i\'m using Netbeans 6.8 and build simple Maven web application project.
create Entity and main file for persist Entity [also create persist unit] and use EclipsLink.
This problems generally occures when there is no hibernate-jpa-2.0-api jar in classpath so first add this jar in your classpath. if still you are getting excption then go to C:\Users\(system user name)\.m2\repository\org\hibernate; delete folder javax. after that again download jars from pom.xml it will work.
first make sure hibernate-jpa-2.0-api jar downloaded and it is available in your build path. after that right click on project-->Build Path--->configure build path--->deployment Assembly--->Add--->java build path entries--->select the available jar(hibernate-jpa-2.0-api)--->next--->finish.
Just incase you stumble upon this. Another problem could be you have a project importing another project the parent project is the one who is supposed to hold all the libraries.
Click Here to download hibernate-jpa-2.0-api-1.0.1.Final.jar and put it into the project library, your application will work fine. Good luck:)
I had the same problem and I solved it by changing the dependencies to the following :
    <dependency>
        <groupId>org.eclipse.persistence</groupId>
        <artifactId>javax.persistence</artifactId>
        <version>2.2.1</version>
    </dependency>
    <dependency>
        <groupId>org.eclipse.persistence</groupId>
        <artifactId>org.eclipse.persistence.jpa</artifactId>
        <version>2.7.3</version>
    </dependency>
    <dependency>
        <groupId>javax</groupId>
        <artifactId>javaee-web-api</artifactId>
        <version>7.0</version>
        <scope>provided</scope>
    </dependency>
                                                                        In your pom.xml you need to add a dependency to JPA libraries.