Filters are components that sit between a request and the intended endpoint of that request. The endpoint could be a static or dynamic resource such as an HTML page, JSP, or a Servlet.
Filters also apply to "fictitious" resources. A request that would typically generate a 404 error can pass through a Filter and the same Filter can optionally generate a response for it instead of displaying the error.
- Read the request data on the way to the endpoint.
- Wrap the request before passing it on.
- Wrap the response before passing it on.
- Manipulate the response data on the way back to the output.
- Return errors to the client.
- Request dispatch to another resource and ignore the original
URL.
- Generate its own response before returning to the client.
This makes Filters very powerful, in fact, so powerful that developers have to be very careful when using them. When using
Filters, the developer is much closer to the HTTP protocol than with a Servlet and must understand the protocol in much more detail. For example, when using Servlets, a developer can pretty much ignore the HTTP content-length header, but when using Filters and producing data, the Filter writer may have to know the size of the content and set the value of that header.
A Filter is a component that intercepts a request sent to a resource in a Web Application. Filters exist as part of a chain, with the last link in the chain being the requested resource. A Filter can choose to pass the request on, in which case the request will be forwarded to either the next Filter in the Filter chain or, if this is the last Filter in the chain, to the requested resource. The Filter also sees the response before it is returned to the client.
Source Code of index.jsp
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<form action="test.jsp">
<input type="submit">
</form>
</body>
</html>
Source Code of test.jsp
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<h1>Test Page</h1>
</body>
</html>
Source Code of LogFilter.java
\
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
// Implements Filter class
public class LogFilter implements Filter {
public void init(FilterConfig config) throws ServletException {
// Get init parameter
String testParam = config.getInitParameter("test-param");
// Print the init parameter
System.out.println("Test Param: " + testParam);
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws java.io.IOException, ServletException {
// Get the IP address of client machine.
String ipAddress = request.getRemoteAddr();
// Log the IP address and current timestamp.
System.out.println("IP " + ipAddress + ", Time "
+ new Date().toString());
// Pass request back down the filter chain
chain.doFilter(request, response);
}
public void destroy() {
System.out.println("Destroy Called !");
}
}
Source Code of web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://java.sun.com/xml/ns/javaee"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
id="WebApp_ID" version="3.0">
<display-name>JSPFilter</display-name>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
<filter>
<filter-name>LogFilter</filter-name>
<filter-class>LogFilter</filter-class>
<init-param>
<param-name>test-param</param-name>
<param-value>Initialization Paramter</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>LogFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
</web-app>
Happy Exploring!
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