EJB查詢語言

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)，通常被稱爲按名稱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. 一個基於控制檯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 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 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 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();
}
}

• 只要在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.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，系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 Testing
   **********************
   Welcome to Book Store
   **********************
   Options
3. Add Book
4. Exit
   Enter Choice: 2
   Book(s) entered so far: 1
5. learn Testing
   BUILD SUCCESSFUL (total time: 15 seconds)