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Transcript t13 

Introduction to J2EE
CS 236700: Software Design
Winter 2004-2005/T13
Based on: Advanced Web Applications Development, cs236606
© Eliezer Dekel, Sara Porat, Michael Factor, Gal Shachor: IBM Labs in Haifa
© Tal Cohen: The Computer Science Department, the Technion
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Preface
 A huge scope
 Complex terminology
 Our goals:
 An overview of J2EE, J2EE services
 Understanding the EJB framework
 Deployment is a significant issue
 Will not be discussed
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Introduction (1/4): Enterprise Application
 Enterprise Application
 Running the information and activity of big organizations
 Government agencies, Banks, Retailers
 Must support various demands
 High-availability
 Scalability
 Backwards compatibility
 Multiple clients from remote sites
 Consistent data
 Security
 Usually, designed around a client-server architecture
 A stand-alone design cannot support remote access
 A peer-to-peer design has security flaws and is more difficult to
program
• A “leader” usually simplifies any distributed algorithm
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Introduction (2/4): A Three-tier application
 Typical design of an Enterprise application:
First Tier
Client
Second Tier
Server
Third Tier
Database
 This is a Three-tier application
 Quite similar the Model-View-Controller pattern
 The key difference: the brains of the system is located inside the 2nd tier
 Usually called: Business Logic
 Many times the client is just web-browser
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Introduction (3/4): Application Server
 The Business Logic layer (2nd tier)
 Is usually the most complicated tier to design
 The other two tiers are complex, but their design is almost
mechanical
 Typically called: Application Server
 It servers as a platform which invokes different applications
 The platform provides core services to these applications
 Concurrency,
 Networking,
 Transactions
 Naming
 Security
 HTTP Connectivity
 etc.
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Introduction (4/4): The Motivation
Let a developer focus on the
domain-specific programming. The
general concerns are handles by
the underlying platform
 This goal has motivated the development of many
Application Server products
 J2EE is a standard (by sun) for an application server
 Has several implementations: JBoss, Web-Sphere, WebLogic
 Note that an application server addresses the same
needs as an operating system
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J2EE in Action: Banking System
First Tier
Browser
HTTP
Second Tier
Third Tier
Account
Deposit
Servlet
RMI
RMI
Accounts EJB
Employees EJB
JDBC
Database
JDBC
J2EE Server
 The applications: Account EJB, Employees EJB, Account Deposit Servlet
 The J2EE server provide many useful services (as mentioned before)
 The applications need to invoke these services…
 …But not to implement them
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The J2EE internal architecture
 Three basic types of applications
 JSP – Java Server Page
 Servlets
 EJB – Enterprise Java Beans
 Containers: Each manages a certain type of application
 EJB Container
 Servlet Container (handles JSPs as well)
 Fundamental technologies used by J2EE:
Reflection, Dynamic Compilation, Serialization, RMI
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JSP (1/2)
 Purpose: A simplified way to generate dynamic web
content
 What is it?
 An HTML page with embedded tags and Java Code
 At runtime the JSP page is translated into an HTML page
 The process:
 A JSP page is dynamically converted into a Java code that is
compiled by the J2EE server
 When a user request arrives, it is passed to an instance of this
class, which creates the desired textual output (HTML)
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JSP (2/2)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01
Transitional//EN">
<HTML>
<HEAD>
<%@ page language="java" contentType="text/html;
charset=US-ASCII" pageEncoding="US-ASCII" %>
<TITLE>What’s the time</TITLE></HEAD>
<BODY><h2>
<P> Welcome to what-is-the-time.com<br><br>
<% String s = new java.util.Date().toString();
out.print("Current time=" + s); %>
</P></BODY></HTML>
 Special variables:
d1/time.jsp
 out – A JspWriter object
 session – An HttpSession object of current user session.
 request – An HttpServletRequest object associated with
current user request
 response – An HttpServletResponse object
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Servlets (1/2)
 Purpose: A standard way for a Java program to handle
HTTP requests
 What is it?
 A Servlet is a Java class that extends HttpServlet
 When the client sends a request to the server, it is forwarded to
an instance of the appropriate servlet class
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Servlets (2/2)
import java.io.*;
import javax.servlet.ServletException;
import javax.servlet.http.*;
public class MyServlet extends HttpServlet
{
public void doPost(HttpServletRequest req,
HttpServletResponse resp)
throws ServletException, IOException
{
doGet(req, resp);
}
public void doGet(HttpServletRequest req,
HttpServletResponse resp)
throws ServletException, IOException
{
resp.setContentType("text/html");
PrintWriter out = resp.getWriter();
out.println("<html><body><h2>");
out.println("Your Name is " + req.getParameter("name"));
out.println("</body></html>");
}
}
d1/MyServlet.html
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EJBs
 Purpose: Provide the means for encapsulation and
reuse of business logic and/or persistent data
 What is it?
 The EJB mechanism is a realization of the component model
 A reusable software unit (like a class), which is dynamically
manipulated by the J2EE server
 Why is it needed?
 Many times the same business logic is used by several distinct
elements in the J2EE. For instance:
 In a banking system, both the withdraw-servlet and the
deposit-servlet need to use the “set-balance” functionality
 We are speaking of a distributed environment with persistent
data
 => it is not possible to invoke the setBalance() method on
an instance of the Account class
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Types of EJBs
 Session bean: Represents a single client
 Relatively short lived
 Does not represent shared data in a database
 Does not generally survive EJB server crashes
 Entity bean: Represent shared, persistent data
 (The data is usually stored in a database)
 Can be long lived
 Survives EJB server crashes
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The EJB mechanism (1/3)
 Interaction: EJB container and an existing EJBs
 Pooling: Reuse of idle EJBs
 Sharing: Let two clients use a single EJB instance
 Usually used with a stateless session EJB
 Passivation/Activation: Save to file/reload from file
 An idle EJB may become passive due to memory considerations
 Etc.
 EJBs are (of course) implemented as instances of classes
 Option 1:
 The J2EE standard will define a dedicated class, which implements
these methods
 Each EJB class will have to extend this dedicated class
 The flaw: In a single inheritance language, this approach rules-out the
possibility of inheritance between EJBs
 Option 2:
 Let the server “paste” the required methods into the EJB class
 (See next slide)
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The EJB mechanism (2/3)
API Interfaces
Remote
extends
Bean Developer
EJBObject
getEJBHome()
getHandle()
getPrimaryKey()
isIdentical(EJBOBject)
remove()
extends
Generated
“Remote interface”
Cart
addBook(String)
removeBook(String)
implements
“Bean class”
CartBean
setSessionContext(..)
ejbCreate(String)
ejbActivate()
ejbPassivate()
ejbRemove()
addBook(String)
removeBook(String)
delegates
“EJBObject implementation”
implementation”
“EJBObject
CartImpl
CalculatorImpl
addBook(String)
add(int,int)
removeBook(String)
sub(int,int)
getEJBHome()
getEJBHome()
getHandle()
getHandle()
getPrimaryKey()
getPrimaryKey()
isIdentical(EJBObject)
isIdentical(EJBObject)
remove()
remove()
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The EJB mechanism (3/3)
 Outline of steps
 Programmer writes an interface for the business logic
 Programmer writes a class which implements this interface
 But it does not have to declare “implements X”
 The EJB container creates a class which implements the interface
 This class forwards the calls to the actual class
 Additional code is inserted into this class
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The EJB initialization mechanism (1/2)
 Creation of a new EJB
 Client code can’t just use new MyEjbClass() to create an EJB
 Why?
1. The name of the actual implementation is not known
2. Many times the EJB container just recycles an existing instance
 => Outline of steps:
 The programmer defines “initialization-methods” in his bean class
 Named: ejbCreate(…)
 The programmer defines the interface (called: Home Interface) for a
factory class
 Parameters to methods must match the ejbCreate() methods
 The EJB container creates a class which implements the Home
Interface
 This class forwards the calls to the appropriate ejbCreate()
method
 Additional code is inserted into this class
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The EJB initialization mechanism (2/2)
API Interfaces
Remote
extends
Bean Developer
Generated
EJBHome
getEJBMetaData()
getHomeHandle()
remove(Handle)
remove(Object)
extends
“Bean class”
CartBean
setSessionContext(..)
ejbCreate(String)
ejbActivate()
ejbPassivate()
ejbRemove()
addBook(String)
removeBook(String)
“Home interface”
CartHome
create(String)
implements
delegates
“EJBHome implementation”
CartHomeImpl
create(String)
getEJBMetaData()
getHomeHandle()
remove(Handle)
remove(Object)
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A Stateful Session EJB (1/4)
 We’d like to model a “Shopping-cart” in an on-line book
store
 The cart is represented by a stateful session bean
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A Stateful Session EJB (2/4)
public interface Cart extends EJBObject
{
public void addBook(String title)
throws RemoteException;
public void removeBook(String title)
throws RemoteException;
public Vector getContents()
throws RemoteException;
}
 Our business logic:
 Add/remove book to cart
 Observe the contents of the cart
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A Stateful Session EJB (3/4)
import
import
import
import
java.io.Serializable;
java.rmi.RemoteException;
javax.ejb.CreateException;
javax.ejb.EJBHome;
public interface CartHome extends EJBHome
{
Cart create(String name) throws RemoteException;
Cart create(String name, String id)
throws RemoteException;
}
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A Stateful Session EJB (4/4)
public class CartBean implements SessionBean
{
private String name_;
private String id_;
private Vector books_;
public void ejbCreate(String name) { ejbCreate(name, "0"); }
public void ejbCreate(String name, String id) {
name_ = name;
id_ = id;
books_ = new Vector();
}
public Vector getContents() { return books_; }
public void addBook(String title) { books_.addElement(title); }
public void removeBook(String title) {
books_.removeElement(title);
}
public CartBean() {}
public
public
public
public
void
void
void
void
ejbRemove() {}
ejbActivate() {}
ejbPassivate() {}
setSessionContext(SessionContext sc) {}
}
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A simple EJB client
import
import
import
import
java.util.*;
javax.naming.Context;
javax.naming.InitialContext;
javax.rmi.PortableRemoteObject;
public class CartClient {
public static void main(String[] args) throws Throwable
{
Context initial = new InitialContext();
Object objref =
initial.lookup("java:comp/env/ejb/SimpleCart");
CartHome home = (CartHome)
PortableRemoteObject.narrow(objref, CartHome.class);
Cart shoppingCart = home.create(“Graham Chapman", "123");
shoppingCart.addBook("Last of the Mohicans");
}
}
 Uses JNDI classes to locate the home interface
 See: Object objref = initial.lookup("...");
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