Spring MVC and Dao Test

I'm developing a web application using Spring MVC runing on glassfish.

Globaly every is going well but the test purpose.

Since for the webapp i have to decalre in web.xml

Code:

---

        <persistence-context-ref>
                <persistence-context-ref-name>MyApp_JTA</persistence-context-ref-name>
                <persistence-unit-name>MyApp_JTA</persistence-unit-name>
        </persistence-context-ref>

---

to make it able to identify persistence unit name in the DAO class

Code:

---

        @PersistenceContext(unitName = "MyApp_JTA")
        private EntityManager em;

---

but this make the test using JUnit fails since it uses another persistence unit that doesn't need to be hard coded the as shown above.

So any suggestion ? thanks