Servlet filter:
Servlet filters are the objects that are used to perform some filtering tasks. A filter can be applied to a servlet, JSP, or HTML.
Servlet filters are mainly used for the following tasks:
- Pre-processing: A servlet filter is used for pre-processing of requests before it accesses any resource on the server side.
- Post-processing: A servlet filter is used for post-processing the response before it is sent back to the client.
How to create a filter?
Implement javax.servlet.Filter interface to create a filter.
Filter interface:
To create a filter you have to implement a filter interface. The filter interface is in javax.servlet package javax.servlet.Filter. It provides the life cycle methods of a filter.
Methods of filter interface:
1. init(FilterConfig config): This method is used to initialize the filter. It is called only once by the web container.
Syntax:
public void init(FilterConfig config)
2. doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain): This method is used for performing pre-processing and post-processing tasks. It is called every time a request/response comes for a resource to which the filter is mapped.
Syntax:
public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)
3. destroy(): This method is called only once by the web container when the filter is taken out of the service.
Syntax:
public void destroy()
FilterChain interface:
FilterChain object is used to call the next filter or a resource if it is the last filter in filter chaining.
Method of FilterChain interface:
1. doFilter(HttpServletRequest request, HttpServletResponse response): This method is used to call the next filter in filter chaining.
Syntax:
public void doFilter(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
How to define a filter in web.xml?
<filter> attribute is used to define a filter in web.xml.
Syntax:
//Other attributes. <filter> <filter-name>filterName </filter-name> <filter-class>filterClass</filter-class> </filter> <filter-mapping> <filter-name>filterName</filter-name> <url-pattern>urlPattern</url-pattern> </filter-mapping> //Other attributes. </web-app>
Example:
LoginFilter.java
import java.io.IOException; import java.io.PrintWriter; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.RequestDispatcher; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; /** * This class is used as a filter. * @author W3schools360 */ public class LoginFilter implements Filter { private static final long serialVersionUID = 1L; public void init(FilterConfig filterConfig) throws ServletException { } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); //get parameters from request object. String userName = request.getParameter("userName").trim(); String password = request.getParameter("password").trim(); //check for null and empty values. if(userName == null || userName.equals("") || password == null || password.equals("")){ out.print("Please enter both username " + "and password. "); RequestDispatcher requestDispatcher = request.getRequestDispatcher("/login.html"); requestDispatcher.include(request, response); }//Check for valid username and password. else if(userName.equals("jai") && password.equals("1234")){ chain.doFilter(request, response); }else{ out.print("Wrong username or password. "); RequestDispatcher requestDispatcher = request.getRequestDispatcher("/login.html"); requestDispatcher.include(request, response); } } public void destroy() { } }
WelcomeServlet.java
import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; /** * This class is used to print login message * if user logged in successfully. */ public class WelcomeServlet extends HttpServlet { private static final long serialVersionUID = 1L; //no-argument constructor public WelcomeServlet() { } protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.print("<h1>You are logged in successfully.</h1>"); out.close(); } }
login.html
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Login</title> <form action="WelcomeServlet" method="POST"> Username:<input type="text" name="userName"> <br><br> Password:<input type="password" name="password"> <br><br> <input type="submit" value="login"> </form>
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" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemalocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5"> <servlet> <servlet-name>WelcomeServlet</servlet-name> <servlet-class> com.w3schools.business.WelcomeServlet </servlet-class> </servlet> <servlet-mapping> <servlet-name>WelcomeServlet</servlet-name> <url-pattern>/WelcomeServlet</url-pattern> </servlet-mapping> <filter> <filter-name>LoginFilter</filter-name> <filter-class> com.w3schools.business.LoginFilter </filter-class> </filter> <filter-mapping> <filter-name>LoginFilter</filter-name> <url-pattern>/WelcomeServlet</url-pattern> </filter-mapping> <welcome-file-list> <welcome-file>login.html</welcome-file> </welcome-file-list> </web-app>