title: When and why JPA entities should implement the Serializable interface?
body: <p>The question is in the title. Below I just described some of my thoughts and findings.</p>
  <p>When I had a very simple domain model (3 tables without any relations), all my entities did NOT implement the <code>Serializable</code> interface.</p>
  <p>But when the domain model became more complex, I got a <code>RuntimeException</code>, saying that one of my entities didn't implement <code>Serializable</code>.</p>
  <p>I use Hibernate as a JPA implementation, and I wonder:</p>
  <ol>
  <li>Is it a vendor-specific requirement/behavior?</li>
  <li>What happens with my serializable entities? Should they be serializable for storing or for transferring?</li>
  <li>At which moment it becomes necessary to make my entity serializable?</li>
  </ol>