Why does JPA require a no-arg constructor for domain objects ?

Question

Why does JPA require a no-arg constructor for domain objects ? I am using eclipselink and just got this exception during deployment.

Exception [EclipseLink-63] (Eclipse Persistence Services-1.1.0.r3639-SNAPSHOT): 
org.eclipse.persistence.exceptions.DescriptorException

Exception Description: The instance creation method   
[com.me.model.UserVO.<Default Constructor>], with no parameters, 
  does not exist, or is not accessible.
Internal Exception: java.lang.NoSuchMethodException: 
  com.me.model.UserVO.<init>()
Descriptor: RelationalDescriptor(com.me.model.UserVO --> 
  [DatabaseTable(user)])

Answer 1

Because it often happens that the JPA provider has to instantiate your domain object dynamically. It cannot do so, unless there is a no-arg constructor - it can't guess what the arguments should be.