Servlet

FAQs

Every Filter in Java has to implement Filter interface either directly or indirectly.

Filter interface defines the following 3 methods

1.init():

public void init(FilterConfig config)throws ServletException

This method will be executed only once to perform initialization activities.

Example01

JCode Cell

1
2	Filter
3	FilterConfig
4	FilterChain
5	Filter(I):
6

FAQs

public void destroy()

This method will be executed only once to perform clean up activities just before taking the filter

from out of service.

Example02

JCode Cell

1
2	destroy()
3

FAQs

public void doFilter(ServletRequest req,ServletResponse resp,FilterChain fc)throws SE,IOE

Total filtering logic we have to define in this method only.

This method will be executed for every request.

By using FilterChain object we can forward the request to the next level.(it may be Servlet or

another Filter)

Example03

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2	doFilter()
3

FilterConfig(I)

returns the logical name of the filter which is configured in web.xml by using <filter-name> tag.

Example04

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2	public String getFilterName()
3

FilterConfig(I)

Example05

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2	public String getInitParameter(String name)
3	public Enumeration getInitParameterNames()
4	public ServletContext getServletContext()
5

DemoFilter.java

;

Example06

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1
2	import javax.servlet.*;
3	import java.io.*;
4	public class DemoFilter implements Filter
5	{
6	public void init(FilterConfig conf) throws ServletException
7	{
8	}
9	public void doFilter(ServletRequest req,ServletResponse resp,FilterChain fc) throws ServletException,IOException
10	{
11	PrintWriter out = resp.getWriter();
12	out.println("<h1>This line added by Demo Filter before processing the request</h1>")
13

Output


    <h1>This line added by Demo Filter before processing the request</h1>

DemoFilter.java

Example07

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1
2	fc.doFilter(req,resp);
3	out.println("<h1>This line added by Demo Filter after processing the request</h1>");
4	}
5	public void destroy()
6	{
7	}
8	}
9

Output


    <h1>This line added by Demo Filter after processing the request</h1>

TargetServlet.java

web.xml:

Example08

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1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class TargetServlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	out.println("<h1>This is the Target Servlet</h1>");
11	}
12	}
13

Output


    <h1>This is the Target Servlet</h1>

TargetServlet.java

Analysis:	Request	Request
Browser	Target

Servlet

Response	Response

Demo Filter

Whenever we are sending the request to TargetServlet,web container checks is there any filter

configured for this servlet or not.

If any Filter configured,web container forwards the request to the Filter instead of Servlet.

After completing Filtering logic,Filter forwards the request to the TargetServlet.

After processing that request by TargetServlet , the response will be forwarded to the Filter

instead of browser.

After executing Filtering logic , Filter forwards total response to the browser.

http://localhost:7777/filter1/test

Example09

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1
2	<web-app>
3
4	<servlet>
5	<servlet-name>TargetServlet</servlet-name>
6	<servlet-class>TargetServlet</servlet-class>
7	</servlet>
8
9	<servlet-mapping>
10	<servlet-name>TargetServlet</servlet-name>
11	<url-pattern>/test1</url-pattern>
12	</servlet-mapping>
13
14	<filter>
15	<filter-name>DemoFilter</filter-name>
16	<filter-class>DemoFilter</filter-class>
17	</filter>
18
19	<filter-mapping>
20	<filter-name>DemoFilter</filter-name>
21	<url-pattern>/test1</url-pattern>
22	</filter-mapping>
23
24	</web-app>
25

Configuring Filter in web.xml

We can map Filter either for a Particular url-pattern or to a particular Servlet or to the total web

application.

Filter-Mapping to a Particular url-pattern:

Example10

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2	<web-app>
3	<filter>
4	<filter-name>
5	<filter-class>
6	<init-param>
7	<param-name>
8	<param-value>
9	</init-param>
10	</filter>
11	..
12	</web-app>
13

Configuring Filter in web.xml

If the request is coming with the specified url-pattern then automatically this filter will be

executed.

Filter-mapping to a Particular Servlet:

Example11

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1
2	<filter-mapping>
3	<filter-name>DemoFilter</filter-name>
4	<url-pattern>/test1</url-pattern>
5	</filter-mapping>
6

Configuring Filter in web.xml

Filter-Mapping for Total web application:

Example12

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2	<filter-mapping>
3	<filter-name>DemoFilter</filter-name>
4	<servlet-name>TargetServlet</servlet-name>
5	</filter-mapping>
6

Configuring Filter in web.xml

For any request to the web application whether it is for servlet or jsp, this Filter will be executed.

Note:

Mapping Filter to the total web application is possible from Servlet 2.5V onwards.

<dispatcher> tag:

A servlet can get the request in one of the following 4 ways

FORWARD

Intermediate INCLUDE

Servlet

Target

REQUEST	Servlet

Intermediate ERROR

Servlet

Example13

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1
2	<filter-mapping>
3	<filter-name>DemoFilter</filter-name>
4	<url-pattern>*</url-pattern>
5	</filter-mapping>
6

Configuring Filter in web.xml

By default Filter concept is applicable only for direct end user REQUEST and not applicable for RD's

forward,include calls and error page calls.

If we want to extend for remaining cases also then we should go for <dispatcher> tag.

<dispatcher> tag introduced in Servlet 2.4V.

The allowed values for <dispatcher> tag are:

REQUEST:

Filter will be executed for direct end user request.

This is default value

FORWARD:

Filter will be executed for RD's forward() call.

INCLUDE:

Filter will be executed for RD's include() call

ERROR:

Filter will be executed for Error page call

Example14

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2	A Request Directly from Browser(REQUEST)
3	By RequestDispatcher's forward() call(FORWARD)
4	By RequestDispatcher's include() call(INCLUDE)
5	By Error Page Call(ERROR)
6

Configuring Filter in web.xml

If we want to execute Filter for direct end user's request and RD's forward() call, then we have to

configured filter as follows...

Example15

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2	<error-page>
3	<exception-type>java.lang.AE</exception-type>
4	<location>/test</location>
5	</error-page>case-1:
6

Configuring Filter in web.xml

In this case Filter won't be executed for RD's include call and error page calls.

Case-2:

Example16

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2	<filter-mapping>
3	<filter-name>DemoFilter</filter-name>
4	<url-pattern>/test1</url-pattern>
5	<dispatcher>REQUEST</dispatcher>
6	<dispatcher>FORWARD</dispatcher>
7	</filter-mapping>
8

Configuring Filter in web.xml

In this case Filter will be executed only for RD's include call and in the remaining 3 cases filter wont

be executed.

Example17

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2	<filter-mapping>
3	....
4	<dispatcher>INCLUDE</dispatcher>
5	</filter-mapping>
6

Demo Program for <dispatcher> tag

Example18

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2	<web-app>
3
4	<servlet>
5	<servlet-name>FirstServlet</servlet-name>
6	<servlet-class>FirstServlet</servlet-class>
7	</servlet>
8
9	<servlet>
10	<servlet-name>TargetServlet</servlet-name>
11	<servlet-class>TargetServlet</servlet-class>
12	</servlet>
13	<servlet>
14	<servlet-name>TargetServlet1</servlet-name>
15	<servlet-class>TargetServlet1</servlet-class>
16	</servlet>
17
18	<servlet-mapping>
19	<servlet-name>FirstServlet</servlet-name>
20	<url-pattern>/test1</url-pattern>
21	</servlet-mapping>
22
23	<servlet-mapping>
24	<servlet-name>TargetServlet</servlet-name>
25	<url-pattern>/test2</url-pattern>
26	</servlet-mapping>
27	<servlet-mapping>
28	<servlet-name>TargetServlet1</servlet-name>
29	<url-pattern>/test3</url-pattern>
30	</servlet-mapping>
31	<filter>
32	<filter-name>DemoFilter</filter-name>
33	<filter-class>DemoFilter</filter-class>
34	</filter>
35
36	<filter-mapping>
37	<filter-name>DemoFilter</filter-name>
38	<servlet-name>TargetServlet</servlet-name>
39	<dispatcher>REQUEST</dispatcher>
40	<dispatcher>FORWARD</dispatcher>
41	<dispatcher>INCLUDE</dispatcher>
42	<dispatcher>ERROR</dispatcher>
43	</filter-mapping>
44
45	<error-page>
46	<exception-type>java.lang.ArithmeticException</exception-type>
47	<location>/test2</location>
48	</error-page>
49	</web-app>
50

TargetServlet.java

Example19

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2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class TargetServlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	out.println("<h1>This is the Target Servlet</h1>");
11	}
12	}
13

Output


    <h1>This is the Target Servlet</h1>

TargetServlet1.java

Example20

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1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class TargetServlet1 extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	out.println(10/0);
11	}
12	}
13

FirstServlet.java

Example21

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2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class FirstServlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	RequestDispatcher rd = req.getRequestDispatcher("/test2");
10	rd.include(req,resp);
11	//rd.forward(req,resp);
12	}
13	}
14

DemoFilter.java

;

Example22

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1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class DemoFilter implements Filter
6	{
7	public void init(FilterConfig conf) throws ServletException
8	{
9	}
10	public void doFilter(ServletRequest req,ServletResponse resp,FilterChain fc) throws ServletException,IOException
11	{
12	PrintWriter out = resp.getWriter();
13	out.println("<h1>This line added by Demo Filter before processing the request</h1>")
14

Output


    <h1>This line added by Demo Filter before processing the request</h1>

DemoFilter.java

Example23

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1
2	fc.doFilter(req,resp);
3	out.println("<h1>This line added by Demo Filter after processing the request</h1>");
4	}
5	public void destroy()
6	{
7	}
8	}
9

Output


    <h1>This line added by Demo Filter after processing the request</h1>

Demo Program for FilterChain

Example24

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1
2	<web-app>
3
4	<filter>
5	<filter-name>LogFilter</filter-name>
6	<filter-class>LogFilter</filter-class>
7	</filter>
8
9	<filter>
10	<filter-name>DemoFilter</filter-name>
11	<filter-class>DemoFilter</filter-class>
12	</filter>
13
14	<filter-mapping>
15	<filter-name>LogFilter</filter-name>
16	<url-pattern>/test1</url-pattern>
17	</filter-mapping>
18
19	<filter-mapping>
20	<filter-name>DemoFilter</filter-name>
21	<url-pattern>/test1</url-pattern>
22	</filter-mapping>
23
24	<servlet>
25	<servlet-name>FirstServlet</servlet-name>
26	<servlet-class>FirstServlet</servlet-class>
27	</servlet>
28
29	<servlet-mapping>
30	<servlet-name>FirstServlet</servlet-name>
31	<url-pattern>/test1</url-pattern>
32	</servlet-mapping>
33
34	</web-app>
35

FirstServlet.java

Example25

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2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class FirstServlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	out.println("<h1>This is Target Servlet</h1>");
11	}
12	}
13

Output


    <h1>This is Target Servlet</h1>

DemoFilter.java

Example26

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1
2	import javax.servlet.*;
3	import java.io.*;
4	public class DemoFilter implements Filter
5	{
6	public void init(FilterConfig conf) throws ServletException
7	{
8	}
9	public void doFilter(ServletRequest req,ServletResponse resp,FilterChain fc) throws ServletException,IOException
10	{
11	PrintWriter out = resp.getWriter();
12	out.println("<h1> This Line added by DemoFilter before processing of the request</h1>");
13	fc.doFilter(req,resp);
14	out.println("<h1> This Line added by DemoFilter After processing of the request</h1>");
15	}
16	public void destroy()
17	{
18	}
19	}
20

Output


    <h1> This Line added by DemoFilter before processing of the request</h1>
    <h1> This Line added by DemoFilter After processing of the request</h1>

LogFilter.java

RequestURL()+" at "+ new Date());

Example27

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2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.util.*;
5	import java.io.*;
6	public class LogFilter implements Filter
7	{
8	private FilterConfig config;
9	public void init(FilterConfig config) throws ServletException
10	{
11	this.config= config;
12	}
13	public void doFilter(ServletRequest req,ServletResponse resp,FilterChain fc) throws ServletException,IOException
14	{
15	PrintWriter out = resp.getWriter();
16	out.println("<h1> This Line added by log Filter before processing of the request</h1>");
17	ServletContext context = config.getServletContext();
18	HttpServletRequest req1 = (HttpServletRequest)req;
19	context.log("A request is coming from "+req1.getRemoteHost()+" for URL :"+req1.get
20

Output


    <h1> This Line added by log Filter before processing of the request</h1>

LogFilter.java

Note:

In filter3 demo program, the log file is available in the following location:

Tomcat/logs/localhost.2017-02-26.txt

Example28

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1
2	fc.doFilter(req,resp);
3	out.println("<h1> This Line added by Log Filter After processing of the request</h1>");
4	}
5	public void destroy()
6	{
7	config= null;
8	}
9	}
10

Output


    <h1> This Line added by Log Filter After processing of the request</h1>

Web Container's Rule for ordering of Filters in FilterChain

Method only.	be another Filter OR Servlet)
3) It is call back method because Web Container	3) It is not call back method because we have to
calls this Method automatically.	call explicitly this Method.

Wrappers

Sometimes it is required to alter request and response information in the filters. We can

implement this by using wrapper classes.

i.e we can use wrappers inside filters to alter request and response information.

Eg1:

Within the filter,we have to convert end user's resume from word format to pdf format.

Eg 2:

Within the filter,we have to compress the response and that compressed response we can send to

the browser,so that we can reduce download time.

There are 2 types of wrappers

Request wrappers
Response wrappers
Request wrappers:

To alter request information

There are two request wrapper classes...

ServletRequest (I)

ServletRequestWrapper (C) HttpServletRequest (I)

HttpServletRequestWrapper (C)

ServletRequestWrapper
HttpServletRequestWrapper
Response wrappers:

To alter response information

ServletResponse (I)

ServletResponseWrapper (C)	HttpServletResponse (I)

HttpServletResponseWrapper (C)

There are two response wrapper classes

ServletResponseWrapper
HttpServletResponseWrapper

Example29

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1
2	Total Filtering Logic, we have to define in this 2) To forward Request to the next Level (It may
3

Demo Program for Request Wrapper

Example30

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1
2	<html>
3	<body bgcolor=green text=white><center><h1>Durga Software Solutions</h1></center>
4	<form action = "/wrapper/test1" >
5	<h1>Enter Any Word :<input type=text name=word></h1>
6	<input type=submit>
7	</form>
8	</body>
9	</html>
10

CustomizedRequest.java

Example31

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1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	public class CustomizedRequest extends HttpServletRequestWrapper
5	{
6	public CustomizedRequest(HttpServletRequest req)
7	{
8	super(req);// to make original request information available to the Parent class
9	}
10	public String getParameter(String word)
11	{
12	String word1 = super.getParameter(word);
13	if(word1.equals("JAVA") \| word1.equals("SCJP") \| word1.equals("SCWCD") \| word1.equals("SAT"))
14	return "SLEEP";
15	else
16	return word1;
17	}
18	}
19

BadWordFilter.java

Example32

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class BadWordFilter implements Filter
6	{
7	public void init(FilterConfig conf) throws ServletException
8	{
9	}
10	public void doFilter(ServletRequest req,ServletResponse resp,FilterChain fc) throws ServletException,IOException
11	{
12	CustomizedRequest req1 = new CustomizedRequest((HttpServletRequest)req);
13	fc.doFilter(req1,resp);
14	}
15	public void destroy()
16	{
17	}
18	}
19

TargetServlet.java

web.xml:

Example33

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1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class TargetServlet extends HttpServlet
6	{
7
8	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
9	{
10	PrintWriter out = resp.getWriter();
11	String word = req.getParameter("word");
12	out.println("<h1>Hi your typed word is :"+word+"</h1>");
13	}
14	}
15

TargetServlet.java

Flow of Program-Execution:

Enter Any Word: 1 23

Submit

Login.html	Target Servlet

5 4

Backward Filter

End user entered the word and click the submit button.

Filter creates a CustomizedRequest object by using wrapper class.

Filter forwards that customized request object to the TargetServlet instead of original request.

Within the servlet if we are performing any operation o the request object,our own customized

behaviour will be reflected.

TargetServlet prepares the response and forwards to Filter.

Filter forwards that response inturn to the end user.

Note:

We are not required to configure anything related to wrapper in web.xml.

Conclusions:

Filter object will be created by web container automatically. For this web container always calls

public no-arg constructor.Hence every Filter class should compulsary contains public no-arg

constructor. It may be explicitly provided by programmer or default constructor generated by

compiler.

Filter object will be created automatically by the web container at the time of application

deployment or at the time of server startup. Hence <load-on-startup> is not required for the

filters.

Usage of filter is nothing but following

Intercepting Filter Design Pattern.Hence it is recommended to use Filters concept in our

application.

Usage of Wrappers is nothing but following Decorator design pattern. Hence it is

recommended to use Wrappers in our application.

@WebFilter Annotation in Servlet 3.0V:

It is the replacement for filter configurations in web.xml

import javax.servlet.annotation.*;

@WebFilter(filterName="DemoFilter", urlPatterns="/test")

public class DemoFilter implements Filter

{

....

}

Example34

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1
2	<web-app>
3	<filter>
4	<filter-name>BadWordFilter</filter-name>
5	<filter-class>BadWordFilter</filter-class>
6	</filter>
7	<filter-mapping>
8	<filter-name>BadWordFilter</filter-name>
9	<servlet-name>TargetServlet</servlet-name>
10	</filter-mapping>
11
12	<servlet>
13	<servlet-name>TargetServlet</servlet-name>
14	<servlet-class>TargetServlet</servlet-class>
15	</servlet>
16
17	<servlet-mapping>
18	<servlet-name>TargetServlet</servlet-name>
19	<url-pattern>/test1</url-pattern>
20	</servlet-mapping>
21	</web-app>
22

Nearby lessons

Servlet - Examples: Filters & Wrappers

01FAQs

02FAQs

03FAQs

04FilterConfig(I)

05FilterConfig(I)

06DemoFilter.java

07DemoFilter.java

08TargetServlet.java

09TargetServlet.java

10Configuring Filter in web.xml

11Configuring Filter in web.xml

12Configuring Filter in web.xml

13Configuring Filter in web.xml

14Configuring Filter in web.xml

15Configuring Filter in web.xml

16Configuring Filter in web.xml

17Configuring Filter in web.xml

18Demo Program for <dispatcher> tag

19TargetServlet.java

20TargetServlet1.java

21FirstServlet.java

22DemoFilter.java

23DemoFilter.java

24Demo Program for FilterChain

25FirstServlet.java

26DemoFilter.java

27LogFilter.java

28LogFilter.java

29Web Container's Rule for ordering of Filters in FilterChain

30Demo Program for Request Wrapper

31CustomizedRequest.java

32BadWordFilter.java

33TargetServlet.java

34TargetServlet.java

FAQs

FAQs

FAQs

FilterConfig(I)

FilterConfig(I)

DemoFilter.java

DemoFilter.java

TargetServlet.java

TargetServlet.java

Configuring Filter in web.xml

Configuring Filter in web.xml

Configuring Filter in web.xml

Configuring Filter in web.xml

Configuring Filter in web.xml

Configuring Filter in web.xml

Configuring Filter in web.xml

Configuring Filter in web.xml

Demo Program for <dispatcher> tag

TargetServlet.java

TargetServlet1.java

FirstServlet.java

DemoFilter.java

DemoFilter.java

Demo Program for FilterChain

FirstServlet.java

DemoFilter.java

LogFilter.java

LogFilter.java

Web Container's Rule for ordering of Filters in FilterChain

Demo Program for Request Wrapper

CustomizedRequest.java

BadWordFilter.java

TargetServlet.java

TargetServlet.java