Rule Definition
When Entity bean instance is to be passed by value as a detached object (e.g., through a remote interface), the entity class must implement the Serializable interface. Also, In some cases an OptimisticLockException will be thrown and wrapped by another exception, such as a RemoteException, when VM boundaries are crossed. Entities that may be referenced in wrapped exceptions should be Serializable so that marshalling will not fail. A consequence of not following this rule when a non serializable entity is passed by value is to get an exception.

Remediation
Implement Serializable and verify that each its instance variables implement serializable unless they inherit from a class that is serializable, is a transient field or a Java primitives..

Violation Code Sample

@Entity
public class ABean  { // VIOLATION
    private String str;
    private aClass field;
}

Fixed Code Sample

@Entity
public class ABean implements Serializable { // FIXED
    private String str;
    private aClass field;
}

JSR-000220 Enterprise JavaBeans v.3.0 specification version 3.0 p 17 and p 57 http://techpolesen.blogspot.com/2007/11/10-tips-on-jpa-domain-modelling.html