EJB3.0，EJB查询语言编写自定义的查询非常方便，不用担心底层数据库的详细信息。这是很相似的HQL(Hibernate查询语言)，通常被称为按名称EJBQL。
要理解熟悉ejb的EJBQL，要做好以下几项工作。
步骤 1. 在数据库中创建表.
步骤 2. 创建一个无状态EJB.
步骤 3. 更新无状态的EJB。添加添加记录并获得通过实体管理器从数据库中记录的方法。
步骤 4. 一个基于控制台应用程序客户端将访问无状态EJB的持久化数据库中的数据。
创建数据库表
创建表 books 在默认数据库 postgres.
CREATE TABLE books (
id integer PRIMARY KEY,
name varchar(50)
);
创建模型类
public class Book implements Serializable{
private int id;
private String name;
public Book(){
}
public int getId() {
return id;
}
...
}
创建无状态EJB
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
public void addBook(Book book) {
//persist book using entity manager
}
public List getBooks() {
//get books using entity manager
}
...
}
构建EJB模块后，我们需要一个无状态的bean，我们将在下一节要创建客户端来访问。
示例应用程序
让我们创建一个测试EJB应用程序来测试EJB的数据库访问机制。
StepDescription
1Create a project with a name EjbComponent under a package com.tutorialspoint.entity as explained in the EJB - Create Application chapter. You can also use the project created in EJB - Create Application chapter as such for this chapter to understand ejb data access concepts.
2Create Book.java under package com.tutorialspoint.entity and modify it as shown below.
3Create LibraryPersistentBean.java and LibraryPersistentBeanRemote as explained in the EJB - Create Application chapter and modify them as shown below.
4Clean and Build the application to make sure business logic is working as per the requirements.
5Finally, deploy the application in the form of jar file on JBoss Application Server. JBoss Application server will get started automatically if it is not started yet.
6Now create the ejb client, a console based application in the same way as explained in theEJB - Create Application chapter under topic Create Client to access EJB. Modify it as shown below.
EJBComponent (EJB Module)
Book.java
package com.tutorialspoint.entity;
import java.io.Serializable;
public class Book implements Serializable{
private int id;
private String name;
public Book(){
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
LibraryPersistentBeanRemote.java
package com.tutorialspoint.stateless;
import com.tutorialspoint.entity.Book;
import java.util.List;
import javax.ejb.Remote;
@Remote
public interface LibraryPersistentBeanRemote {
void addBook(Book bookName);
List getBooks();
}
LibraryPersistentBean.java
package com.tutorialspoint.stateless;
import com.tutorialspoint.entity.Book;
import java.util.List;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
public LibraryPersistentBean(){
}
@PersistenceContext(unitName="EntityEjbPU")
private EntityManager entityManager;
public void addBook(Book book) {
entityManager.persist(book);
}
public List getBooks() {
//create an ejbql expression
String ejbQL = "From Book b where b.name like ?1";
//create query
Query query = entityManager.createQuery(ejbQL);
//substitute parameter.
query.setParameter(1, "%test%");
//execute the query
return query.getResultList();
}
}
只要在JBoss部署 EjbComponent项目，会注意到jboss的日志。
JBoss已经自动为我们的会话bean创建一个JNDI条目 -LibraryPersistentBean/remote.
我们将使用这个查询字符串来获得远程类型的业务对象 -com.tutorialspoint.stateless.LibraryPersistentBeanRemote
JBoss应用服务器的日志输出
...
16:30:01,401 INFO [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:
LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
LibraryPersistentBean/remote-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
16:30:02,723 INFO [SessionSpecContainer] Starting jboss.j2ee:jar=EjbComponent.jar,name=LibraryPersistentBeanRemote,service=EJB3
16:30:02,723 INFO [EJBContainer] STARTED EJB: com.tutorialspoint.stateless.LibraryPersistentBeanRemote ejbName: LibraryPersistentBean
16:30:02,731 INFO [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:
LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
LibraryPersistentBean/remote-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
...
EJBTester (EJB Client)
jndi.properties
java.naming.factory.initial=org.jnp.interfaces.NamingContextFactory
java.naming.factory.url.pkgs=org.jboss.naming:org.jnp.interfaces
java.naming.provider.url=localhost
这些属性是用来初始化InitialContext对象的Java命名服务
InitialContext的对象将被用于查找无状态会话bean
EJBTester.java
package com.tutorialspoint.test;
import com.tutorialspoint.stateless.LibraryPersistentBeanRemote;
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.List;
import java.util.Properties;
import javax.naming.InitialContext;
import javax.naming.NamingException;
public class EJBTester {
BufferedReader brConsoleReader = null;
Properties props;
InitialContext ctx;
{
props = new Properties();
try {
props.load(new FileInputStream("jndi.properties"));
} catch (IOException ex) {
ex.printStackTrace();
}
try {
ctx = new InitialContext(props);
} catch (NamingException ex) {
ex.printStackTrace();
}
brConsoleReader =
new BufferedReader(new InputStreamReader(System.in));
}
public static void main(String[] args) {
EJBTester ejbTester = new EJBTester();
ejbTester.testEntityEjb();
}
private void showGUI(){
System.out.println("**********************");
System.out.println("Welcome to Book Store");
System.out.println("**********************");
System.out.print("Options 1. Add Book 2. Exit Enter Choice: ");
}
private void testEntityEjb(){
try {
int choice = 1;
LibraryPersistentBeanRemote libraryBean =
LibraryPersistentBeanRemote)
ctx.lookup("LibraryPersistentBean/remote");
while (choice != 2) {
String bookName;
showGUI();
String strChoice = brConsoleReader.readLine();
choice = Integer.parseInt(strChoice);
if (choice == 1) {
System.out.print("Enter book name: ");
bookName = brConsoleReader.readLine();
Book book = new Book();
book.setName(bookName);
libraryBean.addBook(book);
} else if (choice == 2) {
break;
}
}
List booksList = libraryBean.getBooks();
System.out.println("Book(s) entered so far: " + booksList.size());
int i = 0;
for (Book book:booksList) {
System.out.println((i+1)+". " + book.getName());
i++;
}
} catch (Exception e) {
System.out.println(e.getMessage());
e.printStackTrace();
}finally {
try {
if(brConsoleReader !=null){
brConsoleReader.close();
}
} catch (IOException ex) {
System.out.println(ex.getMessage());
}
}
}
}
EJBTester做以下任务。
jndi.properties中加载和初始化的InitialContext对象。
在testStatefulEjb()方法，JNDI名称进行查找 - "LibraryStatelessSessionBean/remote" 以获得远程业务对象(状态EJB)。
然后用户显示一个库存储的用户界面和他/她被要求输入选择。
如果用户输入1，系统要求输入书籍名称和节省了使用无状态的会话bean addBook()方法。会话Bean坚持这本书中通过EntityManager的调用数据库。
如果用户输入2，系统检索书使用状态会话Bean getBooks()方法和退出。
然后另一个JNDI名称进行查找 - "LibraryStatelessSessionBean/remote"获得远程业务对象(状态EJB)再次列出书籍。
运行客户端访问EJB
在项目资源管理器中找到EJBTester.java。右键点击上EJBTester类，并选择run file.
在Netbeans控制台验证以下输出。
run:
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 1
Enter book name: Learn Testing
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 2
Book(s) entered so far: 1
1. learn Testing
BUILD SUCCESSFUL (total time: 15 seconds)
(编辑：雷林鹏 来源：网络|侵删)