Example analysis of Filter in JavaWeb Servlet

This article mainly introduces the relevant information about the detailed explanation of Filter filter in JavaWeb Servlet. I hope this article can help everyone and let everyone fully understand how to use it. Friends in need can refer to

JavaWeb Servlet Detailed explanation of Filter filter

1. Brief description

Filter filter filters all web resources of the web server to achieve some special functions (Permission access control, filtering sensitive words, compressing response information). Filter can check and modify the requests and responses of the Servlet container. It cannot generate request requests and responses by itself. It only provides filtering function. (Before the Servlet is called, the Request object is checked to modify its related information. The Servlet is called. After checking the Response and modifying its related information), the Filter object is resident on the server.

2.Lifecycle

## (1). Start the server, read the configuration file web.xml, load the Filter class and instantiate it, and call init (); (3). If the application is stopped or reloaded, destroy() is called.

3.chain filter chain

The two filters play different filtering roles. The server will assemble them into a chain in the order defined by the filters in web. It can be interrupted at any time during the execution process. As long as chain.doFilter() is not executed, subsequent filters and requested content will not be executed.

4. Filter configuration

(1). Inherit the Filter interface and override the doFilter() method (2). Configuration of web.xml to filter operations that meet the requirements

About url-pattern configuration instructions>>

/*
 * MyFilter.java
 * 过滤器
 */
package com.baidu.web;

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;
import javax.servlet.http.HttpServletResponse;

public class MyFilter implements Filter{

 @Override
 public void destroy() {
 }

 @Override
 public void doFilter(ServletRequest request, ServletResponse response,
 FilterChain chain) throws IOException, ServletException {
 //对请求request进行处理
 HttpServletResponse resp = (HttpServletResponse)response;
 HttpServletRequest req = (HttpServletRequest) request;
 String username = (String) req.getSession().getAttribute("username");
 if(req.getRequestURI().contains("Buy") || req.getRequestURI().contains("collect")){
 if(username==null || username.equals("")){ //满足过滤要求，进行处理
 req.getRequestDispatcher("/login.jsp").forward(req, resp);
 return;
 }
 }
 chain.doFilter(req, resp);
 //对相应response进行处理
 }

 @Override
 public void init(FilterConfig filterConfig) throws ServletException {
 //filterConfig，通过filterConfig的getServletContext() 方法可以获得 ServletContext
 }
}

<filter>
 <filter-name>filter</filter-name>
 <filter-class>com.baidu.web.MyFilter</filter-class>
 </filter>
 <filter-mapping>
 <filter-name>filter</filter-name>
 <url-pattern>/*</url-pattern>
 </filter-mapping>

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