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.
Related
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.
I do not know the difference between spring-boot-maven-plugin and maven-compiler-plugin.
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>${java.version}</source>
<target>${java.version}</target>
<encoding>UTF-8</encoding>
</configuration>
</plugin>
</plugins>
</build>
Is that mean Spring Boot Maven Plugin's feature include maven-compiler-plugin?
I just use Spring Boot Maven Plugin is ok, do not need add 2 plugins??
"Spring Boot Maven Plugin provides Spring Boot support in Maven, letting you package executable jar or war archives and run an application “in-place”."
"Maven Compiler Plugin is used to compile the sources of your project."
maven-compiler-plugin has two goals. Both are already bound to their proper phases within the Maven Lifecycle and are therefore, automatically executed during their respective phases.
compiler:compile is bound to the compile phase and is used to compile the main source files.
compiler:testCompile is bound to the test-compile phase and is used to compile the test source files.
To understand more about maven build lifecycle - http://maven.apache.org/guides/introduction/introduction-to-the-lifecycle.html#Lifecycle_Reference
maven-compiler-plugin usages -
To compile source code using -source and -target javac Options
To compile source code using a different JDK
To compile source code using Memory Allocation Enhancement
To Pass Compiler Arguments
Most commonly used to define source and target versions.
Sometimes you may want to compile a certain project to a different version than what you are currently using. The javac can accept such command using -source and -target. maven-compiler-plugin can also be configured to provide these options during compilation.
For example, if you want to use the Java 8 language features (-source 1.8) and also want the compiled classes to be compatible with JVM 1.8 (-target 1.8), you can either add the two following properties, which are the default property names for the plugin parameters:
<project>
[...]
<properties>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
[...]
</project>
or configure the plugin directly:
<project>
[...]
<build>
[...]
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.1</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
[...]
</build>
[...]
</project>
Technically we can use both spring-boot-maven-plugin and maven-compiler-plugin in combination if the requirement is to create an executable jar as well as make sure source and target code have a specific version (which is accomplished by including maven-compiler-plugin).
In my case, i didn't use in combination but when my java project is a spring boot app that needs to run as a micro-service etc then we need an executable jar as build output so used spring boot maven plugin (only) but my other java project that consists of spring beans or components and is going to be used as a spring enabled library in other external apps but not required to run on its own but had to make sure source and target versions are specified then normal "mvn package" generated jar should work. For that maven compiler plugin (only) should do the job.
I'm using Maven Javadoc Plugin like this
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<version>3.1.1</version>
<executions>
<execution>
<id>attach-javadocs</id>
<goals>
<goal>jar</goal>
</goals>
</execution>
</executions>
</plugin>
I have multiple JDK versions installed and want to be able to build a project with any of them. The above configuration works fine with Java 8, but it fails with Java 11 with the following error
Failed to execute goal
org.apache.maven.plugins:maven-javadoc-plugin:3.1.1:jar
(attach-javadocs) on project ...: MavenReportException: Error while
generating Javadoc: Unable to find javadoc command: The environment
variable JAVA_HOME is not correctly set.
The obvious solution is to set JAVA_HOME. But as I mentioned, I have multiple versions of JDK, so reconfiguring JAVA_HOME every time I want to use another version wouldn't be convenient at all.
Another solution (from Unable to find javadoc command - maven) is to add the following configuration to the plugin:
<configuration>
<javadocExecutable>${java.home}/bin/javadoc</javadocExecutable>
</configuration>
This makes it work with Java 11, but breaks Java 8 because the javadoc location is ${java.home}/../bin/javadoc in that case.
Ideally I always want to use javadoc from the directory where the java executable that Maven uses is, but I haven't found a way to do it with Maven Javadoc Plugin.
Maven profiles may be able to help.
Add something like this to your POM.
<profiles>
<profile>
<id>jdk-8-config</id>
<activation>
<jdk>1.8</jdk>
</activation>
<properties>
<javadocExecutable>${java.home}/../bin/javadoc</javadocExecutable>
</properties>
</profile>
<profile>
<id>jdk-11-config</id>
<activation>
<jdk>11</jdk>
</activation>
<properties>
<javadocExecutable>${java.home}/bin/javadoc</javadocExecutable>
</properties>
</profile>
</profiles>
JDK ranges are also possible, read more on those in the linked doc.
Maven will pick up the version of Java being used to run itself, activate the correct profile, and define the property correct.
A caveat - typically we build with JAVA_HOME set, or via Jenkins which can be configured to define JAVA_HOME per job. This approach works well in those cases.
You could also investigate Maven toolchains. I have no experience with those, other than reading they help make it easier to define tool locations on various machines.
I have been searching for a solution to my Maven and Eclipse JRE version issue, where JRE 1.5 is being forced in the Eclipse project somehow even when I want different.
During my searches, I have noticed that there appear to be two different solutions to this. The first solution suggests using something like the snippet below
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.6</source>
<target>1.6</target>
</configuration>
</plugin>
</plugins>
</build>
The second solution suggests using just two properties:
<properties>
<maven.compiler.source>1.6</maven.compiler.source>
<maven.compiler.target>1.6</maven.compiler.target>
</properties>
Why are the two possible solutions and why do they differ?
There's a long way to specify it (by defining a <plugin> configuration in your pom file), which is the normal way to configure plugins, and there is a shorthand using a user property.
As you can find in the documentation.
The -source argument for the Java compiler. Default value is: 1.5.
User property is: maven.compiler.source.
If you only want to specify the source and target level, then there is no difference, but the <plugin> configuration also allows you to specify other configuration settings, restrict the versions of the compiler plugin, etc.
Many Maven plugins allow to use some properties to specify the parameters. The maven.compiler.source and maven.compiler.target properties are specified by the Maven Compiler Plugin.
See the "User Property" sections:
http://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html#source
http://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html#target
http://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html
I created a project with maven2 on eclipse. After I added the hibernate-annotations dependency, i was trying to create a class using hibernate annotations(#Entity, #Table...), but it was giving me this error :
Change project compliance and JRE to 1.5
I fixed it, but cannot understand why it requires this as long as my jdk is 1.6.
Thank in advance!
Check that the settings for the maven compiler plugin is set to 1.5 or 1.6 as well. If I'm not mistaken maven 2 defaults to 1.4.
Something like this:
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.1</version>
<configuration>
<source>1.5</source>
<target>1.5</target>
</configuration>
</plugin>
While javamonkey79's solution is the standard way to do it, there is also a property-based solution, but it's not the one fgysin suggests:
<properties>
<maven.compiler.source>1.6</maven.compiler.source>
<maven.compiler.target>1.6</maven.compiler.target>
</properties>
Reference (Maven compiler plugin):
<source> parameter
<target> parameter
BTW, the reason is that the maven compiler plugin builds a command line call to javac in which it specifies source and target version explicitly (overriding javac's default settings). And previous versions of the compiler plugin had their own defaults set to 1.3. However, starting from plugin version 2.3, 1.5 is the default source and target version.
Add the following lines to the POM:
<properties>
<java.version>1.6</java.version>
</properties>
Edit:
Ah, too bad, just saw that those are custom properties... Won't work.