EJB访问数据库

EJB 3.0持久性机制来访问容器管理的数据库中的数据库相关的操作。开发人员访问数据库可以直接使用JDBC API调用EJB的业务方法。

为了证明在ejb的数据库访问，我们要做好以下几项工作。

步骤 1. 在数据库中创建表.
步骤 2. 创建一个无状态EJB.
步骤 3. 更新无状态的EJB。添加添加记录并获得通过实体管理器从数据库中记录的方法。
步骤 4. 一个基于控制台应用程序客户端将访问无状态EJB的持久化数据库中的数据。

创建表

创建一张表 books 在默认的数据库 postgres.

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

创建模型类

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

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

创建无状态EJB

@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
	
   public void addBook(Book book) {
     //persist book using jdbc calls - by gitbook.net
   }    

   public List<Book> getBooks() {        
     //get books using jdbc calls
   }
   ...
}

构建EJB模块后，我们需要一个无状态的bean，我们将在下一节要创建客户端来访问。

实例应用

让我们创建一个测试EJB应用程序来测试EJB的数据库访问机制。

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. 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.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import javax.ejb.Stateless;

@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
    
   public LibraryPersistentBean(){
   }

   public void addBook(Book book) {
      Connection con = null;
      String url = "jdbc:postgresql://localhost:5432/postgres";
      String driver = "org.postgresql.driver";

      String userName = "sa";
      String password = "sa";
      List<Book> books = new ArrayList<Book>();
      try {

         Class.forName(driver).newInstance();
         con = DriverManager.getConnection(url , userName, password);

         PreparedStatement st = 
         con.prepareStatement("insert into book(name) values(?)");
         st.setString(1,book.getName());

         int result = st.executeUpdate();                

      } catch (SQLException ex) {
         ex.printStackTrace();
      } catch (InstantiationException ex) {
         ex.printStackTrace();
      } catch (IllegalAccessException ex) {
         ex.printStackTrace();
      } catch (ClassNotFoundException ex) {
         ex.printStackTrace();
      }    
   }    

   public List<Book> getBooks() {
      Connection con = null;
      String url = "jdbc:postgresql://localhost:5432/postgres";
      String driver = "org.postgresql.driver";
   
      String userName = "sa";
      String password = "sa";
      List<Book> books = new ArrayList<Book>();
      try {

         Class.forName(driver).newInstance();
         con = DriverManager.getConnection(url , userName, password);

         Statement st = con.createStatement();
         ResultSet rs = st.executeQuery("select * from book");

         Book book;
         while (rs.next()) {
            book = new Book();
            book.setId(rs.getInt(1));                 
            book.setName(rs.getString(2));
            books.add(book);
         }
      } catch (SQLException ex) {
         ex.printStackTrace();
      } catch (InstantiationException ex) {
         ex.printStackTrace();
      } catch (IllegalAccessException ex) {
         ex.printStackTrace();
      } catch (ClassNotFoundException ex) {
         ex.printStackTrace();
      }
      return books;
   }
}

只要你在JBoss部署 EjbComponent项目，注意到jboss的日志。
JBoss已经自动为我们的会话bean创建一个JNDI条目 -LibraryPersistentBean/remote.
我们将使用这个查询字符串来获得远程类型的业务对象 -com.tutorialspoint.stateless.LibraryPersistentBeanRemote

JBoss应用服务器的日志输出

...
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.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

这些属性是用来初始化InitialContext对象的Java命名服务
InitialContext的对象将被用于查找无状态会话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做以下任务。

jndi.properties中加载和初始化的InitialContext对象。
在testStatefulEjb（）方法，JNDI名称进行查找 - "LibraryStatelessSessionBean/remote"以获得远程业务对象（状态EJB）。
然后用户显示一个库存储的用户界面和他/她被要求输入选择。
如果用户输入1，系统要求输入书籍名称和可以节省使用无状态的会话bean addBook（）方法。会话Bean坚持这本书中通过EntityManager的调用数据库。
如果用户输入2，系统检索书使用状态会话Bean getBooks（）方法和退出。
然后另一个JNDI名称进行查找 - "LibraryStatelessSessionBean/remote" 获得远程业务对象（状态EJB）再次列出书籍。

运行客户端访问EJB

在项目资源管理器中找到EJBTester.java。右键点击上EJBTester类，并选择run file.

在Netbeans控制台验证以下输出。

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)