EJB訪問數據庫

EJB 3.0持久性機制來訪問容器管理的數據庫中的數據庫相關的操作。開發人員訪問數據庫可以直接使用JDBC API調用EJB的業務方法。

爲了證明在ejb的數據庫訪問,我們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。

創建表

創建一張表 books 在默認的數據庫 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 jdbc calls - by yiibai.com } public List<Book> getBooks() { //get books using jdbc calls } ... }

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。

實例應用

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

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項目,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可以節省使用無狀態的會話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 Java
    **********************
    Welcome to Book Store
    **********************
    Options
  3. Add Book
  4. Exit
    Enter Choice: 2
    Book(s) entered so far: 1
  5. learn java
    BUILD SUCCESSFUL (total time: 15 seconds)