EJB實體關係

EJB 3.0中提供的選項來定義像一對一的數據庫實體關係/映射,一對多,多對一和多對多關係。以下是相關的註釋。

  • OneToOne - 對象都具有一對一的關係。例如,乘客可以在時間旅行使用一張票。

  • OneToMany - 對象是具有一對多的關係。例如,一個父親可以有多個孩子。

  • ManyToOne - 對象有多對一的關係。舉例來說,多個孩子對一個母親。

  • ManyToMany - 對象是多對多的關係。舉例來說,一本書可以多發作者,一個作者可以寫多本書。

在這裏,我們將演示如何使用多對多的映射。要代表多對多的關係,三表是必需的。

  • Book - 書籍記錄表

  • Author - 作者Author表記錄

  • Book_Author - BOOK_AUTHOR上述Book和Author表的表具有關聯。

創建表

創建表book author, book_author 在默認數據庫 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 );

創建實體類

@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<Author> authors; ... }

Use ManyToMany annotation in Book Entity

@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<Author> getAuthors() { return authors; } ... }

實例應用

讓我們創建一個測試EJB應用程序來測試EJB3.0實體關係對象。

Step

描述

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 theEJB - Create Application chapter under topic Create Client to access EJB.

EJBComponent (EJB Module)

Author.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<Author> 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<Author> 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<Author> 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<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; @Stateless public class LibraryPersistentBean implements LibraryPersistentBeanRemote { public LibraryPersistentBean(){ } @PersistenceContext(unitName="EjbComponentPU") private EntityManager entityManager; public void addBook(Book book) { entityManager.persist(book); } public List<Book> getBooks() { return entityManager.createQuery("From Book").getResultList(); } }

  • 只要你在JBoss部署 EjbComponent項目,會注意到jboss的日誌。

  • JBoss已經自動爲我們的會話bean創建一個JNDI條目 -LibraryPersistentBean/remote.

  • 我們將使用這個查詢字符串來獲得遠程類型的業務對象 -com.tutorialspoint.interceptor.LibraryPersistentBeanRemote

JBoss應用服務器的日誌輸出

... 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 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

  • These properties are used to initialize the InitialContext object of java naming service

  • InitialContext object will be used to lookup stateless session 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

  1. Add Book
  2. Exit Enter 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<Author> authors = new HashSet<Author>(); authors.add(author); book.setAuthors(authors); 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()); System.out.print("Author: "); Author[] authors = (Author[])books.getAuthors().toArray(); for(int j=0;j<authors.length;j++){ System.out.println(authors[j]); } 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做以下任務。

  • jndi.properties中加載和初始化的InitialContext對象。

  • 在testInterceptedEjb()方法,JNDI查找名稱 - 的「LibraryPersistenceBean/遠程」獲得遠程業務對象(無狀態的EJB)。

  • 然後用戶顯示一個庫存儲的用戶界面和他/她被要求輸入選擇。

  • 如果用戶輸入1,系統要求輸入書籍名稱和節約使用無狀態的會話bean addBook()方法的書。會話Bean在數據庫中存儲的書。

  • 如果用戶輸入2,系統檢索書使用狀態會話Bean getBooks()方法和退出。

運行客戶端訪問EJB

在項目資源管理器中找到EJBTester.java。右鍵點擊上EJBTester類,並選擇run file.

在Netbeans控制檯驗證以下輸出。

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
  3. Add Book
  4. Exit Enter Choice: 2 Book(s) entered so far: 1
  5. learn html5 Author: Robert BUILD SUCCESSFUL (total time: 21 seconds)