can a class in a jar use a class in a different jar

Question

I am new to java so not sure I fully understand jar files.

I want to put some common code in a library jar, which I then use from applications that are in different jars

Answer 1

If the Class-Path attribute of the manifest of the application jar file is

mylibrary-0.0.1-SNAPSHOT.jar

and the two jar files are in the same directory, then running

java -jar myapp-0.0.1-SNAPSHOT.jar should work.

You can also set the classpath explicitely, and set the main class explicitely, which is much easier, and doesn't rely on relative paths in the manifest.

java -cp myapp-0.0.1-SNAPSHOT.jar:mylibrary-0.0.1-SNAPSHOT.jar org.myorg.myapp.Main

You should not use the -classpath and the -jar options in the same command. It's one or the other.

Answer 2

This should be marked as a duplicate of this question I just came across

It is the exact problem and solution, which I could not find with all my previous searching until I knew exactly what I was searching for, i.e. Executable JAR ignores its own Class-Path attribute

So not a java problem at all but a maven problem and the usual sparse maven documentation sheds no light on the problem at all. Unfortunately the sample config I copied included the setting, which by default is turned off. Final solution, don't use indexing with the maven plugin.

Answer 3

Try some modification in the pom.xml

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <configuration>
        <archive>
            <manifest>
                <mainClass>org.myorg.myapp.Main</mainClass>
                <addClasspath>true</addClasspath>
            </manifest>
        </archive>
    </configuration>
</plugin>

And after this if you still have problems just unpack your myapp-0.0.1-SNAPSHOT.jar using any unzip tool. After unzipping it check the path of mylibrary-0.0.1-SNAPSHOT.jar in the MANIFEST file in META-INF folder.