Why Can\'t Eclipse Find My XML Classes After Switching to JDK 10?

Eclipse Encounters Issues Locating XML Classes Post JDK 10 Build Path Switch

While working on a Maven project within Eclipse, a user encountered difficulties after modifying the project's Java Build Path to JDK 10. Specifically, Eclipse became unable to locate XML-related classes, such as javax.xml.xpath.XPath, org.w3c.dom.Document, or org.xml.sax.SAXException. Interestingly, the issue only affected XML classes from the Maven dependency xml-apis-1.4.01.

Despite the inability to access these classes during the Eclipse build process, a Maven build executed without error. Additionally, using Ctrl-LeftClick on one of the allegedly missing classes allowed the user to locate and open it within Eclipse's editor. Thus, it appeared that the problem was isolated to the Eclipse build.

To rectify the situation, the user attempted various solutions, including:

• Cleaning the project
• Employing different Eclipse versions (Oxygen and Photon)
• Utilizing both JDK 8 and JDK 10 with Eclipse
• Adjusting the project's Compiler Compliance level (which remained problematic)

None of these measures resolved the issue.

Understanding the Root Cause and Finding Solutions

The underlying issue stems from the assumption that the project migrating from Java 1.8 lacks a module-info.java file, resulting in compilation within an "unnamed module." Within this module, the code evaluates observable named and unnamed modules, including "java.xml" from the JRE System Library. The latter provides access to packages like java.xml.xpath.

Concurrent to this, the project uses xml-apis.java as a dependency, which offers an alternate set of packages with identical names (e.g., java.xml.xpath). As both sets of packages reside in the unnamed module, they conflict, violating the requirement of "unique visibility" in Java 1.8.

To resolve the conflict, two solutions are available:

1. Add a module-info.java file to the project, allowing control over which modules (either java.xml or xml.apis) it utilizes.
2. Exclude java.xml from observable modules. In Eclipse, this can be achieved through the "Modularity Details" dialog, found under the "Contents" tab. By moving java.xml from "Explicitly included modules" to "Available modules," the conflict is avoided.

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