我是 Java EE 的新手。我试图让一些第一个示例运行(JPA)。我正在使用 Glassfish v3。问题是我没有让 App Server 注入 EntityManager。听到的是一个示例http://www.adam-bien.com/roller/abien/entry/ejb_3_persistence_jpa_for ,我使用 JSP 客户端对其进行了扩展。
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;
}
}
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:
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);
}
}
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:
<%@ page import="beans.BookServiceBean" %>
<%@ page import="beans.Book" %>
<html>
<body>
<%
BookServiceBean bs = new BookServiceBean();
Book b = new Book("Superman");
bs.createOrUpdate(b);
%>
</body>
</html>
如果我运行该示例,我会在 createOrUpdate() 方法中得到 java.lang.NullPointerException,因此 entityManager 显然没有正确注入。几天来,我一直在尝试寻找补救措施,我们将不胜感激。
谢谢
马塞尔