pom.xml looks like this

<?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>

    <parent>
        <groupId>com.lapots.breed.platform.cloud</groupId>
        <artifactId>java-cloud-sample</artifactId>
        <version>0.0.1-SNAPSHOT</version>
    </parent>

    <artifactId>java-cloud-rest-api</artifactId>
    <description>Demo project for Spring Boot</description>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <executions>
                    <execution>
                        <goals>
                            <goal>build-info</goal>
                        </goals>
                        <configuration>
                            <additionalProperties>
                                <encoding.source>${project.build.sourceEncoding}</encoding.source>
                                <encoding.reporting>${project.reporting.outputEncoding}</encoding.reporting>
                                <java.source>${maven.compiler.source}</java.source>
                                <java.target>${maven.compiler.target}</java.target>
                            </additionalProperties>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

But when I try to run mvn clean package I get

INFO] java-cloud-rest-api ................................ FAILURE [  1.060 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.104 s
[INFO] Finished at: 2017-09-08T17:15:46+03:00
[INFO] Final Memory: 27M/331M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.6.2:compile (default-compile) on project java-cloud-rest-api: Fatal error compiling: invalid flag: --module-path -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <goals> -rf :java-cloud-rest-api

What is the problem? (maybe I should split maven project into separate modules and each maven module split into java modules)

Answer 1

This failure occurs when maven installed on your machine is configured with java8 as the default java version. You can confirm this by executing

mvn -version

on your terminal and checking the Java version stated in the configuration.

To resolve the version to a newer and supported version like java9 at the moment, you can create/edit the mavenrc(on MacOS) file on your machine:

vi ~/.mavenrc

to include these

export PATH
export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home/
export PATH=${PATH}:$JAVA_HOME/bin

Once you save this configuration, you can confirm maven should be using Java version 9 using the same command and then your project shall build without the stated error.

Answer 2

I had this error with the Maven compiler plugin 3.8.1 when a project was defining a module-info.java and Java 8 was used with Maven to compile the project. It seems that as soon a module-info.java is seen by the Maven compiler plugin it is adding the flag --module-path to the command line which is unknown to the Java 8 compiler. A way to exclude this file and compile the project without module support is to exclude the module-info.java from compilation based on a Maven profile:

    <profiles>
        <profile>
            <id>java8</id>
            <activation>
                <jdk>1.8</jdk>
            </activation>
            <build>
                <plugins>
                    <plugin>
                        <groupId>org.apache.maven.plugins</groupId>
                        <artifactId>maven-compiler-plugin</artifactId>
                        <version>3.8.1</version>
                        <configuration>
                            <excludes>
                                <exclude>module-info.java</exclude>
                            </excludes>
                        </configuration>
                    </plugin>
                </plugins>
            </build>
        </profile>