📅 最后修改于: 2020-11-16 06:15:34 🧑 作者: Mango
EJB 3.0是EJB 2.0中使用的实体bean，在很大程度上已由持久性机制代替。现在，实体bean是具有与表的映射关系的简单POJO。
以下是持久性API中的关键角色-
实体-代表数据存储记录的持久对象。可序列化是一件好事。
EntityManager-持久性接口，可对持久性对象(实体)执行诸如添加/删除/更新/查找之类的数据操作。它还有助于使用查询界面执行查询。
持久性单元(persistence.xml) -持久性单元描述了持久性机制的属性。
数据源(* ds.xml) -数据源描述了与数据存储相关的属性，例如连接URL。用户名，密码等
为了演示EJB持久性机制，我们需要执行以下任务-
步骤1-在数据库中创建表。
步骤2-创建与表相对应的实体类。
步骤3-创建数据源和持久性单元。
步骤4-创建具有EntityManager实例的无状态EJB。
步骤5-更新无状态EJB。添加方法以添加记录并通过实体管理器从数据库获取记录。
步骤6-基于控制台的应用程序客户端将访问无状态EJB，以将数据持久存储在数据库中。
在默认数据库postgres中创建一个表簿。
CREATE TABLE books (
id integer PRIMARY KEY,
name varchar(50)
);
//mark it entity using Entity annotation
//map table name using Table annotation
@Entity
@Table(name="books")
public class Book implements Serializable{
private int id;
private String name;
public Book() {
}
//mark id as primary key with autogenerated value
//map database column id with id field
@Id
@GeneratedValue(strategy= GenerationType.IDENTITY)
@Column(name="id")
public int getId() {
return id;
}
...
}
PostgresDS
jdbc:postgresql://localhost:5432/postgres
org.postgresql.driver
sa
sa
5
20
5
java:/PostgresDS
false
org.hibernate.ejb.HibernatePersistence
java:/PostgresDS
false
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
//pass persistence unit to entityManager.
@PersistenceContext(unitName="EjbComponentPU")
private EntityManager entityManager;
public void addBook(Book book) {
entityManager.persist(book);
}
public List getBooks() {
return entityManager.createQuery("From Books").getResultList();
}
...
}
构建EJB模块之后，我们需要一个客户端来访问无状态bean，这将在下一部分中创建。
让我们创建一个测试EJB应用程序以测试EJB持久性机制。
| 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 persistence 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 chapters and modify them as shown below. |
| 4 |
Create jboss-ds.xml in EjbComponent > setup folder and persistence.xml in EjbComponent > src > conf folder. These folders can be seen in files tab in Netbeans. Modify these files as shown above. |
| 5 |
Clean and Build the application to make sure business logic is working as per the requirements. |
| 6 |
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. |
| 7 |
Now create the EJB client, a console based application in the same way as explained in the EJB – Create Application chapter under topic Create Client to access EJB. Modify it as shown below. |
package com.tutorialspoint.entity;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.EntityListeners;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
@Entity
@Table(name="books")
public class Book implements Serializable{
private int id;
private String name;
public Book() {
}
@Id
@GeneratedValue(strategy= GenerationType.IDENTITY)
@Column(name="id")
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;
}
}
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();
}
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;
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
public LibraryPersistentBean() {
}
@PersistenceContext(unitName="EjbComponentPU")
private EntityManager entityManager;
public void addBook(Book book) {
entityManager.persist(book);
}
public List getBooks() {
return entityManager.createQuery("From Book").getResultList();
}
}
在JBOSS上部署EjbComponent项目后，请注意jboss日志。
JBoss已经为我们的会话bean- LibraryPersistentBean / remote自动创建了一个JNDI条目。
我们将使用此查找字符串获取类型为com.tutorialspoint.stateless.LibraryPersistentBeanRemote的远程业务对象。
...
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
...
java.naming.factory.initial=org.jnp.interfaces.NamingContextFactory
java.naming.factory.url.pkgs=org.jboss.naming:org.jnp.interfaces
java.naming.provider.url=localhost
这些属性用于初始化Java命名服务的InitialContext对象。
InitialContext对象将用于查找无状态会话bean。
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 \n1. Add Book\n2. Exit \nEnter 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执行以下任务。
从jndi.properties加载属性并初始化InitialContext对象。
在testStatefulEjb()方法中，使用名称“ LibraryStatefulSessionBean /远程”完成jndi查找，以获取远程业务对象(有状态ejb)。
然后，向用户显示一个库存储用户界面，并要求他/她输入选择。
如果用户输入1，则系统要求输入书名并使用无状态会话bean addBook()方法保存书。会话Bean通过EntityManager调用将书籍保留在数据库中。
如果用户输入2，则系统使用有状态会话bean getBooks()方法检索书籍并退出。
然后，使用名称“ LibraryStatelessSessionBean / remote”完成另一个jndi查找，以再次获取远程业务对象(无状态EJB)，并完成了书的清单。
在项目资源管理器中找到EJBTester.java。右键单击EJBTester类，然后选择运行文件。
在Netbeans控制台中验证以下输出-
run:
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 1
Enter book name: Learn Java
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 2
Book(s) entered so far: 1
1. learn java
BUILD SUCCESSFUL (total time: 15 seconds)
访问EJB之前，请重新启动JBoss。
在项目资源管理器中找到EJBTester.java。右键单击EJBTester类，然后选择运行文件。
在Netbeans控制台中验证以下输出。
run:
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 1
Enter book name: Learn Spring
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 2
Book(s) entered so far: 2
1. learn java
2. Learn Spring
BUILD SUCCESSFUL (total time: 15 seconds)
上面显示的输出表明，书籍正被存储在永久性存储中，并且已从数据库中检索到。