Filter

Chapter 1 basic use of filter

1. Initial filter

• Filter - filter:

• Filter is a component of J2EE Servlet module
• The function of Filter is to block URL s in a unified way
• Filter is usually used for global processing at the application level

• Three elements of filter development:
  *Any filter must implement the javax. Servlet. Fiter interface
  *Write the function code of the Filter in the doFilter() method of the Filter interface
  *Configure the filter in web.xml to indicate the scope of intercepting URL
  

2. Develop the first filter

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
  <display-name>first-filter</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <!-- filter Tags are used to indicate which class is a filter and are loaded automatically when the application starts -->
  <filter>
  	<filter-name>MyFirstFilter</filter-name>
  	<filter-class>com.imooc.filter.MyFirstFilter</filter-class>
  </filter>
  <!-- <filter>
  	<filter-name>CharacterEncodingFilter</filter-name>
  	<filter-class>com.imooc.filter.CharacterEncodingFilter</filter-class>
  	<init-param>
  		<param-name>encoding</param-name>
  		<param-value>GBK</param-value>
  	</init-param>
  	<init-param>
  		<param-name>p1</param-name>
  		<param-value>v1</param-value>
  	</init-param>
  	<init-param>
  		<param-name>p2</param-name>
  		<param-value>v2</param-value>
  	</init-param>
  </filter> -->
  <!-- 
  	filter-mapping Labels are used to describe filter pairsURLThere are two main points of application:
  	1. filter-name Filter name and filter.filter-name Bring into correspondence with
  	2. url-pattern Describe the range of action of the filter,/* For all URL Filtering
   -->
  <filter-mapping>
  	<filter-name>MyFirstFilter</filter-name>
  	<url-pattern>/*</url-pattern>
  </filter-mapping>
  <!-- <filter-mapping>
  	<filter-name>CharacterEncodingFilter</filter-name>
  	<url-pattern>/*</url-pattern>
  </filter-mapping> -->
</web-app>

MyFirstFilter.java
```javascript
package com.imooc.filter;

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;
 //To be a filter, any java class must implement the javax. Servlet. Fiter interface
public class MyFirstFilter implements Filter {

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		System.out.println("Filter destroyed");;
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		// TODO Auto-generated method stub
		System.out.println("Filter in effect");
		chain.doFilter(request, response);
	}

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		// TODO Auto-generated method stub
		System.out.println("Filter initialized successfully");
	}

}