创建HttpFilter与理解多个Filter代码的执行顺序

1.自定义的HttpFilter，实现Filter接口

HttpFilter

package com.aff.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;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

//自定义的HttpFilter，实现Filter接口
public abstract class HttpFilter implements Filter {
    /**
     * 用于保存FilterConfig 对象
     */

    private FilterConfig filterConfig;

    // 不建议子类直接覆盖，若直接覆盖将导致 FilterConfig 成员变量初始化失败
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
        init();
    }

    // 供子类直接继承的初始化方法，可以通过 getFilterConfig() 获取 FilterConfig 对象
    protected void init() {
        // 直接返回init(ServletConfig) 的FilterConfig 对象
    }

    public FilterConfig getFilterConfig() {
        return filterConfig;
    }

    /*
     * 原生的doFilter 方法， 在方法内部把ServletRequest 和ServletResponse 转为了
     * HttpServletRequest 和 HttpServletResponse， 并调用了
     * dofilter(HttpServletRequest request, HttpServletResponse response,FilterChain chain)
     *
     * 若编写 Filter 的过滤发不能发 不见一丝直接继承该方法，
     * 而建议继承 doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)
     */
    @Override
    public void doFilter(ServletRequest req, ServletResponse reqs, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) reqs;

        doFilter(request, response, chain);
    }

    // 抽象方法，为http请求指定，必须实现的方法
    public abstract void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)
            throws IOException, ServletException;

    // 空的destroy()
    @Override
    public void destroy() {
    }
}

2.理解多个Filter代码的执行顺序

HelloFilter

package com.aff.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;

public class HelloFilter implements Filter {

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        System.out.println("init");

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        System.out.println("1. Before  HelloFilter's  chain.doFilter... ");

        chain.doFilter(request, response);

        System.out.println(" 2.After  HelloFilter's  chain.doFilter... ");
    }

    @Override
    public void destroy() {
        System.out.println("destroy");

    }

}


SecondFilter

package com.aff.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;

public class SecondFilter implements Filter {

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        System.out.println("seond init。。。");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        System.out.println(" 3.Before  SecondFilter's  chain.doFilter... ");

        // 放行
        chain.doFilter(request, response);
        system.out.println();
        System.out.println(" 4.After  SecondFilter's  chain.doFilter... ");
    }

    @Override
    public void destroy() {
        System.out.println("second   destroy。。。");

    }
}

test.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
    <h3>Test Page</h3>
    <%
        System.out.print("5.Test.jsp");
    %>
</body>
</html>

以上的代码执行顺序为

1. Before  HelloFilter's  chain.doFilter... 
 3.Before  SecondFilter's  chain.doFilter... 
5.Test.jsp
 4.After  SecondFilter's  chain.doFilter... 
 2.After  HelloFilter's  chain.doFilter...

All that work will definitely pay off