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: , jav

Answer 1

Since I'll bet lots of people will be running into this problem with modular Java, I'll help and give the real answer. This error happens when you have a dependency in your project that contains code using packages that are also in the modules being referenced by your project. If your project has set the source compatibility to something like Java 12, it will start enforcing the rule, that has been there all along in Java. "Don't use packages that belong to the JDK in your own code." Unfortunately, lots of developers and vendors have done that over the years. Can't do that anymore. If you set your project to Java 12 source compatibility, Eclipse adds the JDK modules which include everything "java." and "javax." and even "jdk.", "org.w3c.". These packages may be in use by your dependencies or their transitive dependencies.

How to fix: You need to look at which package its complaining about and expand the "Projects and External Dependencies" node in the Package Explorer. Find out which dependency is using that package. Then you can simply exclude that dependency from your project. Or you could get the source of that dependency, if available, and rebuild the jar with changed packages. Otherwise you have to remove that dependency and find a replacement for that technology. Pain huh?

If its a transitive dependency you can often just exclude it. Here is an example of that for Gradle based projects.

configurations {
    all*.exclude group: 'xml-apis'
}