First, it's kind of weird, to see you run java -jar "app" and not java -jar app.jar
Second, to make a jar executable... you need to jar a file called META-INF/MANIFEST.MF
the file itself should have (at least) this one liner:
Main-Class: com.mypackage.MyClass
Where com.mypackage.MyClass is the class holding the public static void main(String[] args) entry point.
Note that there are several ways to get this done either with the CLI, Maven, Ant or Gradle:
For CLI, the following command will do: (tks @dvvrt)
jar cmvf META-INF/MANIFEST.MF <new-jar-filename>.jar <files to include>
For Maven, something like the following snippet should do the trick. Note that this is only the plugin definition, not the full pom.xml:
<build>
<plugins>
<plugin>
<!-- Build an executable JAR -->
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<version>3.1.0</version>
<configuration>
<archive>
<manifest>
<addClasspath>true</addClasspath>
<classpathPrefix>lib/</classpathPrefix>
<mainClass>com.mypackage.MyClass</mainClass>
</manifest>
</archive>
</configuration>
</plugin>
</plugins>
</build>
(Pick a <version> appropriate to your project.)
For Ant, the snippet below should help:
<jar destfile="build/main/checksites.jar">
<fileset dir="build/main/classes"/>
<zipfileset includes="**/*.class" src="lib/main/some.jar"/>
<manifest>
<attribute name="Main-Class" value="com.acme.checksites.Main"/>
</manifest>
</jar>
Credits Michael Niemand -
For Gradle:
plugins {
id 'java'
}
jar {
manifest {
attributes(
'Main-Class': 'com.mypackage.MyClass'
)
}
}