EJB查询语言
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。
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步骤 1. 在数据库中创建表.
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步骤 2. 创建一个无状态EJB.
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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。
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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<Book> 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。
| Step | Description |
|---|---|
| 1 | Create 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. |
| 2 | Create Book.java under package com.tutorialspoint.entity and modify it as shown below. |
| 3 | Create LibraryPersistentBean.java and LibraryPersistentBeanRemote as explained in the EJB - Create Application chapter and modify them as shown below. |
| 4 | Clean and Build the application to make sure business logic is working as per the requirements. |
| 5 | Finally, 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. |
| 6 | Now 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<Book> 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<Book> 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();
}
}
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只要在JBoss部署 EjbComponent项目，会注意到jboss的日志。
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JBoss已经自动为我们的会话bean创建一个JNDI条目 -LibraryPersistentBean/remote.
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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
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这些属性是用来初始化InitialContext对象的Java命名服务
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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<Book> 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。
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jndi.properties中加载和初始化的InitialContext对象。
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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）。
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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。
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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。
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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。
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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)
