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EJB查詢語言

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EJB3.0，EJB查詢語言編寫自定義的查詢非常方便，不用擔心底層數據庫的詳細信息。這是很相似的HQL(Hibernate查詢語言)，通常被稱為按名稱EJBQL。

要理解熟悉ejb的EJBQL，要做好以下幾項工作。

  • 步驟 1. 在數據庫中創建表.

  • 步驟 2. 創建一個無狀態EJB.

  • 步驟 3. 更新無狀態的EJB。添加添加記錄並獲得通過實體管理器從數據庫中記錄的方法。

  • 步驟 4. 一個基於控製台應用程序客戶端將訪問無狀態EJB的持久化數據庫中的數據。

創建數據庫表

創建表 books 在默認數據庫 postgres.

CREATE TABLE books (
   id     integer PRIMARY KEY,
   name   varchar(50)
);

創建模型類

public class Book implements Serializable{
    
   private int id;
   private String name;

   public Book(){        
   }
   
   public int getId() {
      return id;
   }
   ...
}

創建無狀態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
   }
   ...
}

構建EJB模塊後，我們需要一個無狀態的bean，我們將在下一節要創建客戶端來訪問。

示例應用程序

讓我們創建一個測試EJB應用程序來測試EJB的數據庫訪問機製。

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();
   }   
}

  • 隻要在JBoss部署 EjbComponent項目，會注意到jboss的日誌。

  • JBoss已經自動為我們的會話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.stateless.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.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

  • 這些屬性是用來初始化InitialContext對象的Java命名服務

  • InitialContext的對象將被用於查找無狀態會話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做以下任務。

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

  • 在testStatefulEjb（）方法，JNDI名稱進行查找 - "LibraryStatelessSessionBean/remote" 以獲得遠程業務對象（狀態EJB）。

  • 然後用戶顯示一個庫存儲的用戶界麵和他/她被要求輸入選擇。

  • 如果用戶輸入1，係統要求輸入書籍名稱和節省了使用無狀態的會話bean addBook（）方法。會話Bean堅持這本書中通過EntityManager的調用數據庫。

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

  • 然後另一個JNDI名稱進行查找 - "LibraryStatelessSessionBean/remote"獲得遠程業務對象（狀態EJB）再次列出書籍。

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控製台驗證以下輸出。

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)


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