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From "Robert Scholte" <>
Subject Re: Building jar targeting multiple Java versions, including 9
Date Wed, 31 Aug 2016 14:39:52 GMT
Hi Paul,

no problem to move it to this thread. It is indeed about "The Maven Way",  
although we may need to start from the beginning, explaining the issue  
we're facing.

Let's use ASM as an example, their 6.0_ALPHA has been built like this,  
although without Maven.
ASM is an all purpose Java bytecode manipulation and analysis framework.  
IIRC their code is compatible with Java 1.2, but they've also added a  
module-info for those who want to use this dependency within a  
Java9/Jigsaw project.
module-info MUST be built with Java9 with source/target or release = 9.  
Other sources must be built with an older JDK with source/target 1.2

There are several ways to solve this:
- multi-module project and multi release jar like Paul suggest. However,  
IIRC the specs say that the module-info MUST exist in the root.
- 2 source folders, src/main/java and src/main/jigsaw, both writing to  
target/classes. Here it is quite clear what happens per source-folder.
- 1 source folder and all the magic of calling javac twice in the  
maven-compiler-plugin. I started with this, but I don't like it. Details  
are below.
- 1 source folder and 2 execution blocks (one excluding module-info, one  
only including module-info).

We shouldn't be looking at Maven alone, but also at IDE support. AFAIK  
Netbeans and IntelliJ simply call Maven. Eclipse is probably hard due to  
the m2eclipse extensions.

Now back to Pauls suggestion. I don't think we make developers very happy  
if they are advised to have a multimodule project just to be able to  
compile the module-info file.
I am aware that this is mainly an issue for library builders, end users  
simply build everything with Java9, no problems there. From library  
builders you can expect that they don't mind adding extra configuration to  
build their jar, but forcing them to set up a Maven multimodule project is  
not really nice.
I would expect the module-info close to the related sourcefiles, so I  
would prefer in the same (single) MavenProject.

*Unless* IDEs are so strong with handling multi release jars that it looks  
like I'm adjusting the module-info, even though it is actually located  
somewhere else.

So let's see the opinions from others.


On Wed, 31 Aug 2016 15:59:04 +0200, Paul Benedict <>  

> Robert, I'm responding to dev@maven so we can discuss Maven  
> philosophies...
> I believe the pattern should be based on a multi-module project. Each
> module should target the expected JDK version. Then introduce a new  
> "mrjar"
> type for the parent that knows how to bind them all together into a
> Multi-Release JAR.
> Cheers,
> Paul
> On Wed, Aug 31, 2016 at 6:10 AM, Robert Scholte <>
> wrote:
>> I've been working on the implementation of this in the
>> maven-compiler-plugin, but I'm not really pleased with the result.
>> The problem is that in the worst case scenario we have to work with 3
>> different versions of Java:
>> - The Maven Runtime (set as JAVA_HOME)
>> - JDK for the
>> - JDK for all other source files.
>> The example below worked because all three were set to JDK9.
>> But based on the source/target of 1.6 I cannot predict which JDK is  
>> used,
>> only that it is at least JDK6. Should the plugin switch to another JDK?
>> And if you want to compile with source/target 1.5 or less, you're in
>> trouble. There's something called toolchain, where you can specify the
>> JavaHome per version, but in case of maven-compiler-plugin it assumes  
>> that
>> all java-related plugins and execution blocks want to use the same
>> toolchain through the whole Maven project.
>> The good news is that for the maven-jdeps-plugin I improved this part in
>> Maven 3.3.1, since this plugin only works with Java8 and above, which
>> doesn't have to be the same JDK to compile the sources with. Now you can
>> simple say: I want the toolchain for version X. This feature needs to be
>> added to the plugin.
>> That said I think I will write a recipe for this. This is first of all  
>> an
>> issue for library writers who want to have their jar compatible with
>> multiple Java versions for their end users.
>> Result: One javac call per execution block as it was meant to be.
>> thanks,
>> Robert
>> On Fri, 26 Aug 2016 15:31:07 +0200, Oliver Gond┼ża <>
>> wrote:
>> Thank you all for your suggestions. I managed to get the project to  
>> build
>>> with following maven setup:
>>> ```
>>>    <!--
>>>          When using compiler from java 8 and older, ignore module files
>>> altogether.
>>>          Otherwise, use 2 phase compilation to build
>>>            - all classes for target version and
>>>            - with 9+ source and target level
>>>       -->
>>>    <build>
>>>      <plugins>
>>>        <plugin>
>>>          <groupId>org.apache.maven.plugins</groupId>
>>>          <artifactId>maven-compiler-plugin</artifactId>
>>>          <version>3.5.1</version>
>>>          <configuration>
>>>            <source>1.6</source>
>>>            <target>1.6</target>
>>>          </configuration>
>>>          <executions>
>>>            <execution>
>>>              <id>default-compile</id>
>>>              <configuration>
>>>                <excludes>
>>>                  <exclude>**/</exclude>
>>>                </excludes>
>>>              </configuration>
>>>            </execution>
>>>          </executions>
>>>        </plugin>
>>>      </plugins>
>>>    </build>
>>>    <profiles>
>>>      <profile>
>>>        <id>jigsaw</id>
>>>        <activation>
>>>          <jdk>[1.9,)</jdk>
>>>        </activation>
>>>        <build>
>>>          <plugins>
>>>            <plugin>
>>>              <groupId>org.apache.maven.plugins</groupId>
>>>              <artifactId>maven-compiler-plugin</artifactId>
>>>              <version>3.5.1</version>
>>>              <configuration>
>>>                <source>1.9</source>
>>>                <target>1.9</target>
>>>              </configuration>
>>>              <executions>
>>>                <execution>
>>>                  <id>module-infos</id>
>>>                  <phase>compile</phase>
>>>                  <goals>
>>>                    <goal>compile</goal>
>>>                  </goals>
>>>                  <configuration>
>>>                    <includes>
>>>                      <include>**/</include>
>>>                    </includes>
>>>                  </configuration>
>>>                </execution>
>>>              </executions>
>>>            </plugin>
>>>          </plugins>
>>>        </build>
>>>      </profile>
>>>    </profiles>
>>> ```
>>> It does compile with older javac versions as a bonus. Given this is
>>> nothing else than using `-source 9 -target 9` for if
>>> present, I dare to say maven-compiler-plugin can be adapted to figure  
>>> this
>>> out on its own.

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