Java Tutorial/Servlet/Filter

Filter Response

   <source lang="java">

import java.util.*; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.security.*; public class MyServlet implements Filter {

 private FilterConfig filterConfig = null;
 public void doFilter(ServletRequest request, ServletResponse response,
   FilterChain chain)
   throws IOException, ServletException 
 {
   chain.doFilter(request, response);
       response.setContentType("text/html");
       PrintWriter out = response.getWriter();
   out.println("The filter got this message: " + filterConfig.getInitParameter("message"));
 }
 public void destroy() { }
 public void init(FilterConfig filterConfig) {
   this.filterConfig = filterConfig;
 }

}</source>

Filter that uses a response wrapper to convert all output to uppercase

   <source lang="java">

import java.io.CharArrayWriter; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpServletResponseWrapper; public class UpperCaseFilter implements Filter {

 protected FilterConfig config;
 public void init(FilterConfig config) throws ServletException {
   this.config = config;
 }
 public void destroy() {
 }
 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
     throws ServletException, IOException {
   ServletResponse newResponse = response;
   if (request instanceof HttpServletRequest) {
     newResponse = new CharResponseWrapper((HttpServletResponse) response);
   }
   chain.doFilter(request, newResponse);
   if (newResponse instanceof CharResponseWrapper) {
     String text = newResponse.toString();
     if (text != null) {
       text = text.toUpperCase();
       response.getWriter().write(text);
     }
   }
 }

} class CharResponseWrapper extends HttpServletResponseWrapper {

 protected CharArrayWriter charWriter;
 protected PrintWriter writer;
 protected boolean getOutputStreamCalled;
 protected boolean getWriterCalled;
 public CharResponseWrapper(HttpServletResponse response) {
   super(response);
   charWriter = new CharArrayWriter();
 }
 public ServletOutputStream getOutputStream() throws IOException {
   if (getWriterCalled) {
     throw new IllegalStateException("getWriter already called");
   }
   getOutputStreamCalled = true;
   return super.getOutputStream();
 }
 public PrintWriter getWriter() throws IOException {
   if (writer != null) {
     return writer;
   }
   if (getOutputStreamCalled) {
     throw new IllegalStateException("getOutputStream already called");
   }
   getWriterCalled = true;
   writer = new PrintWriter(charWriter);
   return writer;
 }
 public String toString() {
   String s = null;
   if (writer != null) {
     s = charWriter.toString();
   }
   return s;
 }

} Simple response wrapper that utilizes a local CharArrayWriter for output

<filter>

 <filter-name>Upper Case Filter</filter-name>
 <filter-class>com.omh.filters.UpperCaseFilter</filter-class>

</filter> <filter-mapping>

 <filter-name>Upper Case Filter</filter-name>
 <url-pattern>/*</url-pattern>

</filter-mapping>

• /</source>

Servlet Filter Init Parameter

   <source lang="java">

import java.util.*; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.security.*; public class MyServlet implements Filter {

 private FilterConfig filterConfig = null;
 public void doFilter(ServletRequest request, ServletResponse response,
   FilterChain chain)
   throws IOException, ServletException 
 {
   System.out.println("The message is: " + filterConfig.getInitParameter("message"));
   chain.doFilter(request, response);
 }
 public void destroy() { }
 public void init(FilterConfig filterConfig) {
   this.filterConfig = filterConfig;
 }

}</source>

Simple filter for measuring servlet response times

   <source lang="java">

import java.io.IOException; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServletRequest; public class ResponseTimerFilter implements Filter {

 protected FilterConfig config;
 public void init(FilterConfig config) throws ServletException {
   this.config = config;
 }
 public void destroy() {
 }
 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
     throws ServletException, IOException {
   long startTime = System.currentTimeMillis();
   chain.doFilter(request, response);
   long elapsed = System.currentTimeMillis() - startTime;
   String name = "servlet";
   if (request instanceof HttpServletRequest) {
     name = ((HttpServletRequest) request).getRequestURI();
   }
   config.getServletContext().log(name + " took " + elapsed + " ms");
 }

} <filter>

 <filter-name>Timing Filter</filter-name>
 <filter-class>com.omh.filters.ResponseTimerFilter</filter-class>

</filter> <filter-mapping>

 <filter-name>Timing Filter</filter-name>
 <url-pattern>/*</url-pattern>

</filter-mapping>

• /</source>

Use Filter to Check Date

   <source lang="java">

import java.util.*; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.security.*; public class MyServlet implements Filter {

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

   GregorianCalendar calendar = new GregorianCalendar();
   Date date1 = new Date();
   calendar.setTime(date1);
   int hour = calendar.get(Calendar.HOUR_OF_DAY);
   if(hour < 9 || hour > 17) {   
       chain.doFilter(request, response);
   } else {
       response.setContentType("text/html");
       PrintWriter out = response.getWriter();
       out.println("<HTML>");
       out.println("<HEAD>");
       out.println("<TITLE>");
       out.println("Get Back to Work!");
       out.println("</TITLE>");
       out.println("</HEAD>");
       out.println("<BODY>");

out.println("

Get Back to Work!

");

       out.println("Sorry, that resource is not available now.");
       out.println("</BODY>");
       out.println("</HTML>");
   }    
 }
 public void destroy() 
 { 
 }
 public void init(FilterConfig filterConfig) 
 {
 }

}</source>

Use Filter to check Request Parameter

   <source lang="java">

import java.util.*; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.security.*; public class MyServlet implements Filter {

 private FilterConfig filterConfig = null;
 public void doFilter(ServletRequest request, ServletResponse response,
   FilterChain chain)
   throws IOException, ServletException 
 {
   String password = ((HttpServletRequest) request).getParameter("password");
    
   if(password.equals("opensesame")) {   
       chain.doFilter(request, response);
   } else {
       response.setContentType("text/html");
       PrintWriter out = response.getWriter();
       out.println("<HTML>");
       out.println("<HEAD>");
       out.println("<TITLE>");
       out.println("Incorrect Password");
       out.println("</TITLE>");
       out.println("</HEAD>");
       out.println("<BODY>");

out.println("

Incorrect Password

");

       out.println("Sorry, that password was incorrect.");
       out.println("</BODY>");
       out.println("</HTML>");
   }    
 }
 public void destroy() { }
 public void init(FilterConfig filterConfig) {
   this.filterConfig = filterConfig;
 }

}</source>

Use Filter to log Configuration

   <source lang="java">

import java.util.*; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.security.*; public class MyServlet implements Filter {

 private FilterConfig filterConfig = null;
 public void doFilter(ServletRequest request, ServletResponse response,
   FilterChain chain)
   throws IOException, ServletException 
 {
   long start = System.currentTimeMillis();
   String address =  request.getRemoteAddr();
   String file = ((HttpServletRequest) request).getRequestURI();
       
   chain.doFilter(request, response);
   filterConfig.getServletContext().log(
       "User access! " +      
       " User IP: " + address +      
       " Resource: " + file + 
       " Milliseconds used: " + (System.currentTimeMillis() - start) 
   );
 }
 public void destroy() { }
 public void init(FilterConfig filterConfig) {
   this.filterConfig = filterConfig;
 }

}</source>

Your First Filter

   <source lang="java">

import java.util.*; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.security.*; public class MyServlet implements Filter {

 public void doFilter(ServletRequest request, ServletResponse response,
   FilterChain chain)
   throws IOException, ServletException 
 {
   System.out.println("In filter");
   chain.doFilter(request, response);
   System.out.println("Leaving filter");
 }
 public void destroy() 
 { 
 }
 public void init(FilterConfig filterConfig) 
 {
 }

}</source>