Servlet Writing Filters

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

One or more Servlet filters can be attached to a Servlet or a group of Servlets. Servlet filters can also be attached to JavaServer Pages (JSP) files and HTML pages. All attached Servlet filters are called before the Servlet is invoked.

A Servlet filter is a Java class that can be used in Servlet programming and can achieve the following purposes:

• Intercept client requests before they access backend resources.
• Process server responses before they are sent back to the client.

Various types of filters suggested by the specification:

• Authentication Filters.
• Data compression Filters.
• Encryption Filters.
• Filters that trigger resource access events.
• Image Conversion Filters.
• Logging and Auditing Filters.
• MIME-TYPE Chain Filters.
• Tokenizing Filters.
• XSL/T Filters, which transform XML content.

Filters are declared with XML tags in the Web deployment descriptor (web.xml) and then mapped to Servlet names or URL patterns in your application's deployment descriptor.

When the Web container starts the Web application, it creates an instance for each filter you declared in the deployment descriptor.

The execution order of filters is consistent with the configuration order in the web.xml configuration file; generally, filters are configured before all Servlets.

Servlet Filter Methods

A filter is a Java class that implements the javax.servlet.Filter interface. The javax.servlet.Filter interface defines three methods:

No.	Method & Description
1	public void doFilter (ServletRequest, ServletResponse, FilterChain) This method performs the actual filtering operation. When the client request method matches the URL set for the filter, the Servlet container will first call the filter's doFilter method. FilterChain allows the user to access subsequent filters.
2	public void init(FilterConfig filterConfig) When the Web application starts, the Web server creates an instance of the Filter and calls its init method, reads the web.xml configuration, and completes the object initialization, thereby preparing for intercepting subsequent user requests (the filter object is created only once, and the init method is also executed only once). Through the parameter of the init method, developers can obtain the FilterConfig object representing the current filter configuration information.
3	public void destroy() The Servlet container calls this method before destroying the filter instance, and in this method releases the resources occupied by the Servlet filter.

Using FilterConfig

The init method of Filter provides a FilterConfig object.

For example, the web.xml file is configured as follows:

<filter>
    <filter-name>LogFilter</filter-name>
    <filter-class>com.example.test.LogFilter</filter-class>
    <init-param>
        <param-name>Site</param-name>
        <param-value>Example</param-value>
    </init-param>
</filter>

Use the FilterConfig object in the init method to obtain parameters:

public void  init(FilterConfig config) throws ServletException {
    // 获取初始化参数
    String site = config.getInitParameter("Site"); 
    // 输出初始化参数
    System.out.println("网站名称: " + site); 
}

Servlet Filter Example

The following is a Servlet filter example that will output the website name and address. This example gives you a basic understanding of Servlet filters. You can use the same concepts to write more complex filter applications:

package com.example.test;

//导入必需的 java 库
import javax.servlet.*;
import java.util.*;

//实现 Filter 类
public class LogFilter implements Filter  {
    public void  init(FilterConfig config) throws ServletException {
        // 获取初始化参数
        String site = config.getInitParameter("Site"); 

        // 输出初始化参数
        System.out.println("网站名称: " + site); 
    }
    public void  doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws java.io.IOException, ServletException {

        // 输出站点名称
        System.out.println("站点网址：http://www.example.com");

        // 把请求传回过滤链
        chain.doFilter(request,response);
    }
    public void destroy( ){
        /* 在 Filter 实例被 Web 容器从服务移除之前调用 */
    }
}

Here we use the previously mentioned DisplayHeader.java as an example:

//导入必需的 java 库
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")

//扩展 HttpServlet 类
public class DisplayHeader extends HttpServlet {

    // 处理 GET 方法请求的方法
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {
        // 设置响应内容类型
        response.setContentType("text/html;charset=UTF-8");

        PrintWriter out = response.getWriter();
        String title = "HTTP Header 请求实例 - Example实例";
        String docType =
            "<!DOCTYPE html> \n";
            out.println(docType +
            "<html>\n" +
            "<head><meta charset=\"utf-8\"><title>" + title + "</title></head>\n"+
            "<body bgcolor=\"#F0F0F0\">\n" +
            "<h1 align=\"center\">" + title + "</h1>\n" +
            "<table width=\"100%\" border=\"1\" align=\"center\">\n" +
            "<tr bgcolor=\"#949494\">\n" +
            "<th>Header 名称</th><th>Header 值</th>\n"+
            "</tr>\n");

        Enumeration headerNames = request.getHeaderNames();

        while(headerNames.hasMoreElements()) {
            String paramName = (String)headerNames.nextElement();
            out.print("<tr><td>" + paramName + "</td>\n");
            String paramValue = request.getHeader(paramName);
            out.println("<td> " + paramValue + "</td></tr>\n");
        }
        out.println("</table>\n</body></html>");
    }
    // 处理 POST 方法请求的方法
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

Servlet Filter Mapping in Web.xml

Defining a filter and then mapping it to a URL or Servlet is roughly the same as defining a Servlet and then mapping it to a URL pattern. In the deployment descriptor fileweb.xmlcreate the following entry for the filter tag:

<?xml version="1.0" encoding="UTF-8"?>  
<web-app>  
<filter>
  <filter-name>LogFilter</filter-name>
  <filter-class>com.example.test.LogFilter</filter-class>
  <init-param>
    <param-name>Site</param-name>
    <param-value>Example</param-value>
  </init-param>
</filter>
<filter-mapping>
  <filter-name>LogFilter</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>
<servlet>  
  <!-- 类名 -->  
  <servlet-name>DisplayHeader</servlet-name>  
  <!-- 所在的包 -->  
  <servlet-class>com.example.test.DisplayHeader</servlet-class>  
</servlet>  
<servlet-mapping>  
  <servlet-name>DisplayHeader</servlet-name>  
  <!-- 访问的网址 -->  
  <url-pattern>/TomcatTest/DisplayHeader</url-pattern>  
</servlet-mapping>  
</web-app>  

The above filter applies to all Servlets, because in the configuration we specified/*If you only want to apply the filter to a few Servlets, you can specify a specific Servlet path.

Now try to invoke any Servlet in the usual way, and you will see the logs generated in the Web server. You can also use a Log4J logger to record the above logs to a separate file.

Next, we visit this example addresshttp://localhost:8080/TomcatTest/DisplayHeader, and then look at the output in the console, as shown below:

Using Multiple Filters

Web applications can define several different filters for specific purposes. Suppose you define two filtersAuthenFilterandLogFilter. You need to create a different mapping as described below; the rest of the processing is roughly the same as explained above:

<filter>
   <filter-name>LogFilter</filter-name>
   <filter-class>com.example.test.LogFilter</filter-class>
   <init-param>
      <param-name>test-param</param-name>
      <param-value>Initialization Paramter</param-value>
   </init-param>
</filter>

<filter>
   <filter-name>AuthenFilter</filter-name>
   <filter-class>com.example.test.AuthenFilter</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>

<filter-mapping>
   <filter-name>AuthenFilter</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>

Order of Filter Application

The order of the filter-mapping elements in web.xml determines the order in which the Web container applies filters to Servlets. To reverse the order of the filters, you only need to reverse the filter-mapping elements in the web.xml file.

For example, the above example will apply LogFilter first, and then AuthenFilter, but the following example will reverse this order:

<filter-mapping>
   <filter-name>AuthenFilter</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>

<filter-mapping>
   <filter-name>LogFilter</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>

---

Explanation of Each Node in web.xml Configuration

• <filter>Specifies a filter.
  • <filter-name>Used to specify a name for the filter; the content of this element cannot be empty.
  • <filter-class>This element is used to specify the fully qualified class name of the filter.
  • <init-param>This element is used to specify initialization parameters for the filter. Its child elements<param-name>specify the parameter name,<param-value>and specify the parameter value.
  • In the filter, you can useFilterConfigthe FilterConfig interface object to access the initialization parameters.
• <filter-mapping>This element is used to set the resources that a Filter is responsible for intercepting. The resources intercepted by a Filter can be specified in two ways: Servlet name and request path for resource access.
  • <filter-name>The child element is used to set the registration name of the filter. This value must be the name of a filter declared in<filter>the <filter> element.
  • <url-pattern>Sets the request path intercepted by the filter (the URL pattern associated with the filter).
• <servlet-name>Specifies the name of the Servlet intercepted by the filter.
• <dispatcher>Specifies the way the resource intercepted by the filter is invoked by the Servlet container; it can beREQUEST,INCLUDE,FORWARDandERRORone of, the default isREQUEST. Users can set multiple<dispatcher>child elements to specify that the Filter intercepts multiple invocation methods of the resource.
• <dispatcher>The values that the child element can be set to and their meanings
  • REQUEST: When the user directly accesses the page, the Web container will invoke the filter. If the target resource is accessed via the include() or forward() methods of RequestDispatcher, then the filter will not be invoked.
  • INCLUDE: If the target resource is accessed via the include() method of RequestDispatcher, then the filter will be invoked. Otherwise, the filter will not be invoked.
  • FORWARD: If the target resource is accessed via the forward() method of RequestDispatcher, then the filter will be invoked. Otherwise, the filter will not be invoked.
  • ERROR: If the target resource is invoked through the declarative exception handling mechanism, then the filter will be invoked. Otherwise, the filter will not be invoked.

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