Why to specify java version in maven build? - java

When I was building my project like :-
mvn clean install -DskipTests
, then it was giving some error.
After that, I just added, -Djdk.version=1.8, then it works fine.
Can someone tell what is the reason for this?


Try something like this in your pom:
<build>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.3</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</pluginManagement>
....
</build>


It may be because you might be missing the maven-compiler-plugin.
Try the following:
<build>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.3</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</pluginManagement>
....
</build>
From Maven 3, it defaults to JDK 1.5. So if you do not include the version, it will take JDK 1.5 as default compiler version.
Since, you mentioned that you used JDK 1.8, and the error disappeared. So if you had maven-compiler-version defined in the pom.xml, the error might be because the version would had not been defined.
So by default, it pointed to JDK 1.5 and it was trying to compile the code which would be defined for JDK 1.8 and not for JDK 1.5.
So, it is better to define the correct java version in the pom.xml


Why to specify java version in maven build?
Because Maven won't try to guess the Java version your project was created with (the one you probably only configured in your IDE).
You need to specify the earliest version supported in your pom.xml with:
<build>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.3</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>

Related

Maven version and java version conflict

I can only use java 1.6 for my project. So maven version also has to be 2.2.1 . Cannot use maven 3.0. But while building I'm getting following error :
Error resolving version for 'org.apache.maven.plugins:maven-jar-plugin':Plugin requires Maven version 3.0
this is the pom:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<version>3.0.0</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
what are the version that we can use?

Changes the source and target to 1.6. If your code does not make use of java8 features then it should work just fine. This will generate a jar that is compatible with any java6 codebase.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<version>3.0.0</version>
<configuration>
<source>1.6</source>
<target>1.6</target>
</configuration>
</plugin>

Eclipse: JRE System Library in Java Build Path reset automatically

I am using Eclipse as IDE. When I right click on the project, then click "Maven Update", my Java version changes to 1.5. Here is what I did so far:
I changed "Java build path" to "workspace default jre 1.8.0_25"
Then changed "java compiler" to 1.8
Then changed "project facets">java>1.8
Lastly Changed pom.xml java version to 1.8
<build>
<plugins>
<plugin>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-maven-plugin</artifactId>
<version>9.1.3.v20140225</version>
</plugin>
<plugin>
<groupId>org.apache.maven.plugin</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.2</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>
How can I keep it to 1.8 version?

There's a typo in the groupId of the maven-compiler-plugin in your pom.xml file: it should be org.apache.maven.plugins.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.2</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
Note that the groupId actually defaults to org.apache.maven.plugins so you could also omit it.
By default, the JRE used by m2e is 1.5, see this question : Eclipse JRE System Library [J2SE-1.5]

enabling the lambda expression

In the program like
entities.stream().filter(m->m.getId()==id).findAny().get();
where entities is a List. After setting all the libraries and other SDKs to Java 8. we are getting the error as:
use -source 8 or higher to enable lambda expressions

This is how solved my problem by adding the below plugin settings in my parent POM file.
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.1</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>

How to create a new classes for each compilation using maven plugin

In case of my project I need to create new classes after each compilation. For compilation I'm using maven compiler plugin 3.1. I tried to use compilerReuseStrategy = alwaysNew option but it didn't make any affect, it always compile only changed classes. Here is plugin declaration in pom.xml:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.1</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
<compilerReuseStrategy>alwaysNew</compilerReuseStrategy>
</configuration>
</plugin>
An I doing something wrong or that's a bug and this option really doesn't work?

If you are talking about the incremental feature fo the maven-compiler-plugin you can change this behaviour by the following configuration:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.1</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
<useIncrementalCompilation>false</useIncrementalCompilation>
</configuration>
</plugin>
The compileReuseStrategy in contradiction is intended to define the behaviour in relationship with multi-threaded running of the compiler.

What about using mvn clean install?

maven: (use -source 5 or higher to enable static import declarations)

How can I use source 5?
I tried
mvn -source 5 test
but it didn't work :-)
When I compile the file by javac, everything works.

You need to configure the maven-compiler-plugin:
<project>
...
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<configuration>
<source>1.5</source>
<target>1.5</target>
</configuration>
</plugin>
...
</plugins>
...
</build>
</project>
EDIT: Changed example to use latest version of the plugin.

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