I’m going to make a summary of the proposed solutions and the one that helped me!
After reading this bug report:
bug in the way NetBeans 6.8 creates the jar for a Java Library Project.
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Create a manifest.mf file in my project root
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Edit manifest.mf. Mine looked something like this:
Manifest-Version: 1.0 Ant-Version: Apache Ant 1.7.1 Created-By: 16.3-b01 (Sun Microsystems Inc.) Main-Class: com.example.MainClass Class-Path: lib/lib1.jar lib/lib2.jar
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Open file /nbproject/project.properties
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Add line
manifest.file=manifest.mf
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Clean + Build of project
Now the .jar is successfully build.
Thank you very much vkraemer