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Lesson 41
Layers and Tiers
How do you structure an application to support such operational requirements as maintainability,
reusability, scalability and robustness? The answer lies in using Layers and Tiers? What different
technologies Java provides to support layered or tiered architectures. The answer to these questions will
remain our focus in this handout. A small case study will also be used to comprehend the concept of layers.
Layers vs. Tiers
Layers are merely logical grouping of the software components that make up the application or service,
whereas Tiers refer to the physical residence of those layers.
In general,
. Layers ­ represents the logical view of application
. Tiers ­ represents physical view of application
However, both terms are used intractably very often. You must be confused what does logical & physical
view mean? Let's elaborate layers and tiers further in detail to differentiate between them.
Layers
The partitioning of a system into layers such that each layer performs a specific type of functionality and
communicates with the layer that adjoin it.
The separation of concerns minimizes the impact of adding services/features to an application. The
application developed in layers also enables tiered distribution (discussed later). Furthermore easier
maintenance, reuse of code, high cohesion & loose coupling sort of additional benefits are also enjoyed by
the use of tiered architecture.
To begin with, layered architecture based on three layers. These are
Presentation Layer
Business Layer
Data Layer
Note:
However, there is no upper limit of number of layers an application can have. Each layer can also be further
break down into several layers depending upon the requirement and size of the application.
The figure given below shows a simplified view of an application and its layers.
As you can see in the figure, users can only interact with the presentation layer. The presentation layer
passes the user request to the business layer, which further passes the request to the data layer. The data
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layer communicates with the data sources (like Database etc.) or other external services in order to
accomplish the user request.
Let's discuss each layer's responsibility in detail:
Presentation Layer
It provides a user interface to the client/user to interact with the application. This is the only part of the
application visible to client.
Its job list includes collecting user's input, validating user's input (on client side using JavaScript like
technologies OR on server side), presenting the results of the request made by the user and controlling the
screen flow (which page/view will be visible to the user).
Business Layer
Also called application layer, it is only concerned with the application specific functionality. It is used to
implement business rules and to perform business tasks.
For example, in a banking system, this layer will provide the functionality of banking functions such as
opening an account, transferring of balance from one account to another,
Calculation of taxes etc.
Data Layer
It is concerned with the management of the data & data sources of the system. Data sources can be
database, XML, web services, flat file etc. Encapsulates data retrieval & storage logic For example, the
address book application needs to retrieve all person records from a database to display them to the user.
Tiers
As mentioned, layers help in building a tiered architecture. Like layers, there is no restriction on using
number of tiers. An application can be based on Single-tier, Two-tier, Three-tier or N-Tier (application
which have more than three tiers). The choice of using a tiered architecture is contingent to the business
requirements and the size of the application etc.
Tiers are physically separated from each other. Layers are spread across tiers to build up an application.
Two or more layers can reside on one tier. The following figure presents a three-tier architectural view of
an application.
The client tier
represents the
client machine
where actually
web browser is
running and
usually displays
HTML. You can
think of a
Presentation as
of two parts; one
is on client side,
for example,
HTML. There is
also a
presentation
layer that is used to generate the client presentation often called server presentation. We'll discuss about it
later.
The server machine can consist on a single server machine or more. Therefore, it is possible web server is
running on one server machine while application server on another. Web server is used to execute web
pages like JSPs whereas application server is used to run special business objects like Enterprise
JavaBeans (discussed later). The web layer and applications server can be on two separate machines or
they can be on same tier as shown in the diagram
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The client tier represents the client mThe database server is often running on a separate tier, i.e. DB
machine often called Enterprise information tier.
Layers Support in Java
The secret of wide spread use of Java lies in providing specific technology for each layer. This not only
eases the development by freeing the programmer for caring operational features but only reduces the
production time of the software.
In the following figure, Presentation is bifurcated into two layers. These are Client Presentation layer and
Server Presentation Layer. What client sees in a browser forms client presentation layer while server
presentation layer includes the Java technology components?
(JSP and Servlets etc.) that are used to generate the client presentation.
Layers Java/J2EE Technology
On business layer, JavaBeans (also referred as Plain
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Suppose, our client is HTML based. Client does some processing on HTML and transports it to web server.
JSP and Servlets are possible technologies that can be used in a web server. However, there are some
Frameworks such as JSF etc that can be used in a web server. The classes which form the presentation layer
reside on web server and of course controllers are also used over here.
If web server, wants to perform some business process, it usually gets help from some business layer
components. The business layer component can be a simple JavaBean (POJO) but in a typical J2EE
architecture, EJBs are used. Enterprise JavaBeans interacts with the database or information system to store
and retrieve data.
EJBs and JSP/Servlets works in two different servers. As you already know, JSP and Servlets runs in a web
server where as EJBs requires an application server. But, generally application server contains the web
server as well.
Application server including web server generally resides on a single tier (machine), which is often called
middle tier. This tier stores and retrieves data from the Enterprise Information Tier (EIS) which is a
separate tier. The response sends back to the client by the middle tier can be HTML, XML etc. This
response can be seen on the separate tier know as client tier.
Case Study: Matrix Multiplication using Layers
Problem Statement
. Calculate product of two matrices of order 2 * 2
. Result of multiplication should be stored in DB as well as shown to the user.
Format
. Input format
-input will be in 4,2,6,5 format separated by commas where 4,2 represents entries of the first row
. Display format
-Displays the matrix as a square
. Storage format for DB
-Matrix will be stored as a string in the database along with the order of the matrix
-The following figure shows the table design that will be used to store the results.
Layer by Layer View
A picture's worth than thousand words. Therefore, before jumping on to code, let's put a glance over layers
that will be used in this small case study. The classes that will be used on each layer and what functionality
each class will perform will also be discussed.
First, look on the following picture that will describe the whole story.
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matrix = new int[2][2];
matrix[0][0] = 0;
matrix[0][1] = 0;
matrix[1][0] = 0;
matrix[1][1] = 0;
}
// setter that takes 4 int values and assigns these to array
public void setMatrix(int w, int x, int y, int z)
{
matrix[0][0] = w;
matrix[0][1] = x;
matrix[1][0] = y;
matrix[1][1] = z;
}
// getter returning a 2D array
public int[ ][ ] getMatrix()
{
return matrix;
}
// used to convert 2D array into
string
public String toString(){ return matrix[0][0] + "," +
matrix[0][1] + "," +
matrix[1][0] + "," +matrix[1][1] ; }
} // end MatrixBean
matrixinput.jsp
This JSP is used to collect the input for two matrices in the form of string such as 2,3,5,8. The data will be
submitted to ControllerServlet from this page.
<html>
<body>
<h2>
Enter Two Matrices of order 2 * 2 to compute Product
</h2>
<h3>
<%--
"controller" is an alias/URL pattern of ControllerServlet -%>
<form name="matrixInput" action="controller" >
First Matrix:
<input type="text" name = "firstMatrix" /> E.g. 2,3,4,1
<br/>
Second Matrix:
<input type="text" name = "secondMatrix" />
<br/>
<input type = "submit" value = "Calculate Product" />
</form>
</h3>
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</body>
</html>
ControllerServlet
This servlet acting as a controller receives the input from matrixinput.jsp. Furthermore, it will interact with
the business layer class MatrixMultiplier to convert the string into a MatrixBean object, and to multiply two
matrices.
package controller;
import bl.*;
import bo.* ;
import java.io.*;
import java.net.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ControllerServlet extends HttpServlet {
// This method only calls processRequest()
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException {
processRequest(request, response);
}
// This method only calls processRequest()
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException {
processRequest(request, response);
}
protected void processRequest(HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException {
// retrieving values from input fields of matrixinput.jsp
String sMatrix1 = req.getParameter("firstMatrix");
String sMatrix2 = req.getParameter("secondMatrix");
// Creating MatrixMultipler object
MatrixMultiplier mm = new MatrixMultiplier();
// Passing Strings to convertToObject() method of MatrixMultiplier
// convertToObject() is used to
convert strings into MatrixBean
MatrixBean fMatrix = mm.convertToObject(sMatrix1);
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MatrixBean sMatrix = mm.convertToObject(sMatrix2);
// passing MatrixBean's objects to multiply() method of  // MatrixMultiplier and receiving the
product matrix in the form  // of MatrixBean
MatrixBean rMatrix = mm.multiply(fMatrix, sMatrix);
// saving results in database
mm.saveResult(rMatrix);
// storing the product of matrices into request, so that it can be
// retrieved on
matrixresult.jsp req.setAttribute("product", rMatrix);
// forwarding request to matrixresult.jsp
RequestDispatcher rd = req.getRequestDispatcher("matrixresult.jsp");
rd.forward(req, res);
} // end processRequest()
} // end ControllerServlet
MatrixMultiplier
The business layer class that's primary job is to calculate product of tow matrices given in the form of
MatrixBean. This class also has a method convertToObject that takes a String and returns back a
MatrixBean object. MatrixMultiplier will also interact with the data layer class MatrixDAOto store results
in the database.
package bl;
import bo.*;
import dal.*;
public class MatrixMultiplier {
//constructor
public MatrixMultiplier( ) {
}
// used to convert a String (like 2,3,4,5) into a MatrixBean object
public MatrixBean convertToObject(String sMatrix){
//splitting received string into tokens by passing "," as delimeter
String tokens[] = sMatrix.split(",");
//creating MatrixBean object
MatrixBean matrixBO = new MatrixBean();
// converting tokens into integers
int w = Integer.parseInt(tokens[0]);
int x = Integer.parseInt(tokens[1]);
int y = Integer.parseInt(tokens[2]);
int z = Integer.parseInt(tokens[3]);
// setting values into MatrixBean object by calling setter
matrixBO.setMatrix(w , x , y, z);
return matrixBO;
} // end convertToObject()
// used to multiply two matrices , receives two MatrixBean objects
// and returns the product in the form of MatrixBean as well
public MatrixBean multiply(MatrixBean fMatrix , MatrixBean sMatrix){
// creating MatrixBean object where product of the matrices will be tored
MatrixBean resultMatrix = new MatrixBean();
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// retrieving two dimensional arrays from MatrixBeans object to // perform multipication
int matrixA[ ][ ] = fMatrix.getMatrix();
int matrixB[ ][ ] = sMatrix.getMatrix();
int matrixC[ ][ ] = resultMatrix.getMatrix();
// code to multiply two matrices
for (int i=0; i<2; i++) {
for (int j=0; j<2; j++) {
for (int k=0; k<2; k++){
matrixC[i][j] += (matrixA[i][k] * matrixB[k][j]);
}
}
}
// storing the product from 2d array to MatrixBean object by
// calling setter
resultMatrix.setMatrix( matrixC[0][0], matrixC[0][1], matrixC[1][0], matrixC[1][1] );
return resultMatrix;
} // end MatrixMulitplier
} // end multiply()
// save results (MatrixBean containg product of two matrices) into // database using DAO
public void saveResult( MatrixBean resultMatrix )
{
MatrixDAO dao = new MatrixDAO();
dao.saveMatrix(resultMatrix);
}
} // end MatrixMulitplier
MatrixDAO
As class name depicts, it is used to store product results into database. Let's look on the code to see how it
is accomplished.
package dal;
import java.util.*; import java.sql.*;
import bo.*; public class MatrixDAO{
private Connection con;
// constructor
public MatrixDAO() throws ClassNotFoundException , SQLException {
establishConnection();
}
// method used to establish connection with db  private void  establishConnection() throws
ClassNotFoundException , SQLException {
// establishing conection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:MatrixDSN";
con = DriverManager.getConnection(conUrl);
}
// used to store MatrixBean into database after converting it to  // a String
public void saveMatrix(MatrixBean matrix)
try
{
String sql = "INSERT INTO Matrix(mOrder, mValues) VALUES (?,?)";
PreparedStatement pStmt = con.prepareStatement(sql);
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// converting MatrixBean into String by calling toString()
String sMatrix = matrix.toString();
// setting order of matrix
pStmt.setString( 1 , "2*2" );
// setting matrix values in the form of string
pStmt.setString( 2 , sMatrix );
pStmt.executeUpdate();
}catch(SQLException sqlex){
System.out.println(sqlex);
}
} // end saveMatrix
// overriding finalize method to release acquired resources
public void finalize( ) {
try{
if(con != null){
con.close();
}
}catch(SQLException sex){
System.out.println(sex);
}
} // end finalize
} // end MatrixDAO class
matrixresult.jsp
Used to display resultant product of two matrices. The code is given below:
<%-- importing "bo" package that contains MatrixBean -%>
<%@ page import="bo.*"%>
<html>
<body>
<h1>The resultant Matrix is </h1>
<%--  retrieving MatrixBean object from request, that was set on
ControllerServlet
--%>
<% MatrixBean productMatrix = (MatrixBean)request.getAttribute("product");
// retrieving values in 2d array so that it can be displayed
int matrix[][] = productMatrix.getMatrix() ;
%>
<%-- displaying MatrixBean's object values -%>
<TABLE>
<TR>
<TD> <%= matrix[0][0] %> </TD>
<TD> <%= matrix[0][1] %> </TD>
</TR>
<TR>
<TD> <%= matrix[1][0] %> </TD>
<TD> <%= matrix[1][1] %> </TD>
</TABLE>
</body>
</html>
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web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
<servlet>
<servlet-name> ControllerServlet </servlet-name>
<servlet-class> controller.ControllerServlet </servlet-class>
</servlet>
<servlet-mapping>
<servlet-name> ControllerServlet </servlet-name>
<url-pattern> /controller </url-pattern>
</servlet-mapping>
</web-app>
References:
. Java A Lab Course by Umair Javed.
. Java Passion by Sang Shin
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