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Create Java 8 and Java 11 build for single API

We are working to upgrade our API from Java 8 to Java 11.
However we need to support Java 8 and Java 11 both at same time because all clients are not ready to move on Java 11.
Is it possible if we can create maven build with Java 8 and Java 11 without duplicate our repository and without any change in pom.xml each time before create build for Java 8 and Java 11?
Generated artifact should have different name to distinguish Java 8 and Java 11 versions something like xyz-jdk8-0.0.1.jar and xyz-jdk11-.0.1.jar

TL;DR
You don't need that, just build to Java 8 and be happy!
Solving
You can use Maven Build Profiles for this:
https://maven.apache.org/guides/introduction/introduction-to-profiles.html
1. Set your properties to Java 11:
<properties>
<maven.compiler.target>11</maven.compiler.target>
<maven.compiler.source>11</maven.compiler.source>
<maven.compiler.release>11</maven.compiler.release>
</properties>
2. Set your final name:
<build>
<finalName>${project.artifactId}-jdk11-${project.version}</finalName>
</build>
3. Add a profile:
<profile>
<id>jdk-8</id>
<properties>
<maven.compiler.target>1.8</maven.compiler.target>
<maven.compiler.release>1.8</maven.compiler.release>
</properties>
<build>
<finalName>${project.artifactId}-jdk8-${project.version}</finalName>
</build>
</profile>
4. Build:
You will need to run 2 builds, one normal, and other activating the JDK 8 profile:
$ mvn ...
$ mvn ... -P jdk-8
Considerations:
Always use JDK 11 as it can confidently build Java 8 targets;
You don't need target and source properties, but if some plugin fails, put it back.

You should:
Remove from you POM, if present, maven.compiler.source and maven.compiler.target.
Set JAVA_HOME=<<JAVA_8_HOME_FOLDER>>.
Build from Java 8 by mvn clean install.
Take the Java 8 artifact.
Set JAVA_HOME=<<JAVA_11_HOME_FOLDER>>.
Build from Java 11 by mvn clean install.
Take the Java 11 artifact.

I found a way to do that, which lies somewhat in the middle of Java 8 and Java 11. The main problem with the above solution occurs in case you have (or want to define) a module-info.java that can't be parsed in Java 8.
I started considering a solution with <classifier>s, likewise Claudio Weiler above answer but did not wanted to bother having two separate builds (and even two separate branches because I wanted to manage module-info).
I also noticed that some libraries like the JAXB API or activation API (jakarta.xml.bind:jakarta.xml.bind-api:2.3.3) shows a module-info when you use it in Java 11, but is compatible with Java 8 😮.
Digging into how the details of how the build of the com.sun.activiation:jakarta.activation library was made, I found the following solution :
compile the module-info.java with Java 11
compile the rest of your code (including tests) with Java 8. Of course, this has the drawback of not being able to use fully Java 11 features...
You thus need to add the following snippet in the <build> section of your pom.xml:
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<executions>
<execution>
<id>default-compile</id>
<configuration>
<source>1.8</source>
<target>1.8</target>
<excludes>
<exclude>module-info.java</exclude>
</excludes>
</configuration>
</execution>
<execution>
<id>module-info-compile</id>
<goals>
<goal>compile</goal>
</goals>
<configuration>
<release>11</release>
<includes>
<include>module-info.java</include>
</includes>
</configuration>
</execution>
<execution>
<id>default-testCompile</id>
<configuration>
<release>11</release>
</configuration>
</execution>
</executions>
</plugin>
I hope that helps.

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.

Two maven versions required in pom.xml - what to do?

I'm trying to build JavaOCR project with mvn clean install. There is used maven-enforcer-plugin in the pom.xml and there are two required Maven versions. (Or maybe I misunderstood something)
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-enforcer-plugin</artifactId>
<version>1.4</version>
<executions>
<execution>
<id>enforce-versions</id>
<goals>
<goal>enforce</goal>
</goals>
<configuration>
<rules>
<requireMavenVersion>
<!--different rules for different issues-->
<!--3.3.x causes `java.lang.NoClassDefFoundError: org/eclipse/aether/spi/connector/Transfer$State` which is caused by certain maven versions, see https://cwiki.apache.org/confluence/display/MAVEN/AetherClassNotFound for details-->
<version>(,3.3)</version>
<!--3.2.x causes `No implementation for org.eclipse.aether.connector.wagon.WagonConfigurator was bound.`-->
<version>(,3.2)</version>
</requireMavenVersion>
</rules>
</configuration>
</execution>
</executions>
</plugin>
Currently I have installed maven 3.3.3 and it throws the java.lang.NoClassDefFoundError error (same as described in the comment in pom.xml), so I cannot build it.
My question is what version should I use to build the project successfully? Is it possible to use two maven versions simultaneously?

According to Maven's Version Range Specification documentation, that's specifying a range, not a specific version. (,3.2) means anything less than version 3.2, so those two configurations are compatible: for example, use version 3.1.
This is reinforced by the comments in the pom file, which say that versions 3.2.x and 3.3.x cause errors, so don't use them.
See also
Version Range Specification

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>

Required Java version of Maven Dependency?

I developed and built my Java application using Maven. I need to support Java 1.6, so I use the following properties:
<maven.compiler.target>1.6</maven.compiler.target>
<maven.compiler.source>1.6</maven.compiler.source>
Still, when I run the application I get an "Unsupported major.minor version" error and I suspect that one of my dependency jars is compiled with a Java version newer than the one I need to support.
My questions:
Is this even possible? I thought Maven would take care of this kind of dependency version problems.
Is there an easy way to find out the minor/major version of all my dependencies? (Would be awesome if it could be shown when doing mvn dependency:tree for instance.)

The problem is that every dependency (maintainer) can decide on it's own which java version is used to compile (1.5, 1.6, 1.7, 1.8 etc) so this is not solvable via Maven. But you can make sure that you don't use dependencies which are using a different Java version than you like to have.
This can be enfored by using Maven Enforcer Plugin by using extra-enforcer-rules:
<project>
[...]
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-enforcer-plugin</artifactId>
<version>1.4.1</version> <!-- find the latest version at http://maven.apache.org/plugins/maven-enforcer-plugin/ -->
<executions>
<execution>
<id>enforce-bytecode-version</id>
<goals>
<goal>enforce</goal>
</goals>
<configuration>
<rules>
<enforceBytecodeVersion>
<maxJdkVersion>1.6</maxJdkVersion>
<excludes>
<exclude>org.mindrot:jbcrypt</exclude>
</excludes>
</enforceBytecodeVersion>
</rules>
<fail>true</fail>
</configuration>
</execution>
</executions>
<dependencies>
<dependency>
<groupId>org.codehaus.mojo</groupId>
<artifactId>extra-enforcer-rules</artifactId>
<version>1.0-beta-5</version>
</dependency>
</dependencies>
</plugin>
</plugins>
</build>
[...]
</project>
This will break your build if you have a dependency which is compiled with a different version of JDK than you want to have.

To answer your two questions:
Yes, this is possible. Looking at the docs, the <maven.compiler.target> and <maven.compiler.source> properties simply tells Maven which version of javac to use to compile your project. And I quote, for your reference:
Note: Merely setting the target option does not guarantee that your
code actually runs on a JRE with the specified version. The pitfall is
unintended usage of APIs that only exist in later JREs which would
make your code fail at runtime with a linkage error. To avoid this
issue, you can either configure the compiler's boot classpath to match
the target JRE or use the Animal Sniffer Maven Plugin to verify your
code doesn't use unintended APIs.
The magic number after the Unsupported major.minor version error actually tells the version of the JRE the class file is compatible with:
J2SE 8 = 52,
J2SE 7 = 51,
J2SE 6.0 = 50,
J2SE 5.0 = 49,
JDK 1.4 = 48,
JDK 1.3 = 47,
JDK 1.2 = 46,
JDK 1.1 = 45
I don't know if there is an easy way to tell the major/minor version of ALL the dependencies (and transitive dependencies) in a project though.
UPDATE: Though I haven't used it before, I wonder the Animal Sniffer Maven plugin will help to sniff out the major/minor version of your dependencies.