Eclipse does not recognize content of persistence.xml

I fixed the problem the following way (I'm using RAD v8.5.1):

1. Windows --> Preferences --> Validation
2. Scroll down to "JPA Validator" and click on the Ellipses under Settings.
3. Click on the "Add Include Group..." button.
4. Click on the new Include Group that appeared to highlight it.
5. Click on the "Add Rule..." button.
6. A dialog menu pops up. Click the "Folder or file name" radio button and click on "Next".
7. Click on "Browse Folder..." and navigate to your src directory (in my case, it was "src/main/resources") within your project (my project was a .war in a multi module app) in your workspace.
8. Click on "Finish".
9. Click on "OK". This will prompt a full build.
10. The error will go away.

Had same problem with error

The persistence.xml file does not have recognized content

Fixed by editing file in eclipse, any edit will do. And after saving error got away.

<persistence xmlns="http://java.sun.com/xml/ns/persistence"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/persistencehttp://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"version="2.0">

Worked for me!