java Glassfish JPA：注入 EntityManager 的问题
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Glassfish JPA: Problems injecting EntityManager
提问by Marcel Menz
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_forwhich I extended with a JSP client.
我是 Java EE 的新手。我试图让一些第一个例子运行（JPA）。我正在使用 Glassfish v3。问题是我没有让应用服务器注入 EntityManager。听到一个例子http://www.adam-bien.com/roller/abien/entry/ejb_3_persistence_jpa_for，我用一个 JSP 客户端扩展了它。
Entity:
实体：
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:
图书服务接口：
package beans;
import javax.ejb.Local;
@Local
public interface BookService {
Book createOrUpdate(Book book);
void remove(Book book);
Book find(Object id);
}
BookServiceBean:
图书服务Bean：
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:
持久性.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:
索引.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.
如果我运行这个例子，我会在 createOrUpdate() 方法中得到一个 java.lang.NullPointerException ，所以 entityManager 显然没有正确注入。几天来我一直试图找到一种补救措施，我们将不胜感激。
Thanks
谢谢
Marcel
马塞尔
回答by Pascal Thivent
You get a NullPointerExceptionbecause you are instantiating your BookServicewith 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.
你得到 a 是NullPointerException因为你BookService用 a实例化你的new()- 这基本上是错误的 - 并且没有任何东西被注入到 EJB 中。EJB 是由容器管理的组件，应该通过注入或查找来获取。
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:
在这里，虽然 JSP 规范允许在脚本中运行任何代码，但实际上并不鼓励从 JSP 调用 EJB，并且 JSP 不支持注入。换句话说，您必须使用查找：
<%@ 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).
但是您应该从 Servlet 或 JSF Managed Bean 调用您的 EJB（并且您的 EJB 可以被注入到此类组件中）。
If you need some samples, have a look at the Java EE Code Samples & Apps.
如果您需要一些示例，请查看Java EE 代码示例和应用程序。
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).
更新：请参阅如何从 POJO 访问本地 EJB 组件？有关 JNDI（尤其是由 EJB 3.1 规范定义的新的可移植全局 JNDI 名称）的更多详细信息，请参阅 EJB 常见问题解答。
回答by Enno Shioji
Try:
尝试：
@PersistenceContext(unitName = "sample")
private EntityManager em;
回答by jsight
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.
当您确实需要让容器注入它时（通过@EJB），您将直接实例化服务 bean。但是，这在 JSP 中不受支持，因此您也必须切换到 servlet。
