This is caused by
- a JAR on the Classpath that contains the package
java.awt
that also exists in the system library but the - JRE System Library is on the Modulepath
In the Java Platform Module System (JPMS) it is not allowed to use the same package in more than one module. If the Modulepath and the Classpath is used, everything on the Classpath is handled as the <unnamed>
module (in your case the package java.awt
exists in the system module java.desktop
and also via the JAR on the Classpath in the module <unnamed>
).
Since the JRE System Library cannot be moved from the Modulepath to the Classpath (see this answer by Stephan Herrmann for details), you only have the following options:
- Set the compiler compliance to 1.8 (as you already mentioned)
- Rebuilt the JAR to avoid Java system library package names inside the JAR (if reflection is used, additional code changes may be necessary):
- If you have the source code, change the package names (e.g. change the package and subpackae
java
tojava_util
andjavax
tojavax_util
) and recreate the JAR - If you have only the
.class
files you have to decompile the.class
files first
- If you have the source code, change the package names (e.g. change the package and subpackae