How to generate an executable . jar using Maven?

Question

Is it necessary to make some extra configuration to generate a Maven executable . jar? How have I never used this technology under development desktop I’m a little lost.

When I build the project (the logs show that you built successfully) and try to run it, nothing happens. To see if there was an exception or something similar, I tried to execute by command line and here’s the message:

Unable to locate or load main class

My file pom.xml is like this:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>kitty.project</groupId>
    <artifactId>Kitty</artifactId>
    <version>1.0</version>
    <packaging>jar</packaging>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>
    <name>Kitty</name>
    <dependencies>
       <!-- Dependências do projeto -->
    </dependencies>
</project>

Is any configuration missing? How can I fix this and generate one .jar executable as if it were a normal Java application?

Answer 1

Yes, it is missing to specify your main class (main class). For this you can use this plugin:

<build>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-jar-plugin</artifactId>
      <configuration>
        <archive>
          <manifest>
            <mainClass>sua.classe.Main</mainClass>
          </manifest>
        </archive>
      </configuration>
    </plugin>
  </plugins>
</build>

Answer 2

To reply from Afonso worked as I expected and generated a .jar that runs my application.

Until then I didn’t know they existed plugins for Maven and, researching a little more found the Maven Assembly Plugin that (in addition to doing the same thing) still allows building a single .jar with all dependencies packaged.

In the case of my application, this seemed more interesting, in the sample page there are some other situations where the plugin can be useful. The settings I used in my file pom.xml were:

<plugins>

   <!-- ... -->

   <plugin>
       <artifactId>maven-assembly-plugin</artifactId>
       <configuration>
          <archive>
              <manifest>
                  <mainClass>br.com.MinhaClasseMain</mainClass>
              </manifest>
          </archive>
          <descriptorRefs>
              <descriptorRef>jar-with-dependencies</descriptorRef>
          </descriptorRefs>
       </configuration>
       <executions>
          <execution>
              <phase>package</phase>
              <goals>
                 <goal>single</goal>
              </goals>
          </execution>
       </executions>
   </plugin>

   <!-- ... -->

</plugins>

Note: it is possible to omit the artifact group since Maven has the org.apache.maven.plugins as standard group. And in this case, it is the same group as the plugin.

Answer 3

Renan also do not know such technology either, but about Java (and not Maven) if I am not mistaken the command java search for a file .class, is not?

To run files jar would need the parameter -jar, as in the example:

java -jar Kitty-1.0.jar