问题
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 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 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?
回答1:
How to specify the JDK version?
1) <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 :
<properties>
<java.version>1.8</java.version>
</properties>
Feel free to use it if you use Spring Boot.
2) Using maven-compiler-plugin
or maven.compiler.source
/maven.compiler.target
properties to specify the source
and the target
are equivalent.
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
and
<properties>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
are equivalent 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.
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.
3) The maven-compiler-plugin 3.6
and later versions provide a new way :
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.0</version>
<configuration>
<release>9</release>
</configuration>
</plugin>
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.
回答2:
None of the above solutions worked for me straight away. So I did the followings:-
Added
<properties> <maven.compiler.target>1.8</maven.compiler.target> <maven.compiler.source>1.8</maven.compiler.source> </properties>
in pom.xml
Went to the
Project Properties > Java Build Path
, then removed the JRE System Library which was pointing toJRE1.5
.Force updated the project.
回答3:
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)
来源:https://stackoverflow.com/questions/38882080/specifying-java-version-in-maven-differences-between-properties-and-compiler-p