Eclipse is confused by imports ("accessible from more than one module")

Question

When referencing simple .jar files, Eclipse shows an error stating:

The package java.awt is accessible from more than one module: <unnamed>, java.desktop

This happens for instance when javax.awt or javax.swing is included in the .jar files.

The simplest example would be the following:

package test;  import javax.swing.JDialog;  public class Test {     public static void main(String[] args) {         new JDialog();     } } 

Adding a .jar file to the classpath with only the folder structure javax/swing (no files needed) will cause the error to appear. I'm using JDK 10/12 (neither works). Setting the compiler compliance to 1.8 makes the whole thing work again. On another machine with Eclipse 2018-09 this works with compiler compliance set to 10.

I'm on Eclipse 2019-03, on a (for testing purposes) freshly installed Eclipse 2018-09 it works fine. Why?

Edit June/2020 (Solution)

As the answers correctly stated, this is a restriction built into Java ages ago and only recently was forced upon us. I came into contact with it while migrating a big project with dozens of dependencies to Maven. There were libraries from around the year 2000! There were 'meta libraries' which consisted of several libraries packaged together. So there was no other way than to identify what was still needed (into the trash with the rest!), update libraries which violate the rules or find replacements for them. This took me many, many hours.

In the end it worked out and we've got a nice Maven project to work with.

Answer 1

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 to java_util and javax to javax_util) and recreate the JAR
  • If you have only the .class files you have to decompile the .class files first