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  • Sep 05, 2018
  • Post Nº 75

Maven JAR Plugin Understanding

A step-by-step guide for understanding Maven JAR Plugin in Java 8 and Java 11.

Maven JAR Plugin is one of the core plugins of the Maven build tool. It is used to build JARs. After reading this post, you’ll understand:

  • What is JAR?
  • What is Maven JAR Plugin?
  • How to invoke Maven JAR Plugin?
  • How to create an executable JAR?
  • Java 11 support

Now, let’s get started!

What is JAR?

Before going further, we must first understand what is JAR.

A JAR (Java ARchive) is a package file format typically used to aggregate many Java class files and associated metadata and resources (text, images, etc.) into one file for distribution. JAR files are archive files that include a Java-specific manifest file. They are built on the ZIP format and typically have a .jar file extension. For more detail, see wikipedia: JAR (file format)

Item Description
Filename extension .jar
Internet media type application/java-archive
Uniform Type Identifier (UTI) com.sun.java-archive
Developed by Netscape, Sun Microsystems, Oracle Corporation
Type of format File archive, data compression
Extended from ZIP

If you have a Java application packaged as a standalone JAR file, you can run it as:

java -jar /path/to/app.jar

What is Maven JAR Plugin?

Maven JAR Plugin is used to build JARs. It contains two goals:

  • jar:jar create a jar file for your project classes inclusive resources.
  • jar:test-jar create a jar file for your project test classes.

When you want to create a JAR with Maven, you need to create a POM file pom.xml with the following content. Note that <packaging> default to “jar”, so you can omit this field:

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>shop</groupId>
  <artifactId>shop-core</artifactId>
  <version>1.0-SNAPSHOT</version>
  <!-- <packaging>jar</packaging>  -->
</project>

How to Invoke JAR Plugin?

Maven JAR Plugin is bound to Maven Build Life Cycle. Both goals jar:jar and jar:test-jar are bound to lifecycle phase: package. Therefore, using any other commands which invode phase package, Maven JAR Plugin will be executed. For example, the following commands can invoke Maven JAR Plugin:

  • mvn clean install
  • mvn clean package
  • mvn clean verify
  • …

Of course, there’re more than that, but I’m not going to list all of them. You can see the plugin’s goal invoked in the logs of Maven’s execution:

maven-jar-plugin-demo-java8 $ mvn package
[INFO] Scanning for projects...
[INFO]
[INFO] --------------------------< shop:shop-parent >--------------------------
[INFO] Building Shop Parent 1.0-SNAPSHOT                                  [1/3]
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ shop-parent ---
[INFO]
[INFO] ---------------------------< shop:shop-api >----------------------------
[INFO] Building Shop API 1.0-SNAPSHOT                                     [2/3]
[INFO] --------------------------------[ jar ]---------------------------------
...
[INFO] --- maven-jar-plugin:3.1.0:jar (default-jar) @ shop-api ---
[INFO] Building jar: /Users/mincong/github/maven-jar-plugin-demo-java8/shop-api/target/shop-api-1.0-SNAPSHOT.jar
[INFO]
[INFO] ---------------------------< shop:shop-core >---------------------------
[INFO] Building Shop Core 1.0-SNAPSHOT                                    [3/3]
[INFO] --------------------------------[ jar ]---------------------------------
...
[INFO] --- maven-jar-plugin:3.1.0:jar (default-jar) @ shop-core ---
[INFO] Building jar: /Users/mincong/github/maven-jar-plugin-demo-java8/shop-core/target/shop-core-1.0-SNAPSHOT.jar

If you want to run the code yourself, check my GitHub mincong-h/maven-jar-plugin-demo-java8.

Create an Executable JAR

Create an executable JAR, requires some customization in Maven. You need to configure Maven Archieve accordinly—you need to tell it which main class to user. This is done via the <mainClass> configuration element. Here’s a sample pom.xml configured to add the classpath and use the class shop.core.Main as the main class:

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-jar-plugin</artifactId>
        <configuration>
          <archive>
            <manifest>
              <addClasspath>true</addClasspath>
              <mainClass>shop.core.Main</mainClass>
            </manifest>
          </archive>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

And you can package the application and execute it like:

$ mvn clean package
$ java -classpath shop-api/target/shop-api-1.0-SNAPSHOT.jar:shop-core/target/shop-core-1.0-SNAPSHOT.jar shop.core.Main
Welcome to Java 8

Note:

  • use semi-colon (;) as module path separator on Windows
  • use colon (:) as module path separator on macOS/Linux

Java 11 Support

Maven JAR plugin works for Java 11 without additional configuration, but be careful about Modules. You can package the application as usual using:

$ mvn package

In regard to execution, you need to care about Java Platform Module System (JPMS): provide the module path (--module-path) and the module (-m, --module) to execute. No classpath is required. In our case, we have 2 modules in the module paths:

Java Module File Path
shop-api shop-api/target/shop-api-1.0-SNAPSHOT.jar
shop-core shop-core/target/shop-core-1.0-SNAPSHOT.jar

Here’re the commands for packaging the app and printing “Welcome to Java 11”:

$ mvn clean package
$ java --module-path shop-api/target/shop-api-1.0-SNAPSHOT.jar:shop-core/target/shop-core-1.0-SNAPSHOT.jar --module shop-core/shop.core.Main
Welcome to Java 11

Note:

  • use semi-colon (;) as module path separator on Windows
  • use colon (:) as module path separator on macOS/Linux

If you want to run the code yourself, check my GitHub mincong-h/maven-jar-plugin-demo-java11.

Conclusion

In this post, we learnt what is JAR, what is Maven JAR Plugin, how to invoke it, and how to create an executable JAR. We also learnt how to use Maven JAR Plugin in Java 11 for Java Platform Module System (JPMS). If you need more information, the full source code are available on GitHub:

Hope you enjoy this post, see you the next time!

References

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