Servlets 简明教程

Servlets - Writing Filters

Servlet 过滤器是可以在 Servlet 编程中用于以下目的的 Java 类 −

Servlet Filters are Java classes that can be used in Servlet Programming for the following purposes −

  1. To intercept requests from a client before they access a resource at back end.

  2. To manipulate responses from server before they are sent back to the client.

规范建议了多种类型的过滤器 −

There are various types of filters suggested by the specifications −

  1. Authentication Filters.

  2. Data compression Filters.

  3. Encryption Filters.

  4. Filters that trigger resource access events.

  5. Image Conversion Filters.

  6. Logging and Auditing Filters.

  7. MIME-TYPE Chain Filters.

  8. Tokenizing Filters .

  9. XSL/T Filters That Transform XML Content.

过滤器部署在部署描述符文件 web.xml 中，然后映射到应用程序部署描述符中的 servlet 名称或 URL 模式。

Filters are deployed in the deployment descriptor file web.xml and then map to either servlet names or URL patterns in your application’s deployment descriptor.

当 Web 容器启动 Web 应用程序时，它将创建在部署描述符中声明的每个过滤器的实例。这些过滤器将按照在部署描述符中声明的顺序执行。

When the web container starts up your web application, it creates an instance of each filter that you have declared in the deployment descriptor. The filters execute in the order that they are declared in the deployment descriptor.

Servlet Filter Methods

过滤器只是一个实现了 javax.servlet.Filter 接口的 Java 类。javax.servlet.Filter 接口定义了如下三个方法 −

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

Sr.No.

Method & Description

1

public void doFilter (ServletRequest, ServletResponse, FilterChain) This method is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain.

2

public void init(FilterConfig filterConfig) This method is called by the web container to indicate to a filter that it is being placed into service.

3

public void destroy() This method is called by the web container to indicate to a filter that it is being taken out of service.

Servlet Filter − Example

下面是 Servlet 过滤器示例，它将打印客户端的 IP 地址和当前日期时间。此示例将让您了解 Servlet 过滤器基础知识，但您可使用相同概念编写更复杂过滤器应用程序 −

Following is the Servlet Filter Example that would print the clients IP address and current date time. This example would give you basic understanding of Servlet Filter, but you can write more sophisticated filter applications using the same concept −

// Import required java libraries
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( ) {
      /* Called before the Filter instance is removed from service by the web container*/
   }
}

按照通常方式编译 LogFilter.java ，并将类文件放在 <Tomcat-installationdirectory>/webapps/ROOT/WEB-INF/classes

Compile LogFilter.java in usual way and put your class file in <Tomcat-installationdirectory>/webapps/ROOT/WEB-INF/classes

Servlet Filter Mapping in Web.xml

过滤器经过定义并映射到 URL 或 Servlet，方式与将 Servlet 定义并映射到 URL 模式大致相同。在部署描述文件 web.xml 中创建过滤器标记以下条目

Filters are defined and then mapped to a URL or Servlet, in much the same way as Servlet is defined and then mapped to a URL pattern. Create the following entry for filter tag in the deployment descriptor file web.xml

<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>

上述过滤器适用于所有 servlet，因为我们在配置中指定 / *。如果您只想对少数 servlet 应用过滤器，则可以指定特定 servlet 路径。

The above filter would apply to all the servlets because we specified /* in our configuration. You can specicy a particular servlet path if you want to apply filter on few servlets only.

现在，尝试以惯常方式调用任何 servlet，您会在 Web 服务器日志中看到生成的日志。您可以使用 Log4J 记录器将上述日志记录到单独文件中。

Now try to call any servlet in usual way and you would see generated log in your web server log. You can use Log4J logger to log above log in a separate file.

Using Multiple Filters

您的 Web 应用程序可能针对特定目的定义多个不同的过滤器。假设您定义了两个过滤器 AuthenFilter 和 LogFilter。除了需要创建不同的映射外（如下所述），其余过程将保持上述说明 −

Your web application may define several different filters with a specific purpose. Consider, you define two filters AuthenFilter and LogFilter. Rest of the process would remain as explained above except you need to create a different mapping as mentioned below −

<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>
   <filter-name>AuthenFilter</filter-name>
   <filter-class>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>

Filters Application Order

web.xml 中 filter-mapping 元素的顺序决定 Web 容器将过滤器应用于 servlet 的顺序。要逆转过滤器的顺序，您只需要逆转 web.xml 文件中的 filter-mapping 元素。

The order of filter-mapping elements in web.xml determines the order in which the web container applies the filter to the servlet. To reverse the order of the filter, you just need to reverse the filter-mapping elements in the web.xml file.

例如，上述示例将首先应用 LogFilter，然后将其应用任何 servlet，但以下示例将逆转顺序 −

For example, above example would apply LogFilter first and then it would apply AuthenFilter to any servlet but the following example would reverse the 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>