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How to use maven with both jigsaw and jre 8 support?

I'm building a java project with maven.
I want to make sure several things:
the built jar CAN run on jre8.
the built jar CAN run on jre9, with module/jigsaw.
the built jar CAN be put on maven central.
How should I configure the maven-compiler-plugin?
thanks.
the original repo is at https://github.com/cyanpotion/SDL_GameControllerDB_Util
right now I can pass 2 and 3, but the output jar seems cannot run on jre8.

A multi-release jar can be used to accomplish this purpose; however, if you are only after Jigsaw module support, a dual compilation configuration of Java 8 and 9 with the maven-compiler-plugin is sufficient (demonstrated below).
The following is a build configuration that is able to maintain JRE 8 support, but is compatible with being used as a module in JRE 9+. This can be tweaked to support a project as far back as JRE 1.6, if necessary.
Overview of the following build configuration:
Enforce a $JAVA_HOME JDK 9+ so that module-info.java can be compiled and validated against the sources (ensures that all module dependencies are correctly referenced).
Changes the default-compile execution to NO-OP. By specifying <release>8</release> (or target/source flags), some IDEs during Maven auto-importation (tested with IntelliJ) will assume the language level for this project is Java 8, and produce errors about the project's module-info.java.
Compiles all sources (including module-info.java) for Java 9; this ensures that the module-info.java includes any dependencies used without the project sources.
Compile all sources (excluding module-info.java) for Java 8, overwriting all previously compiled Java 9 classes with Java 8 classes; this means the only Java 9 class (class level 53+) will be module-info.class. All other classes should now be executable on a compliant JRE 8.
Notes:
A caveat is this will compile sources twice, once for Java 8 and another for Java 9. This may significantly increase build times for larger projects (but negligible for smaller projects). This may resolved by placing module-info.java in another source directory (e.g. src/main/java9) and configuring a multi-release JAR (see the first link at the beginning of this message). Note that for proper IDE auto-importation and marking this additional Java 9 source directory correctly, a NO-OP execution with org.codehaus.mojo:build-helper-maven-plugin can be used.
Only module-info.java will have Java 9 support, all other classes are limited to classes/methods/fields/code available in JDK/JRE 8.
Some tooling may need to be updated in your project configuration if it does not support Java 9 module-info.class. Some older variants of the default Maven plugins do not have sufficient support for modules.
<build>
<plugins>
<!-- ensure the project is compiling with JDK 9+ -->
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-enforcer-plugin</artifactId>
<version>3.0.0-M3</version>
<executions>
<execution>
<id>enforce-jdk9</id>
<goals>
<goal>enforce</goal>
</goals>
<configuration>
<rules>
<requireJavaVersion>
<version>[1.9,)</version>
<message>JDK 9+ is required for compilation</message>
</requireJavaVersion>
</rules>
</configuration>
</execution>
</executions>
</plugin>
<!-- compile sources -->
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.1</version>
<executions>
<!-- disable default phase due to fixed id and position in lifecycle -->
<execution>
<id>default-compile</id>
<phase>none</phase>
<!-- specify source/target for IDE integration -->
<configuration>
<release>9</release>
</configuration>
</execution>
<!-- compile sources with Java 9 to generate and validate module-info.java -->
<execution>
<id>java-9-module-compile</id>
<goals>
<goal>compile</goal>
</goals>
<configuration>
<release>9</release>
</configuration>
</execution>
<!-- recompile sources as Java 8 to overwrite Java 9 class files, except module-info.java -->
<execution>
<id>java-8-compile</id>
<goals>
<goal>compile</goal>
</goals>
<configuration>
<!-- specify JDK 9+ release flag to ensure no classes/methods later than Java 8 are used accidentally -->
<release>8</release>
<!-- exclude module-info.java from the compilation, as it is unsupported by Java 8 -->
<excludes>
<exclude>module-info.java</exclude>
</excludes>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>

I've done it. See my project, the IPAddress Java library. You compile with Java 9 compiler level, then you recompile everything except the module-info.java with Java 8 compile level.. So you have a Java 8 jar with a Java 9 module-info. You can see the compile commands in my ant xml script on GitHub.
There was originally an issue with Android studio that has since been resolved, it would not ignore the module-info. More recent Android studio versions are fine, and so are all Java platforms and environments. Java 9 jres and up will recognize the module-info, Java 8 will not, Java 8 and earlier will ignore it.
My jar is also in maven central, so it is satisfying your 3 requirements. Try it out in your dev environment to see it work, using Java 8 or later jres.
Multi-release jars are not necessary and not worth the trouble.
When I was researching this same question I found this same solution in use with an Apache project, so I am not the only one doing it.

Method introduced in java 9 is compiled using compiler target java 8

I'm experiencing an odd behavior in maven as well as in eclipse itself.
Even though i configured my project to be compiled in Java 1.8, I can compile and run (eclipse) a piece of code that was introduced in Java 9
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
The code line in question:
LocalTime.ofInstant(cal.toInstant(), cal.getTimeZone().toZoneId());
I'm using Oracle's JDK 11 locally for compiling and running in eclipse without any errors. When i package it into a docker container using openjdk:8-jdk-alpine it will boot up, but throw the following Exception when I call the method:
java.lang.NoSuchMethodError: java.time.LocalTime.ofInstant(Ljava/time/Instant;Ljava/time/ZoneId;)Ljava/time/LocalTime
How can I avoid and identify these situations before they go to testing? Am I doing something wrong or is it a bug in the build system or in JDK11?
thanks in advance

The source option specifies that the source code must be compatible with Java 8, the target option that the classes should be compatible with Java 8. However, you will still compile with the Java 11 class library if you build with Java 11 and then you can get errors like the one you have.
There are two good solutions. One is to use the Maven toolchains plugin and build with Java 8. Then you can have multiple Java versions installed and Maven will use the configured one on a per-project basis.
The other is to use the new release and testRelease options. They will build with API classes from the given release. Just add <release>1.8</release>.

If you are using JDK 11, configure your maven pom.xml like that:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.0</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-release-plugin</artifactId>
<version>2.5.3</version>
</plugin>
</plugins>
</build>

Is there a maven jigsaw jlink plugin?

Does maven have a plugin for the new Java 9 jlink I have searched online but have not been able to find anything official from the maven team.

Yes. There has been some progress made to create one on Github/maven-plugins for the same.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jlink-plugin</artifactId>
<version>3.0.0-SNAPSHOT</version>
</plugin>
The plugin in its code reads to be adaptive to JEP-282 and JEP-220 from the proposals.
And though this might look like a link too many answer. There is a working example from #khmarbaise on Github as well for this, which requires a toolchain with -
<configuration>
<jdkHome>/Library/Java/JavaVirtualMachines/jdk1.9.0_ea+170.jdk/Contents/Home</jdkHome>
</configuration>
Plus a side note from the author quoting -
Currently not more than a Proof of Concept. Everything here is speculative!
Edit1:- As shared in the comments, additional details could be found # How to create a Java runtime with Maven.
Edit2:- Dated 10 November, 2018 one can upgrade to using maven-jlink-plugin:3.0.0-alpha-1 and still provide some valuable feedback.

I'm working on ModiTect, general tooling around Java 9 modules. One of the goals of the ModiTect Maven plug-in lets you create module runtime images via jlink:
<plugin>
<groupId>org.moditect</groupId>
<artifactId>moditect-maven-plugin</artifactId>
<version>1.0.0-SNAPSHOT</version>
<executions>
<execution>
<id>create-runtime-image</id>
<phase>package</phase>
<goals>
<goal>create-runtime-image</goal>
</goals>
<configuration>
<modulePath>
<path>${project.build.directory}/modules</path>
</modulePath>
<modules>
<module>com.example.module1</module>
<module>com.example.module2</module>
</modules>
<launcher>
<name>helloWorld</name>
<module>com.example.module1</module>
</launcher>
<outputDirectory>
${project.build.directory}/jlink-image
</outputDirectory>
</configuration>
</execution>
</executions>
</plugin>
The plug-in is under active development right now and must be built from source for the time being (will deploy a first version to Maven Central soon).

there is mvn-jlink plugin which allows to call jdeps and jlink (and any tool provided by jdk), also it can download and unpack needed openjdk version from ADOPT and LIBERICA, such way allows build cross-platform images
<plugin>
<groupId>com.igormaznitsa</groupId>
<artifactId>mvn-jlink-wrapper</artifactId>
<version>1.0.0</version>
<executions>
<execution>
<id>call-jlink</id>
<goals>
<goal>jlink</goal>
</goals>
<configuration>
<jdepsReportPath>${project.build.directory}${file.separator}jdeps.out</jdepsReportPath>
<output>${project.build.directory}${file.separator}preparedJDK</output>
<addModules>
<module>java.compiler</module>
</addModules>
<options>
<option>--compress=2</option>
<option>--no-header-files</option>
<option>--no-man-pages</option>
</options>
</configuration>
</execution>
</executions>
</plugin>

Maybe check out https://github.com/ghackenberg/jigsaw-maven-plugin. The plugin also supports
jdeps --generate-module-info + javac + jar for patching unnamed modules,
jlink for creating runtime images, and
jpackage for creating application installers (only available since JDK 14 though).
You find the plugin documentation on the Github README page.
<plugin>
<groupId>io.github.ghackenberg</groupId>
<artifactId>jigsaw-maven-plugin</artifactId>
<version>1.1.3</version>
</plugin>

Specifying Java version in maven - differences between properties and compiler plugin

I'm not very experienced with Maven and while experimenting with multi-module project I started wondering how can I specify Java version for all my child modules in parent Maven pom. Until today I was using just:
<properties>
<java.version>1.8</java.version>
</properties>
...but when researching I found that you can also specify Java version in Maven compiler plugin, like that:
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
And then wrap this into plugin management tag to enable child poms usage of this. So the first question is this:
What are the differences beetwen setting Java version in properties and in Maven compiler plugin?
I couldn't find clear answer but in process of researching I found that you can also specify Java version in this way:
<properties>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
...which suggest that compiler plugin is there even if I don't explicit declare it. Running mvn package outputs with
maven-compiler-plugin:3.1:compile (default-compile) # testproj ---
...and some other plugins that I didn't declare.
So are those plugins default, hidden part of Maven pom? Are there any differences between setting source/target in properties and in Maven plugin configuration element?
Some other questions are - which way should be used (and when if they are not equal)? Which one is best for multi-module project and what happens if Java version specified in pom is different than version pointed in JAVA_HOME?

How to specify the JDK version?
Use any of three ways: (1) Spring Boot feature, or use Maven compiler plugin with either (2) source & target or (3) with release.
Spring Boot
<java.version> is not referenced in the Maven documentation.
It is a Spring Boot specificity.
It allows to set the source and the target java version with the same version such as this one to specify java 1.8 for both :
1.8
Feel free to use it if you use Spring Boot.
maven-compiler-plugin with source & target
Using maven-compiler-plugin or maven.compiler.source/maven.compiler.target properties are equivalent.
That is indeed :
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
is equivalent to :
<properties>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
according to the Maven documentation of the compiler plugin
since the <source> and the <target> elements in the compiler configuration use the properties maven.compiler.source and maven.compiler.target if they are defined.
source
The -source argument for the Java compiler.
NOTE: Since 3.8.0 the default value has changed from 1.5 to 1.6. Since 3.9.0 the default value has changed from 1.6 to 1.7
Default value is: 1.7.
User property is: maven.compiler.source.
target
The -target argument for the Java compiler.
NOTE: Since 3.8.0 the default value has changed from 1.5 to 1.6. Since 3.9.0 the default value has changed from 1.6 to 1.7
Default value is: 1.6.
User property is: maven.compiler.target.
About the default values for source and target, note that
since the 3.8.0 of the maven compiler, the default values have changed from 1.5 to 1.6.
maven-compiler-plugin with release instead of source & target
The maven-compiler-plugin 3.6 and later versions provide a new way :
org.apache.maven.plugins
maven-compiler-plugin
3.8.0
9
You could also declare just :
<properties>
<maven.compiler.release>9</maven.compiler.release>
</properties>
But at this time it will not work as the maven-compiler-plugin default version you use doesn't rely on a recent enough version.
The Maven release argument conveys release : a new JVM standard option that we could pass from Java 9 :
Compiles against the public, supported and documented API for a
specific VM version.
This way provides a standard way to specify the same version for the source, the target and the bootstrap JVM options.
Note that specifying the bootstrap is a good practice for cross compilations and it will not hurt if you don't make cross compilations either.
Which is the best way to specify the JDK version?
The first way (<java.version>) is allowed only if you use Spring Boot.
For Java 8 and below :
About the two other ways : valuing the maven.compiler.source/maven.compiler.target properties or using the maven-compiler-plugin, you can use one or the other. It changes nothing in the facts since finally the two solutions rely on the same properties and the same mechanism : the maven core compiler plugin.
Well, if you don't need to specify other properties or behavior than Java versions in the compiler plugin, using this way makes more sense as this is more concise:
<properties>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
From Java 9 :
The release argument (third point) is a way to strongly consider if you want to use the same version for the source and the target.
What happens if the version differs between the JDK in JAVA_HOME and which one specified in the pom.xml?
It is not a problem if the JDK referenced by the JAVA_HOME is compatible with the version specified in the pom but to ensure a better cross-compilation compatibility think about adding the bootstrap JVM option with as value the path of the rt.jar of the target version.
An important thing to consider is that the source and the target version in the Maven configuration should not be superior to the JDK version referenced by the JAVA_HOME.
A older version of the JDK cannot compile with a more recent version since it doesn't know its specification.
To get information about the source, target and release supported versions according to the used JDK, please refer to java compilation : source, target and release supported versions.
How handle the case of JDK referenced by the JAVA_HOME is not compatible with the java target and/or source versions specified in the pom?
For example, if your JAVA_HOME refers to a JDK 1.7 and you specify a JDK 1.8 as source and target in the compiler configuration of your pom.xml, it will be a problem because as explained, the JDK 1.7 doesn't know how to compile with.
From its point of view, it is an unknown JDK version since it was released after it.
In this case, you should configure the Maven compiler plugin to specify the JDK in this way :
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
<compilerVersion>1.8</compilerVersion>
<fork>true</fork>
<executable>D:\jdk1.8\bin\javac</executable>
</configuration>
</plugin>
You could have more details in examples with maven compiler plugin.
It is not asked but cases where that may be more complicated is when you specify source but not target. It may use a different version in target according to the source version. Rules are particular : you can read about them in the Cross-Compilation Options part.
Why the compiler plugin is traced in the output at the execution of the Maven package goal even if you don't specify it in the pom.xml?
To compile your code and more generally to perform all tasks required for a maven goal, Maven needs tools. So, it uses core Maven plugins (you recognize a core Maven plugin by its groupId : org.apache.maven.plugins) to do the required tasks : compiler plugin for compiling classes, test plugin for executing tests, and so for... So, even if you don't declare these plugins, they are bound to the execution of the Maven lifecycle.
At the root dir of your Maven project, you can run the command : mvn help:effective-pom to get the final pom effectively used. You could see among other information, attached plugins by Maven (specified or not in your pom.xml), with the used version, their configuration and the executed goals for each phase of the lifecycle.
In the output of the mvn help:effective-pom command, you could see the declaration of these core plugins in the <build><plugins> element, for example :
...
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<version>2.5</version>
<executions>
<execution>
<id>default-clean</id>
<phase>clean</phase>
<goals>
<goal>clean</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>2.6</version>
<executions>
<execution>
<id>default-testResources</id>
<phase>process-test-resources</phase>
<goals>
<goal>testResources</goal>
</goals>
</execution>
<execution>
<id>default-resources</id>
<phase>process-resources</phase>
<goals>
<goal>resources</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.1</version>
<executions>
<execution>
<id>default-compile</id>
<phase>compile</phase>
<goals>
<goal>compile</goal>
</goals>
</execution>
<execution>
<id>default-testCompile</id>
<phase>test-compile</phase>
<goals>
<goal>testCompile</goal>
</goals>
</execution>
</executions>
</plugin>
...
You can have more information about it in the introduction of the Maven lifeycle in the Maven documentation.
Nevertheless, you can declare these plugins when you want to configure them with other values as default values (for example, you did it when you declared the maven-compiler plugin in your pom.xml to adjust the JDK version to use) or when you want to add some plugin executions not used by default in the Maven lifecycle.

None of the solutions above worked for me straight away. So I followed these steps:
Add in pom.xml:
<properties>
<maven.compiler.target>1.8</maven.compiler.target>
<maven.compiler.source>1.8</maven.compiler.source>
</properties>
Go to Project Properties > Java Build Path, then remove the JRE
System Library pointing to JRE1.5.
Force updated the project.

The below steps work for me like charm! so thought to share with everyone.
These are the lines i added in the pom.xml file to work with a basic project. I am using Java 12 (you can replace yours 11, 10, 1.8 etc).
<properties>
<maven.compiler.source>12</maven.compiler.source>
<maven.compiler.target>12</maven.compiler.target>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.1</version>
<configuration>
<release>12</release>
</configuration>
</plugin>
</plugins>
</build>
After changing the pom file please reload your project so that IDE can download/fetch the plugin to the project. (For IntelijIDEA: Right-click on pom.xml -> Go to maven -> Reload project).
please make sure to configure the desire version in your IDE as well.

if you are using IntelliJ idea maven build.

Consider the alternative:
<properties>
<javac.src.version>1.8</javac.src.version>
<javac.target.version>1.8</javac.target.version>
</properties>
It should be the same thing of maven.compiler.source/maven.compiler.target but the above solution works for me, otherwise the second one gets the parent specification (I have a matrioska of .pom)

For NetBeans IDE, changing project properties - (Jersey Service) - Categories > Sources >
Selected 'Source/Binary Format' as 1.8.

Multiple java version in maven build

I am working on one of the modules of a multi-module project. I am developing a java plugin to deploy my gigaspace application in pre-integration-test phase in maven.
The build happens on Teamcity and current JAVA_HOME points to Java 6 on which whole repository is built. Now, when deploying the application in pre-integration-test phase, it needs Java 7 because it uses some 3rd party libraries which were compiled in Java 7.
Is there any way I can somehow use Java 7 for deploying my application in pre-integration-test phase, but use Java 6 for compilation?

It seems that this is not straightforward. If you look at the plugin goal that gets associated with the lifecycle phase named pre-integration-test, then perhaps you can control its execution configuration:
<plugin>
...
<execution>
<id>...</id>
<phase>pre-integration-test</phase>
<goals>
<goal>...</goal>
</goals>
<configuration>
<source>1.7</source>
<target>1.7</target>
</configuration>
</execution>
</executions>
</plugin>
I haven't tried it myself. Also, see another somewhat similar question and its answer here. It's also useful to run mvn help:effective-pom to see what is going on.

If the purpose is to generate JAVA 6 bytecode, then you can use JAVA 7 (JAVA_HOME points to JAVA 7) for the whole lifecycle that include integration tests, but configure the maven-compiler-plugin with the version 1.6:
<properties>
<maven.compiler.source>1.6</maven.compiler.source>
<maven.compiler.target>1.6</maven.compiler.target>
</properties>