Java Web Programming [3/9] : Servlet Advanced

Module 3:
Servlet Advanced

Thanisa Kruawaisayawan
Thanachart Numnonda
www.imcinstitute.com

Objectives
 Including, forwarding to, and redirecting to
other web resources
 Servlet & JDBC
 Session Tracking API
 Scope Objects
 Servlet Listener
 Servlet Filter

2

When to Include another Web resource?
 When it is useful to add static or dynamic
contents already created by another web
resource
 Adding a banner content or copyright information in
the response returned from a Web component

3

How to Include another Web resource?

 Get RequestDispatcher object from ServletContext
object
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher(“/banner");
 Then, invoke the include() method of the
RequestDispatcher object passing request and
response objects
 dispatcher.include(request, response);

4

When to use “Forwarding” to another
Web resource?
 When you want to have one Web component do preliminary
processing of a request and have another component
generate the response
 You might want to partially process a request and then
transfer to another component depending on the nature of
the request

5

How to do “Forwarding” to another Web
resource?
 Get RequestDispatcher object from HttpServletRequest object
 Set “request URL” to the path of the forwarded page
RequestDispatcher dispatcher
= request.getRequestDispatcher(“/template.jsp");
 If the original URL is required for any processing, you can save
it as a request attribute
 Invoke the forward() method of the RequestDispatcher object
 dispatcher.forward(request, response);
6

Redirecting a Request
 Two programming models for
directing a request
 Method 1:
response.setStatus(response.SC_MOVED_PERMANTLY);
response.setHeader("Location", "http://...");

 Method 2:
response.sendRedirect("http://...");
7

What is JDBC?
Standard Java API for accessing
relational database
 Hides database specific details from
application
Part of Java SE (J2SE)
 Java SE 7 has JDBC 4.1

9

JDBC API
 Defines a set of Java Interfaces, which are implemented by
vendor-specific JDBC Drivers
 Applications use this set of Java interfaces for performing
database operations - portability
 Majority of JDBC API is located in java.sql package
 DriverManager, Connection, ResultSet, DatabaseMetaData,
ResultSetMetaData, PreparedStatement, CallableStatement and
Types
 Other advanced functionality exists in the javax.sql package
 DataSource

10

JDBC Driver

 Database specific implemention of JDBC
interfaces
 Everydatabase server has corresponding
JDBC driver(s)
 You can see the list of available drivers
from
 http://industry.java.sun.com/products/jdbc/
drivers
11

Database URL
 Used to make a connection to the database
– Can contain server, port, protocol etc…

jdbc:subprotocol_name:driver_dependant_databasename
– Oracle thin driver
jdbc:oracle:thin:@machinename:1521:dbname
– MySQL
jdbc:mysql://localhost:3306/test

12

MySQL Tools

 mysql-installer-5.5.22.1.msi (Database)
 mysql-workbench-gpi-5.2-40-win32.msi (GUI)
 mysql-connector-java-5.1.19-bin.jar (Driver)
 Copy to C:Program FilesApache Software
FoundationApache Tomcat 7.0.27lib

13

SQL Commands
 CREATE - Builds a new table with the specified field name and
types
 INSERT - Adds a new record to the named table
 SELECT - Retrieves records from the named table
 DELETE - Removes one or more record(s) from the table
 UPDATE - Modified one or more field(s) of particular record(s)
 DROP - Completely removes a table from the database

14

CREATE Statement
 The formal syntax
CREATE TABLE table_name(column [ , column] …)

 For example
CREATE TABLE Books(ISBN varchar(5) not null PRIMATY KEY,
title varchar(50) not null,
author varchar(50), price float);

15

INSERT Statement
INSERT INTO table_name (column [ , column] …) VALUES
(value [, value] …)

 For example
INSERT INTO Books VALUES('11111', 'J2ME on Symbian OS',
'Sam', 40);

16

SELECT Statement
SELECT [table.]column [ , [table].column] …
FROM table [ , table] …
[WHERE [table.]column OPERATOR VALUE
[AND] OR [table.]column OPERATOR VALUE …
[ORDER BY [table.]column [DESC] [ ,[table.]column
[DESC}]

 For example
SELECT * FROM Books WHERE ISBN = '11111';
17

DELETE Statement
DELETE FROM table_name WHERE column OPERATOR value
[AND | OR column OPERATOR value ] …

 For example
DELETE FROM Book WHERE ISBN = '11111';

18

Steps of Using JDBC

1. Load DB-specific JDBC driver
2. Get a Connection object
3. Get a Statement object
4. Execute queries and/or updates
5. Read results
6. Read Meta-data (optional step)
7. Close Statement and Connection objects

19

1. Load DB-Specific Database Driver

 To manually load the database driver and register
it with the DriverManager, load its class file
– Class.forName(<database-driver>)

try {
// This loads an instance of the Pointbase DB Driver.
// The driver has to be in the classpath.
Class.forName("com.mysql.jdbc.Driver");
}catch (ClassNotFoundException cnfe){
System.out.println("" + cnfe);
}
20

2. Get a Connection Object
 DriverManager class is responsible for selecting the
database and and creating the database connection
 Using DataSource is a prefered means of getting a
conection object
 Create the database connection as follows:
try {
Connection connection =
DriverManager.getConnection("jdbc:mysql://localhost:3306/test",
"root", "root");
} catch(SQLException sqle) {
System.out.println("" + sqle);
}
21

DriverManager & Connection

 java.sql.DriverManager
• getConnection(String url, String user, String password) throws
SQLException

 java.sql.Connection
• Statement createStatement() throws SQLException
• void close() throws SQLException
• void setAutoCommit(boolean b) throws SQLException
• void commit() throws SQLException
• void rollback() throws SQLException

22

Sub-Topics
 DataSource interface and DataSource
object
 Properties of a DataSource object
 JNDI registration of a DataSource object
 DataSource object that implements
Connection pooling

23

javax.sql.DataSource Interface and
DataSource Object

 Driver vendor implements the interface
 DataSource object is the factory for creating
database connections

24

Properties of DataSource Object
 A DataSource object has properties that can be modified when
necessary – these are defined in a container's configuration file
 location of the database server
 name of the database
 network protocol to use to communicate with the
server
 The benefit is that because the data source's properties can be
changed, any code accessing that data source does not need to
be changed
 In the GlassFish Application Server, a data source is called a
JDBC resource
25

JNDI Registration of a DataSource (JDBC
Resource) Object
 The JNDI name of a JDBC resource is expected
in the java:comp/env/jdbc subcontext
 For example, the JNDI name for the resource of
a BookDB database could be
java:comp/env/jdbc/BookDB
 Because all resource JNDI names are in the
java:comp/env subcontext, when you specify the
JNDI name of a JDBC resource enter only
jdbc/name. For example, for a payroll database,
specify jdbc/BookDB 26

Why Connection Pooling?
 Database connection is an expensive and limited
resource
 Using connection pooling, a smaller number of
connections are shared by a larger number of clients
 Creating and destroying database connections are
expensive operations
 Using connection pooling, a set of connections are pre-
created and are available as needed basis cutting down
on the overhead of creating and destroying database
connections

27

Connection Pooling & DataSource

 DataSource objects that implement connection
pooling also produce a connection to the
particular data source that the DataSource class
represents
 The connection object that the getConnection
method returns is a handle to a PooledConnection
object rather than being a physical connection
 The application code works the same way

28

3. Get a Statement Object

 Create a Statement Object from Connection
object
• java.sql.Statement
– ResultSet executeQuery(string sql)
– int executeUpdate(String sql)
• Example:
Statement statement = connection.createStatement();
 The
same Statement object can be used for
many, unrelated queries
29

4. Executing Query or Update

 From the Statement object, the 2 most used
commands are
– (a) QUERY (SELECT)
• ResultSet rs = statement.executeQuery("select * from
customer_tbl");

– (b) ACTION COMMAND (UPDATE/DELETE)
• int iReturnValue = statement.executeUpdate("update
manufacture_tbl set name = ‘IBM' where mfr_num = 19985678");

30

5. Reading Results

 Loop through ResultSet retrieving information
 java.sql.ResultSet
 boolean next()
 xxx getXxx(int columnNumber)
 xxx getXxx(String columnName)
 void close()
 The iterator is initialized to a position before the first
row
– You must call next() once to move it to the first row

31

5. Reading Results (cont.)
 Once you have the ResultSet, you can easily
retrieve the data by looping through it
while (rs.next()){
// Wrong this will generate an error
String value0 = rs.getString(0);

// Correct!
String value1 = rs.getString(1);
int value2 = rs.getInt(2);
int value3 = rs.getInt(“ADDR_LN1");
}

32

5. Reading Results (cont.)

 When retrieving data from the ResultSet, use
the appropriate getXXX() method
• getString()
• getInt()
• getDouble()
• getObject()
 Thereis an appropriate getXXX method of
each java.sql.Types datatype

33

6. Read ResultSet MetaData and
DatabaseMetaData (Optional)
 Once you have the ResultSet or Connection
objects, you can obtain the Meta Data about
the database or the query
 This gives valuable information about the data
that you are retrieving or the database that you
are using
– DatabaseMetaData dbmd = connection.getMetaData();
– ResultSetMetaData rsMeta = rs.getMetaData();
• There are approximately 150 methods in the
DatabaseMetaData class.

34

Example
public void showDBInfo() {{
public void showDBInfo()
try {{
try
DatabaseMetaData dbmd == connection.getMetaData();
DatabaseMetaData dbmd connection.getMetaData();
System.out.println("Product: "" ++ dbmd.getDatabaseProductName());
System.out.println("Product: dbmd.getDatabaseProductName());
System.out.println("Version: "" ++ dbmd.getDatabaseProductVersion());
System.out.println("Version: dbmd.getDatabaseProductVersion());
System.out.println ("Stored Procedures: "" ++
System.out.println ("Stored Procedures:
dbmd.supportsStoredProcedures());
dbmd.supportsStoredProcedures());
System.out.println("Transactions: "" ++
System.out.println("Transactions:
dbmd.supportsTransactions());
dbmd.supportsTransactions());
}} catch (SQLException ex) {{
catch (SQLException ex)
ex.getMessage();
ex.getMessage();
}}
}}

35

ResultSetMetaData Example
ResultSetMetaData meta = rs.getMetaData();
//Return the column count
int iColumnCount = meta.getColumnCount();

for (int i =1 ; i <= iColumnCount ; i++){
System.out.println(“Column Name: " + meta.getColumnName(i));
System.out.println(“Column Type" + meta.getColumnType(i));
System.out.println("Display Size: " + meta.getColumnDisplaySize(i) );
System.out.println("Precision: " + meta.getPrecision(i));
System.out.println(“Scale: " + meta.getScale(i) );
}

36

What Are They?

 PreparedStatement
• SQL is sent to the database and compiled or prepared
beforehand
 CallableStatement
• Executes SQL Stored Procedures

37

PreparedStatement Interface
 To call the same SQL statement multiple times, use a
PrepareStatement object.
 Extend PreparedStatement from Statement class.
 public interface PreparedStatement extends Statement
 Advantage
 Using Statement needs to compile every times.
 PreparedStatement will compile only once.

38

Example
public void insertData() {{
public void insertData()
PreparedStatement pstmt;
PreparedStatement pstmt;
String insertCmd == "INSERT INTO Book VALUES(?,?,?,?)";
String insertCmd "INSERT INTO Book VALUES(?,?,?,?)";
try {{
try
pstmt == connection.prepareStatement(insertCmd);
pstmt connection.prepareStatement(insertCmd);
pstmt.setString(1,"11111");
pstmt.setString(1,"11111");
pstmt.setString(2,"J2ME on Symbian OS");
pstmt.setString(2,"J2ME on Symbian OS");
pstmt.setString(3,"Sam");
pstmt.setString(3,"Sam");
pstmt.setDouble(4,40);
pstmt.setDouble(4,40);
pstmt.executeUpdate();
pstmt.executeUpdate();
}} catch (SQLException ex) {{
catch (SQLException ex)
ex.getMessage();
ex.getMessage();
}}
}}

39

CallableStatement
 The interface used to execute SQL stored procedures
 A stored procedure is a group of SQL statements that form
a logical unit and perform a particular task
 A CallableStatement object contains a call to a stored
procedure; it does not contain the stored procedure itself.
 The part that is enclosed in curly braces is the escape
syntax for stored procedures.
CallableStatement cs = connection.prepareCall("{call SHOW_SUPPLIERS}");
ResultSet rs = cs.executeQuery();

40

Servlet & JDBC (cont.)

43

Why Session Tracking?
 Need a mechanism to maintain state across a
series of requests from the same user (or
originating from the same browser) over some
period of time
 Example: Online shopping cart
 Yet, HTTP is stateless protocol
 Each time, a client talks to a web server, it opens a new
connection
 Server does not automatically maintains
“conversational state” of a user
44

Session Tracking Use Cases
 When clients at an online store add an
item to their shopping cart, how does
the server know what’s already in the
cart?
 When clients decide to proceed to
checkout, how can the server determine
which previously created shopping cart
is theirs? 45

A Session Maintains Client Identity and State across
multiple HTTP requests

Client 1

server
Session 1
Session ID 1

Client 2 Session ID 2

Session 2

46

Three “underlying” Session Tracking
Mechanisms
 Cookies
 URL rewriting
 Hidden form fields
 Note that these are just underlying mechanisms of
passing “session id”
 do not provide high-level programming APIs
 do not provide a framework for managing sessions
 This is what Servlet Session Tracking feature provides

47

What is HTTP Cookie?
 Cookie is a small amount of information sent by a
servlet to a Web browser
 Saved by the browser, and later sent back to the
server in subsequent requests
 A cookie has a name, a single value, and optional attributes
 A cookie's value can uniquely identify a client

 Server uses cookie's value to extract information
about the session from some location on the server

48

Cookies as Session Tracking
Mechanism
 Advantages:
 Very easy to implement
 Highly customizable
 Persist across browser shut-downs
 Disadvantages:
 Often: users turn off cookies for privacy or
security reason
 Not quite universal browser support
50

URL Rewriting
 URLs can be rewritten or encoded to include
session information.
 URL rewriting usually includes a session id
 Example:
http://localhost:8080/bookstore1/cashier;jsessionid
=c0o7fszeb1

51

URL Rewriting as Session Tracking
Mechanism
 Advantages:
 Letuser remain anonymous
 They are universally supported(most styles)
 Disadvantages:
 Tediousto rewrite all URLs
 Only works for dynamically created
documents

52

If Cookie is turned off..
 If your application makes use of session
objects
 you must ensure that session tracking is enabled
by having the application rewrite URLs whenever
the client turns off cookies
 by calling the response's encodeURL(URL)
method on all URLs returned by a servlet
 This method includes the session ID in the URL
only if cookies are disabled; otherwise, it returns
the URL unchanged 53

Example: response.encodeURL()
out.println("<strong><a href="" +
response.encodeURL(request.getContextPath() + "/cashier") +
""></a></strong>");

 If cookies are turned off
 http://localhost:8080/bookstore1/cashier;jsessionid=c0o7fszeb1
 If cookies are turned on
 http://localhost:8080/bookstore1/cashier

54

Hidden Form Fields
 Hidden form fields do not display in the
browser, but can be sent back to the server by
submit
<INPUT TYPE=”HIDDEN” NAME=”session” VALUE=”...”>
 Fields can have identification (session id) or just
some thing to remember (occupation)
 Servlet reads the fields using req.getParameter()

55

Hidden Form Fields as Session Tracking
Mechanism
 Advantages:
 Universally
supported.
 Allow anonymous users
 Disadvantages:
 Only works for a sequence of dynamically
generated forms.
 Breaks down with static documents, emailed
documents, bookmarked documents.
 No browser shutdowns. 56

HttpSession
 Toget a user's existing or new session
object:
 HttpSession session = request.getSession(true);
 "true"
means the server should create a new
session object if necessary
 HttpSession is Java interface

 Container creates a object of HttpSession type

57

Example: Getting HttpSession Object
public class CatalogServlet extends HttpServlet {
public void doGet (HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {

// Get the user's session and shopping cart
HttpSession session = request.getSession(true);
...
out = response.getWriter();
...
}
}
...

58

HttpSession Java Interface
 Contains Methods to
 View and manipulate information about a session,
such as the session identifier, creation time, and
last accessed time
 Bind objects to sessions, allowing user
information to persist across multiple user
connections

59

HttpSession Methods
 getId()
 Returns the unique identifier
 isNew()
 Determines if session is new to client (not page)
 getCreationTime()
 Returns time at which session was first created
 getLastAccessedTime()
 Returns time at which the session was last sent from the
client
 invalidate()
 Invalidate the session and unbind all objects associated
with it
60

Store and Retrieve of Attribute
 To stores values:
 session.setAttribute("cart", cart);
 To retrieves values:
 session.getAttribute("cart");

61

Session Timeout
 Used when an end-user can leave the browser without
actively closing a session
 Sessions usually timeout after 30 minutes of inactivity
 Product specific
 A different timeout may be set by server admin
 getMaxInactiveInterval(), setMaxInactiveInterval()
methods of HttpSession interface
 Gets or sets the amount of time, session should go without
access before being invalidated
62

Issues with “Stale” Session Objects

 The number of “stale” session objects that are in “to
be timed out” could be rather large
 Example
 1000 users with average 2 minutes session time, thus 15000
users during the 30 minutes period
 4K bytes of data per session
 15000 sessions * 4K -= 60M bytes of session data
 This is just for one Web application

 Could have an performance impact
 Use the data space in Session object with care
63

Session Invalidation
 Can be used by servlet programmer to end a session
proactively
 when a user at the browser clicks on “log out” button
 when a business logic ends a session (“checkout” page in the example
code in the following slide)
 public void invalidate()
 Expire the session and unbinds all objects with it
 Caution
 Remember that a session object is shared by multiple servlets/JSP-
pages and invalidating it could destroy data that other servlet/JSP-
pages are using
64

Example: Invalidate a Session
public class ReceiptServlet extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
...
scart = (ShoppingCart)
session.getAttribute("examples.bookstore.cart");
...
// Clear out shopping cart by invalidating the session
session.invalidate();

// set content type header before accessing the Writer
response.setContentType("text/html");
out = response.getWriter();
...
}
}

65

Scope Objects
 Enables sharing information among
collaborating web components via attributes
maintained in Scope objects
 Attributes are name/object pairs
 Attributes maintained in the Scope objects
are accessed with
 getAttribute() & setAttribute()
 4 Scope objects are defined
 Web context, session, request, page
66

Four Scope Objects: Accessibility
 Web context (ServletContext)
 Accessible from Web components within a Web
context
 Session
 Accessible from Web components handling a request
that belongs to the session
 Request
 Accessible from Web components handling the
request
 Page
 Accessible from JSP page that creates the object 67

Four Scope Objects: Class
 Web context
 javax.servlet.ServletContext

 Session
 javax.servlet.http.HttpSession

 Request
 javax.servlet.http.HttpServletRequest

 Page
 javax.servlet.jsp.PageContext

68

Session, Application Scope
Client 1

server
Session 1
Session ID 1

Client 2 Session ID 2

Session 2

Client 1

server

application

Client 2

Session, Request, Page Scope
client

request
response request response

Page 1 forward Page 2 Page 3 Page 4
forward

Page scope Page scope Page scope Page scope

request scope
request scope

Session scope

Scope of Objects

71

What is ServletContext For?
 Used by servlets to
 Set and get context-wide (application-wide) object-
valued attributes
 Get request dispatcher
 To forward to or include web component
 Access Web context-wide initialization parameters set in
the web.xml file
 Access Web resources associated with the Web context
 Log
 Access other misc. information

72

Scope of ServletContext
 Context-wide scope
 Shared by all servlets and JSP pages within a "web
application"
 Why it is called “web application scope”
 A "web application" is a collection of servlets and content
installed under a specific subset of the server's URL
namespace and possibly installed via a *.war file
 All servlets in BookStore web application share same
ServletContext object
 There is one ServletContext object per "web application"
per Java Virtual Machine
73

Example: Getting Attribute Value
from ServletContext
public class CatalogServlet extends HttpServlet {
private BookDB bookDB;
public void init() throws ServletException {
// Get context-wide attribute value from
// ServletContext object
bookDB = (BookDB)getServletContext().
getAttribute("bookDB");
if (bookDB == null) throw new
UnavailableException("Couldn't get database.");
}
} 74

What is Servlet Request?
 Contains data passed from client to servlet
 All servlet requests implement ServletRequest interface
which defines methods for accessing
 Client sent parameters
 Object-valued attributes
 Locales
 Client and server
 Input stream
 Protocol information
 Content type
 If request is made over secure channel (HTTPS) 75

Requests
data,
client, server, header
servlet itself
Request Servlet 1

Servlet 2

Response Servlet 3

Web Server 76

Servlet Lifecycle Events
 Support event notifications for state changes
in
 ServletContext
 Startup/shutdown

 Attribute changes
 HttpSession
 Creationand invalidation
 Changes in attributes

77

Listener Interfaces
 ServletContextListener
 contextInitialized/Destroyed(ServletContextEvent)
 ServletContextAttributeListener
 attributeAdded/Removed/Replaced(ServletContextAttributeEvent)
 HttpSessionListener
 sessionCreated/Destroyed(HttpSessionEvent)
 HttpSessionAttributeListener
 attributedAdded/Removed/Replaced(HttpSessionBindingEvent)
 HttpSessionActivationListener
 Handles sessions migrate from one server to another
 sessionWillPassivate(HttpSessionEvent)
 sessionDidActivate(HttpSessionEvent) 78

Listener Registration
 Web container
 creates an instance of each listener class
 registers it for event notifications before processing first
request by the application
 Registers the listener instances according to
 the interfaces they implement

 the order in which they appear in the deployment

descriptor web.xml
 Listeners are invoked in the order of their registration
during execution 79

Steps for Implementing Servlet Lifecycle
Event
1. Decide which scope object you want to receive an event
notification
2. Implement appropriate interface
3. Override methods that need to respond to the events of interest
4. Obtain access to important Web application objects and use them
5. Configure web.xml accordingly
6. Provide any needed initialization parameters

80

Example: Context Listener
public final class ContextListener
implements ServletContextListener {
private ServletContext context = null;

public void contextInitialized(ServletContextEvent event) {
context = event.getServletContext();

try {
BookDB bookDB = new BookDB();
context.setAttribute("bookDB", bookDB);
} catch (Exception ex) {
context.log("Couldn't create bookstore
database bean: " + ex.getMessage());
}

Counter counter = new Counter();
context.setAttribute("hitCounter", counter);
counter = new Counter();
context.setAttribute("orderCounter", counter);
}

81

Example: Context Listener
public void contextDestroyed(ServletContextEvent event) {
context = event.getServletContext();
BookDB bookDB =(BookDB)context.getAttribute("bookDB");
bookDB.remove();
context.removeAttribute("bookDB");
context.removeAttribute("hitCounter");
context.removeAttribute("orderCounter");
}
}

82

Listener Configuration
<web-app>
<display-name>Bookstore1</display-name>
<description>no description</description>

<filter>..</filter>
<filter-mapping>..</filter-mapping>
<listener>
<listener-class>listeners.ContextListener</listener-class>
</listener>
<servlet>..</servlet>
<servlet-mapping>..</servlet-mapping>
<session-config>..</session-config>
<error-page>..</error-page>
...
</web-app>
83

What are Java Servlet Filters?
 New component framework for intercepting and
modifying requests and responses
 Filters
can be chained and plugged in to the system during
deployment time
 Allows range of custom activities:
 Marking access, blocking access
 Caching, compression, logging
 Authentication, access control, encryption
 Introduced in Servlet 2.3 (Tomcat 4.0)
 Content transformations
84

What Can a Filter Do?
 Examine the request headers
 Customize the request object if it wishes to modify
request headers or data
 Customize the response object if it wishes to modify
response headers or data
 Invoke the next entity in the filter chain
 Examine response headers after it has invoked the
next filter in the chain
 Throw an exception to indicate an error in processing
85

How Servlet Filter Work?
Servlet Servlet
Container Filter Chain

Filter 1 Filter 2 Filter N

doFilter(
User Servlet service(
ServletRequest,
implemented container ServletRequest,
filters filter
ServletResponse,
FilterChain)
ServletResponse) 86

javax.servlet.Filter Interface
 init(FilterConfig)
 called only once when the filter is first initialized
 get ServletContext object from FilterConfig object and
save it somewhere so that doFilter() method can access it
 read filter initialization parameters from FilterConfig
object through getInitParameter() method
 destroy()
 called only once when container removes filter object
 close files or database connections

87

How Filter Chain Works
 Multiple filters can be chained
 order is dictated by the order of <filter> elements in the web.xml
deployment descriptor
 The first filter of the filter chain is invoked by the
container
 via doFilter(ServletRequest req, ServletResponse res, FilterChain
chain)
 the filter then perform whatever filter logic and then call the next
filter in the chain by calling chain.doFilter(..) method
 The last filter's call to chain.doFilter() ends up calling
service() method of the Servlet
88

Steps for Building a Servlet Filter
 Decide what custom filtering behavior you want to
implement for a web resource
 Create a class that implements Filter interface
 Implement filtering logic in the doFilter() method
 Call the doFilter() method of FilterChain object

 Configure the filter with Target servlets and JSP
pages
 use <filter> and <filter-mapping> elements

89

Example: HitCounterFilter
public final class HitCounterFilter implements Filter
{
private FilterConfig filterConfig = null;

public void init(FilterConfig filterConfig)
throws ServletException {
this.filterConfig = filterConfig;
}
public void destroy() {
this.filterConfig = null;
}

// Continued in the next page...

90

Example: HitCounterFilter
public void doFilter(ServletRequest request,
ServletResponse response, FilterChain chain)
throws IOException, ServletException {

if (filterConfig == null) return;
StringWriter sw = new StringWriter();
PrintWriter writer = new PrintWriter(sw);
Counter counter = (Counter)
filterConfig.getServletContext().getAttribute("hitCounter");
writer.println("The number of hits is: " + counter.incCounter());

// Log the resulting string
writer.flush();
filterConfig.getServletContext().log(sw.getBuffer().toString());
...
chain.doFilter(request, wrapper);
...
}
} 91

HitCounterFilter Configuration
<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app PUBLIC '-//Sun Microsystems, Inc.//DTD Web
Application 2.3//EN' 'http://java.sun.com/dtd/web-app_2_3.dtd'>

<web-app>
<display-name>Bookstore1</display-name>
<description>no description</description>

<filter>
<filter-name>HitCounterFilter</filter-name>
<filter-class>filters.HitCounterFilter</filter-class>
</filter>

<filter-mapping>
<filter-name>HitCounterFilter</filter-name>
<url-pattern>/enter</url-pattern>
</filter-mapping>
... 92

Configuration in web.xml
 <filter>
 <filter-name>: assigns a name of your choosing to the filter
 <filter-class>: used by the container to identify the filter class
 </filter>
 <filter-mapping>
 <filter-name>: assigns a name of your choosing to the filter
 <url-pattern>: declares a pattern URLs (Web resources) to which
the filter applies
 </filter-mapping>

93

Concurrency Issues on a Servlet
 The service() method of a servlet instance can be
invoked by multiple clients (multiple threads)
 Servlet programmer has to deal with concurrency
issue
 shared data needs to be protected
 this is called “servlet synchronization”

 use of synchronized block

94

Many Threads,
One Servlet Instance
Web Server

request
thread

request thread

request thread
Servlet
thread
request
thread

request Servlet container
95

Use of synchronized block

 Synchronized blocks are used to
guarantee only one thread at a time
can execute within a section of code

synchronized(this) {
myNumber = counter + 1;
counter = myNumber;
}
...
synchronized(this) {
counter = counter + 1 ;
}
96

Thank you

thananum@gmail.com
www.facebook.com/imcinstitute
www.imcinstitute.com

97

Java Web Programming [3/9] : Servlet Advanced

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Java Web Programming [3/9] : Servlet Advanced