how to generate barcode in vb.net 2008 Accessing an application-managed EntityManager outside the container in Java

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12.3.3 Accessing an application-managed EntityManager outside the container
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Developers write all sorts of applications. You might have some applications, such as a Swing or SWT application, for which you d prefer to use JPA outside the EJB 3 container. Or perhaps you just want to utilize web containers like Tomcat or Jetty that are not Java EE 5 containers, or one of the other lightweight containers that do not have support for a container-managed EntityManager or JTA transactions. Another case in which you might want to use JPA outside the EJB 3 container is when you re testing your entities. You can use the approach we discuss here in conjunction with a test framework such as JUnit and to test entities outside the container.
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Effectively integrating EJB 3 across your application tiers
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It s simple to use JPA without an EJB 3 container. All you need to do is create a persistence unit and create an EntityManager from the EntityManagerFactory obtained from the Persistence class. Let s look at an example. Defining a persistence unit As you know from chapters 9 and 11, a persistence unit requires a DataSource configured in the application server that is used by the persistence provider to connect to the database. Normally, the Java EE container will provide this, but if you are using JPA outside the container, you must provide the JDBC configuration as vendor-specific properties in the persistence.xml file. Listing 12.5 shows a sample persistence.xml that illustrates the JDBC configuration for the GlassFish server. The property names will vary depending on which persistence provider you re using.
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Listing 12.5 JPA outside a container: an example persistence.xml
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<persistence xmlns = "http://java.sun.com/xml/ns/persistence" version = "1.0"> <persistence-unit name = "actionBazaar"> <provider>oracle.toplink.essentials.PersistenceProvider</provider> <class>actionbazaar.persistence.Bid</class> <class>actionbazaar.persistence.Item</class> <properties> <property name = "toplink.jdbc.driver" value = "oracle.jdbc.OracleDriver"/> <property name = "toplink.jdbc.url" value = "jdbc:oracle:thin:@//localhost:1521/ORCL"/> <property name = "toplink.jdbc.user" value = "scott"/> <property name = "toplink.jdbc.password" value = "tiger"/> </properties> </persistence-unit> JDBC configuration </persistence>
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Creating an application-managed EntityManager To perform CRUD operations on entities, you need to grab an instance of an EntityManager. An EntityManager may not be available in the JNDI registry or accessible by dependency injection when using JPA outside the Java EE container. In such cases, you have to create an EntityManager from the EntityManagerFactory that can be created from the javax.persistence.Persistence factory class. This is typically the case when you re trying to use JTA in environments such as Java SE or when you re using a web container like Tomcat. (JTA transactions are unavailable outside the Java EE container.)
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Using JPA from the web tier
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Listing 12.6 shows an example of how you can use entities outside the container when JTA transactions are not available.
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Listing 12.6 Persistence outside the container: a Java SE class using the EJB 3 JPA
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public class PlaceBidBeanJavaSE { private static EntityManagerFactory emf; private static EntityManager em; public static void main(String[] args) { String userId = "idiot"; Long itemId = new Long(100); Double bidPrice = 2001.50; try Creates { EntityManagerFactory //Create EntityManagerFactory emf = Persistence.createEntityManagerFactory("actionBazaar"); getEntityManager(); em.getTransaction().begin(); addBid(userId, itemId, bidPrice); em.getTransaction().commit(); } finally { closeEntityManager(); emf.close(); }
Begins EntityTransaction
Commits EntityTransaction
private static void getEntityManager() { em = emf.createEntityManager(); } Creates EntityManager
private static void closeEntityManager() { em.close(); } Closes EntityManager
Note that we are using the javax.persistence.Persistence class b to create an instance of EntityManagerFactory. Persistence is the bootstrap class outside the Java EE container that allows us to access the EntityManagerFactory by invoking the createEntityManagerFactory method. You can create an EntityManager instance c after initializing the factory. Keep in mind that you have to use the EntityTransaction interface to begin and commit a transaction d and e. Finally we close the EntityManager f.
Effectively integrating EJB 3 across your application tiers
When running JPA outside the container, you have to make sure you include all the required files in the CLASSPATH. Check the appropriate documentation for your persistence provider to determine any additional requirements it may have.
12.4 Summary
In this chapter you learned how to use EJBs from other tiers. We first looked at how you can use EJB 3 session beans and JPA from the web tier. We described some design patterns that you can use to build better applications. The ModelView-Controller pattern helps separate code based on lines of responsibility. It is most commonly used in frameworks dealing with user interfaces (such as web frameworks). The Entity Access Object design pattern upgrades the trusted DAO pattern to support EJB 3 entities. EAO reduces tight coupling between application tiers, and provides the ability to change persistence code without impacting business logic. Similarly, the Session Fa ade design pattern reduces the presentation tier s dependencies on the internal workings of the application tier, and improves centralization of transaction management and security. You can access EJB 3 session beans using either dependency injection or JNDI lookup, but you must avoid injecting stateful session beans into multithreaded clients. JPA can be used from web applications without having to depend on session beans. As we showed you in this chapter, you have to use UserTransaction to demarcate transactions when using JPA directly from the web tier. The next chapter discusses performance and scalability issues around real EJB 3 applications.
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