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copied!<p>In short, when declaring <code>@PersistenceUnit</code> on a class, you must add a <code>name="..."</code> that you can use to look up the <code>EntityManagerFactory</code> with <code>new InitialContext("java:...")</code>. Alternatively, you can declare an <code>@PersistenceUnit(...) EntityManagerFactory emf;</code> field in your class and omit the name.</p> <p>All ref annotations are basically the same as <code>@Resource</code>. Per the commons annotations specification:</p> <blockquote> <p>The name element is the JNDI name of the resource. When the Resource annotation is applied on a field, the default value of the name element is the field name qualified by the class name. When applied on a method, the default is the JavaBeans property name corresponding to the method qualified by the class name. When applied on a class, there is no default and the name MUST be specified.</p> </blockquote> <p>The last sentence is relevant, and it makes sense: using <code>@PersistenceUnit</code> (and all other <code>@Resource</code>-like annotations) has two effects:</p> <ol> <li>If you declare the annotation on a field or method, then the container will automatically inject when the instance is created.</li> <li>The reference is inserted into the <code>java:comp</code> namespace using its name. If you declare the annotation on a field or method, the default name is <code>java:comp/env/com.example.ClassName/targetName</code>.</li> </ol> <p>If you declare the annotation on a class, then (1) there's no injection, and (2) there's no default name so there's no way to bind into <code>java:comp/env</code>. In that case, the annotation declaration would be pointless, so it's an error.</p>

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