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📜  EJB-实体关系

📅  最后修改于: 2020-11-16 06:25:28             🧑  作者: Mango

EJB 3.0提供了一些选项来定义数据库实体关系/映射，例如一对一，一对多，多对一和多对多关系。

以下是相关的注释-

  • 一对一-对象具有一对一关系。例如，乘客可以一次使用一张车票旅行。

  • 一对多-对象具有一对多关系。例如，一个父亲可以有多个孩子。

  • 多对一-对象具有多对一关系。例如，有一个孩子的多个孩子。

  • 多对多-对象具有多对多关系。例如，一本书可以有多位作者，而一位作者可以写多本书。

我们将在此处演示ManyToMany映射的用法。为了表示ManyToMany关系，需要以下三个表-

  • 书籍-书籍表，具有书籍记录。

  • 作者-作者表，其中包含作者记录。

  • Book_Author-书籍作者表，具有上述书籍和作者表的链接。

建立表格

创建一个表书的作者，在BOOK_AUTHOR默认数据库的Postgres。

CREATE TABLE book (
   book_id     integer,   
   name   varchar(50)      
);

CREATE TABLE author (
   author_id   integer,
   name   varchar(50)      
);

CREATE TABLE book_author (
   book_id     integer,
   author_id   integer 
);

创建实体类

@Entity
@Table(name="author")
public class Author implements Serializable{
   private int id;
   private String name;
   ...   
}

@Entity
@Table(name="book")
public class Book implements Serializable{
   private int id;
   private String title;
   private Set authors;
   ...   
}

在图书实体中使用ManyToMany批注。

@Entity
public class Book implements Serializable{
   ...
   @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}
      , fetch = FetchType.EAGER)
   @JoinTable(table = @Table(name = "book_author"),
      joinColumns = {@JoinColumn(name = "book_id")},
      inverseJoinColumns = {@JoinColumn(name = "author_id")})
   public Set getAuthors() {
      return authors;
   }
   ...
}

应用范例

让我们创建一个测试EJB应用程序以测试EJB 3.0中的实体关系对象。

Step Description
1

Create a project with a name EjbComponent under a package com.tutorialspoint.entity as explained in the EJB – Create Application chapter. Please use the project created in EJB – Persistence chapter as such for this chapter to understand embedded objects in EJB concepts.
2

Create Author.java under package com.tutorialspoint.entity as explained in the EJB – Create Application chapter. Keep rest of the files unchanged.
3

Create Book.java under package com.tutorialspoint.entity. Use EJB – Persistence chapter as reference. Keep rest of the files unchanged.
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 the EJB – Create Application chapter under topic Create Client to access EJB.

EJBComponent(EJB模块)

作者.java

package com.tutorialspoint.entity;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="author")
public class Author implements Serializable{
    
   private int id;
   private String name;

   public Author() {}

   public Author(int id, String name) {
      this.id = id;
      this.name = name;
   }
   
   @Id  
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="author_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;
   }

   public String toString() {
      return id + "," + name;
   }    
}

Book.java

package com.tutorialspoint.entity;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;

@Entity
@Table(name="book")
public class Book implements Serializable{

   private int id;
   private String name;
   private Set authors;

   public Book() {        
   }

   @Id  
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="book_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;
   }

   public void setAuthors(Set authors) {
      this.authors = authors;
   }    
   
   @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}
      , fetch = FetchType.EAGER)
   @JoinTable(table = @Table(name = "book_author"),
      joinColumns = {@JoinColumn(name = "book_id")},
      inverseJoinColumns = {@JoinColumn(name = "author_id")})
   public Set getAuthors() {
      return authors;
   }
}

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;

@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.interceptor.LibraryPersistentBeanRemote的远程业务对象。

JBoss应用服务器日志输出

...
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.interceptor.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
16:30:02,723 INFO  [SessionSpecContainer] Starting jboss.j2ee:jar=EjbComponent.jar,name=LibraryPersistentBean,service=EJB3
16:30:02,723 INFO  [EJBContainer] STARTED EJB: com.tutorialspoint.interceptor.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.interceptor.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
...

EJBTester(EJB客户端)

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。

EJBTester.java

package com.tutorialspoint.test;
   
import com.tutorialspoint.stateful.LibraryBeanRemote;

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;

import java.util.*;

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.testEmbeddedObjects();
   }
   
   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 testEmbeddedObjects() {

      try {
         int choice = 1; 

         LibraryPersistentBeanRemote libraryBean = 
         (LibraryPersistentBeanRemote)
         ctx.lookup("LibraryPersistentBean/remote");

         while (choice != 2) {
            String bookName;
            String authorName;
            
            showGUI();
            String strChoice = brConsoleReader.readLine();
            choice = Integer.parseInt(strChoice);
            if (choice == 1) {
               System.out.print("Enter book name: ");
               bookName = brConsoleReader.readLine();
               System.out.print("Enter author name: ");
               authorName = brConsoleReader.readLine();               
               Book book = new Book();
               book.setName(bookName);
               Author author = new Author();
               author.setName(authorName);
               Set authors = new HashSet();
               authors.add(author);
               book.setAuthors(authors);

               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());
            System.out.print("Author: ");
            Author[] authors = (Author[])books.getAuthors().toArray();
            for(int j=0;j

EJBTester执行以下任务-

  • 从jndi.properties加载属性并初始化InitialContext对象。

  • 在testInterceptedEjb()方法中，使用名称“ LibraryPersistenceBean /远程”完成jndi查找，以获取远程业务对象(无状态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，则系统会询问书名并使用无状态会话bean addBook()方法保存书。 Session Bean将书存储在数据库中。

  • 如果用户输入2，则系统使用无状态会话bean getBooks()方法检索书籍并退出。

运行客户端以访问EJB

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 html5
Enter Author name: Robert
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 1
1. learn html5
Author: Robert
BUILD SUCCESSFUL (total time: 21 seconds)