Glassfish JPA: Problems injecting EntityManager

StackOverflow https://stackoverflow.com/questions/2359139

문제

I am new to Java EE. I tried to get some first examples running (JPA). I am using Glassfish v3. The trouble is that I don't get the App Server injecting the EntityManager. Hear is one example http://www.adam-bien.com/roller/abien/entry/ejb_3_persistence_jpa_for which I extended with a JSP client.

Entity:


package beans;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

@Entity
public class Book {

    @Id
    @GeneratedValue
    private Long id;
    private String title;
    private String subtitle;

    public Book() {
    }

    public Book(String title) {
        this.title = title;
    }
}

BookService Interface:


package beans;
import javax.ejb.Local;

@Local
public interface BookService {
    Book createOrUpdate(Book book);
    void remove(Book book);
    Book find(Object id);
}

BookServiceBean:


package beans;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Stateless
public class BookServiceBean implements BookService {

    @PersistenceContext
    private EntityManager em;

    public Book createOrUpdate(Book book) {
        return em.merge(book);
    }
    public void remove(Book book) {
        em.remove(em.merge(book));
    }
    public Book find(Object id) {
        return em.find(Book.class, id);
    }
}

persistence.xml:

<persistence>
    <persistence-unit name="sample" transaction-type="JTA">
    <jta-data-source>jdbc/MarcelsDataSource</jta-data-source>
    <properties>
        <property name="eclipselink.ddl-generation" value="create-tables"/>
    </properties>
</persistence-unit>
</persistence>

index.jsp:

<%@ page import="beans.BookServiceBean" %>
<%@ page import="beans.Book" %>
<html>
<body>
<%
    BookServiceBean bs = new BookServiceBean();
    Book b = new Book("Superman");
    bs.createOrUpdate(b);
%>
</body>
</html>

If I run the example I get a java.lang.NullPointerException in the createOrUpdate() method so the entityManager is obviously not injected correctly. I tried to find a remedy for days now and some help would be highly appreciated.

Thanks

Marcel

도움이 되었습니까?

해결책

You get a NullPointerException because you are instantiating your BookService with a new() - which is basically wrong - and nothing gets injected in the EJB. EJB are component that are managed by the container and should be obtained either via injection or with a lookup.

Here, while the JSP spec allows any code to be run in a scriplet, calling an EJB from a JSP is actually not really encouraged and JSPs don't support injection. In other words, you'll have to use a lookup:

<%@ page import="beans.BookService" %>
<%@ page import="beans.Book" %>
<html>
<body>
<%
    BookService bs = (BookService) new InitialContext().lookup("java:module/BookServiceBean")
    Book b = new Book("Superman");
    bs.createOrUpdate(b);
%>
</body>
</html>

But you should call your EJB from a Servlet or a JSF Managed Bean (and your EJB could be injected in such components).

If you need some samples, have a look at the Java EE Code Samples & Apps.

Update: See How do I access a Local EJB component from a POJO? in the EJB FAQ for more details on JNDI (especially the new portable global JNDI names defined by the EJB 3.1 specification).

다른 팁

Try: 

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

You are instantiating the service bean directly, when you really need to be having the container inject it (via @EJB). This isn't supported in a JSP, though, so you'll have to switch to a servlet as well.

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