2.10. Servlet authoring filter

发布时间 ： 2025-10-25 13:33:41 UTC      

Page Views: 9 views

Servlet filters can dynamically intercept requests and responses to transform or use the information contained in the request or response.

You can attach one or more Servlet filters to a Servlet or a set of Servlet.Servlet filters can also be attached to JavaServer Pages (JSP) files and HTML pages. Call all additional Servlet filters before calling Servlet.

Servlet filters are Java classes that can be used for Servlet programming todo the following:

  • Intercept client requests before accessing back-end resources.

  • The responses from the server are processed before they are sent back to theclient.

Various types of filters recommended according to the specification:

  • Authentication Filters.

  • Data compression Filters.

  • EEncryption Filters.

  • Triggers a resource access event filter.

  • Image Conversion Filters.

  • Logging and Auditing Filters.

  • MIME-TYPE Chain Filters.

  • Tokenizing Filters.

  • XSL/T filter (XSL/T Filters) to convert XML content.

The filter uses the Web deployment descriptor web.xml and then map to the Servlet name or URL schema in your application’s deployment descriptor.

When the Web container starts the Web application, it creates an instance ofeach filter that you declare in the deployment descriptor.

The execution order of Filter is similar to that of the web.xml the configuration order in the configuration file is the same, and Filter is generally configured before all Servlet.

2.10.1. Servlet filter method

The filter is an implementation of javax.servlet.Filter the Java class of the interface. javax.servlet.Filter the interface defines three methods:

Serial number

Method & description

1

public void doFilter (ServletRequest, ServletResponse, FilterChain) thismethod completes the actual filtering operation, and when the client requests a URL whose method matches the filter settings, the Servlet container will first call the filter’s doFilter method. FilterChain users access subsequent filters.

2

public void init(FilterConfig filterConfig) when the web application starts, the web server will create an instance object of Filter, call its init method, read the web.xml configuration, and complete the initializationof the object, thus preparing for the interception of subsequent user requests (the filter object will only be created once, and the init method will only be executed once). Developers can obtain a FilterConfig object that represents the current filter configuration information through the parameters of the init method.

3

public void destroy() the Servlet container calls this method before destroying the filter instance, releasing the resources consumed by the Servlet filter in this method.

2.10.2. FilterConfig usage

Filter’s init method provides a FilterConfig object.

Such as the web.xml file configuration is as follows:

<filter> <filter-name>LogFilterfilter-name> <filter-class>com.runoob.test.LogFilterfilter-class> <init-param> <param-name>Siteparam-name> <param-value>Rookie Tutorialparam-value> init-param> filter>    
 In   init   method use   FilterConfig   object to get parameters: 
 
 
 
public void init(FilterConfig config) throws ServletException { // Get initialization parameters String site = config.getInitParameter("Site"); // Output initialization parameters System.out.println("Site Name: " + site); } 
 
 
  
 
  2.10.3.  Servlet filter instance  
 
 The following is an example of a Servlet filter that outputs the name and address of the Web site. This example gives you a basic understanding of Servlet filters, and you can write more complex filter applications using the same concepts: 
 
 
 
package com.runoob.test; //Import necessary Java libraries import javax.servlet.*; import java.util.*; //Implement the Filter class public class LogFilter implements Filter { public void init(FilterConfig config) throws ServletException { // Get initialization parameters String site = config.getInitParameter("Site"); // Output initialization parameters System.out.println("Site Name: " + site); } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws java.io.IOException, ServletException { // Output Site Name System.out.println("Website URL：http://www.runoob.com"); // Send the request back to the filtering chain chain.doFilter(request,response); } public void destroy( ){ /* Called before the Filter instance is removed from the service by the Web container */ } } 
 
 
 
 Use the above mentioned here.   DisplayHeader.java   as an example: 
 
 
 
//Import necessary Java libraries import java.io.IOException; import java.io.PrintWriter; import java.util.Enumeration; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet("/DisplayHeader") //Extend HttpServlet Class public class DisplayHeader extends HttpServlet { // Method for handling GET method requests public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Set response content type response.setContentType("text/html;charset=UTF-8"); PrintWriter out = response.getWriter(); String title = "HTTP Header Request Example - Rookie Tutorial Example"; String docType = " \n"; out.println(docType + "\n" + "\"utf-8\">"</span> <span class="o">+</span> <span class="n">title</span> <span class="o">+</span> <span class="s2">"\n"+ "\"#f0f0f0\">\n" + "
\"center\">" + title + "\n" + "\"100%\" border=\"1\" align=\"center\">\n"+"\"#949494\">\n"+"
  
  Header nameHeader value\n"+ "\n"); Enumeration headerNames = request.getHeaderNames(); while(headerNames.hasMoreElements()) { String paramName = (String)headerNames.nextElement(); out.print("
" + paramName + "\n"); String paramValue = request.getHeader(paramName); out.println(" " + paramValue + "\n"); } out.println("\n"); } // Method for handling POST method requests public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }     
 
  2.10.4.  Servlet filter Mapping in Web.xml  
 
 Define a filter and then map to a URL or Servlet in much the same way as defining a Servlet and then mapping to an URL schema. In the deployment descriptor file   web.xml   create the following entry for the filter tag in: 
 
 
 
   LogFilter com.runoob.test.LogFilter  Site Rookie Tutorial    LogFilter /*    DisplayHeader  com.runoob.test.DisplayHeader   DisplayHeader  /TomcatTest/DisplayHeader   
 
 
 
 The above filter applies to all Servlet because we specify in the configuration   /\*   . If you only want to apply filters to a small number of Servlet, you can specify a specific Servlet path. 
 
 Now try calling any Servlet in the usual way, and you will see the logs generated in the Web server. You can also use the Log4J logger to log the above to a separate file. 
 
 Next, we visit the instance address  http://localhost:8080/TomcatTest/DisplayHeader  , and take a look at the output in the console, as shown below: 
 
 Image0 
 
 
 
  2.10.5.  Use multiple filters  
 
 Web applications can define several different filters for a specific purpose. Suppose you define two filters   AuthenFilter   and   LogFilter   .You need to create a different mapping as described below, and the rest of the processing is roughly the same as explained above: 
 
 
 
<filter> <filter-name>LogFilterfilter-name> <filter-class>com.runoob.test.LogFilterfilter-class> <init-param> <param-name>test-paramparam-name> <param-value>Initialization Paramterparam-value> init-param> filter> <filter> <filter-name>AuthenFilterfilter-name> <filter-class>com.runoob.test.AuthenFilterfilter-class> <init-param> <param-name>test-paramparam-name> <param-value>Initialization Paramterparam-value> init-param> filter> <filter-mapping> <filter-name>LogFilterfilter-name> <url-pattern>/*url-pattern> filter-mapping> <filter-mapping> <filter-name>AuthenFilterfilter-name> <url-pattern>/*url-pattern> filter-mapping>     
 
  2.10.6.  Application order of filter  
 
 In web.xml   filter-mapping   The order of elements determines the order inwhich the Web container applies the filter to the Servlet. To reverse the order of the filters, you only need to use the   web.xml   invert in file   filter-mapping   elements are fine. 
 
 For example, the above example will apply LogFilter first and then AuthenFilter, but the following example will reverse this order: 
 
 
 
<filter-mapping> <filter-name>AuthenFilterfilter-name> <url-pattern>/*url-pattern> filter-mapping> <filter-mapping> <filter-name>LogFilterfilter-name> <url-pattern>/*url-pattern> filter-mapping>     
 
  2.10.7.  web.xml configuring each node description