Why is My EntityManager Throwing a 'Persistence Provider Not Found' Error?

Persistence Provider Not Found for EntityManager

When attempting to establish a connection to a database through an EntityManager and encountering the error "No Persistence provider for EntityManager named [EntityManager name]," it signifies that the persistence.xml file is present and configured correctly, but the appropriate persistence provider library is missing from the classpath.

In this specific case, using TopLink as the persistence provider, the error is due to the absence of the "hibernate-entitymanager.jar" library in the Java Build Path. For newer versions of Hibernate, such as Hibernate 5, "hibernate-core.jar" should be used instead.

Solution:

To resolve this issue, ensure that the appropriate persistence provider library is added to the classpath of the application. This library is typically found in the lib directory of the IDE or application server. For example, in Eclipse:

1. Right-click on the project.
2. Select "Properties."
3. Navigate to "Java Build Path -> Libraries."
4. Click on the "Add External JARs" button.
5. Locate and select the "hibernate-entitymanager.jar" (or "hibernate-core.jar") file.

Alternatively, the library can be manually added to the /lib directory of the application.

After adding the persistence provider library to the classpath, the EntityManager should be able to find and utilize it to establish a connection to the database successfully.

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