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

Question

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    lt properties gt       lt java version gt 1 8 lt  java version gt   lt  properties gt    but when researching I found that you can also specify java version in maven compiler plugin  like that    lt plugins gt       lt plugin gt               lt artifactId gt maven-compiler-plugin lt  artifactId gt           lt configuration gt               lt source gt 1 8 lt  source gt               lt target gt 1 8 lt  target gt           lt  configuration gt       lt  plugin gt   lt  plugins gt    And then wrap this into plugin management tag to enable child poms usage of this  So the first question is 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    lt properties gt       lt maven compiler source gt 1 8 lt  maven compiler source gt       lt maven compiler target gt 1 8 lt  maven compiler target gt   lt  properties gt    which suggest that compiler plugin is there even if I dont 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 beetwen 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

User · Answer

Consider the alternative    lt properties gt       lt javac src version gt 1 8 lt  javac src version gt       lt javac target version gt 1 8 lt  javac target version gt   lt  properties gt    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

User · Answer

How to specify the JDK version  Use any of three ways   1  Spring Boot feature  or use Maven compiler plugin with either  2  source  amp  target or  3  with release  Spring Boot   lt java version gt  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  amp  target  Using maven-compiler-plugin or maven compiler source maven compiler target properties are equivalent   That is indeed    lt plugins gt       lt plugin gt               lt artifactId gt maven-compiler-plugin lt  artifactId gt           lt configuration gt               lt source gt 1 8 lt  source gt               lt target gt 1 8 lt  target gt           lt  configuration gt       lt  plugin gt   lt  plugins gt   is equivalent to    lt properties gt       lt maven compiler source gt 1 8 lt  maven compiler source gt       lt maven compiler target gt 1 8 lt  maven compiler target gt   lt  properties gt   according to the Maven documentation of the compiler plugin since the  lt source gt  and the  lt target gt  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  Default value is  1 6  User property is  maven compiler source    target   The -target argument for the Java compiler  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  amp  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    lt properties gt       lt maven compiler release gt 9 lt  maven compiler release gt   lt  properties gt   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   lt java version gt   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   lt properties gt       lt maven compiler source gt 1 8 lt  maven compiler source gt       lt maven compiler target gt 1 8 lt  maven compiler target gt   lt  properties gt   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    lt plugin gt       lt groupId gt org apache maven plugins lt  groupId gt       lt artifactId gt maven-compiler-plugin lt  artifactId gt       lt configuration gt           lt source gt 1 8 lt  source gt           lt target gt 1 8 lt  target gt           lt compilerVersion gt 1 8 lt  compilerVersion gt                 lt fork gt true lt  fork gt           lt executable gt D  jdk1 8 bin javac lt  executable gt                       lt  configuration gt   lt  plugin gt   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  lt build gt  lt plugins gt  element  for example        lt plugin gt      lt artifactId gt maven-clean-plugin lt  artifactId gt      lt version gt 2 5 lt  version gt      lt executions gt        lt execution gt          lt id gt default-clean lt  id gt          lt phase gt clean lt  phase gt          lt goals gt            lt goal gt clean lt  goal gt          lt  goals gt        lt  execution gt      lt  executions gt    lt  plugin gt    lt plugin gt      lt artifactId gt maven-resources-plugin lt  artifactId gt      lt version gt 2 6 lt  version gt      lt executions gt        lt execution gt          lt id gt default-testResources lt  id gt          lt phase gt process-test-resources lt  phase gt          lt goals gt            lt goal gt testResources lt  goal gt          lt  goals gt        lt  execution gt        lt execution gt          lt id gt default-resources lt  id gt          lt phase gt process-resources lt  phase gt          lt goals gt            lt goal gt resources lt  goal gt          lt  goals gt        lt  execution gt      lt  executions gt    lt  plugin gt    lt plugin gt      lt artifactId gt maven-compiler-plugin lt  artifactId gt      lt version gt 3 1 lt  version gt      lt executions gt        lt execution gt          lt id gt default-compile lt  id gt          lt phase gt compile lt  phase gt          lt goals gt            lt goal gt compile lt  goal gt          lt  goals gt        lt  execution gt        lt execution gt          lt id gt default-testCompile lt  id gt          lt phase gt test-compile lt  phase gt          lt goals gt            lt goal gt testCompile lt  goal gt          lt  goals gt        lt  execution gt      lt  executions gt    lt  plugin gt         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

User · Answer

None of the solutions above worked for me straight away  So I followed these steps   Add in pom xml    lt properties gt       lt maven compiler target gt 1 8 lt  maven compiler target gt       lt maven compiler source gt 1 8 lt  maven compiler source gt   lt  properties gt    Go to Project Properties  gt  Java Build Path  then remove the JRE System Library pointing to JRE1 5   Force updated the project

[java] Specifying java version in maven - differences between properties and compiler plugin

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