Filter入门介绍

1.Filter作用

拦截客户端对服务端的资源请求,如实现统一字符编码/禁止缓存/用户登录等

2.Filter的实现

开发Web应用时,实现了Servlet API接口中Filter接口的类就称为Filter过滤器类,然后在web.xml中配置和映射过滤器

3.执行过程

Web应用程序启动时,Web服务器负责创建Filter类,然后调用其init()方法进行初始化,准备拦截web.xml配置的过滤器地址1)当请求被拦截时,首先执行相应FIlter类的doFilter()方法2)当doFilter()中的过滤代码执行完成后,执行doFilter方法中chain参数的chain.doFilter()方法,此时服务器判断是否有其他的过滤器,如果有则执行另外的过滤器,如果没有则调用目标资源

4.配置

web.xml中配置Filter的格式如下:

<!--注册Filter--><filter>
    <filter-name>LoginFilter</filter-name>    <filter-class>com.lpq.LoginFilter</filter-class>    <init-param>        <param-name>ignorePath</param-name>        <param-value>/login</param-value>    </init-param>
</filter><!--映射Filter--><filter-mapping>    <filter-name>LoginFilter</filter-name>    <url-pattern>/*</url-pattern></filter-mapping>
也可以在配置文件中配置多个Filter,执行的顺序取决于配置的顺序.服务器会将多个Filter组成一个过滤器链.Filter的doFilter()中有一个FilterChain类型的参数chain,当调用chain.doFilter()方法时,可系统判断过滤器链中是否存在其他的过滤器,如果有则继续执行其doFilter()方法,如果没有则调用目标资源

5.以登录为例

FIlter如下

@WebFilter(filterName = "LoginFilter")public class LoginFilter implements Filter {    private Set<String> prefixIgnores=new HashSet<String>();    public void destroy() {        System.out.println("-----构造器销毁-----");        prefixIgnores=null;    }    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {        HttpServletRequest request=(HttpServletRequest)req;        HttpServletResponse response=(HttpServletResponse)resp;        request.setCharacterEncoding("UTF-8");        response.setCharacterEncoding("UTF-8");        if(canIgnore(request)){            chain.doFilter(request,response); //执行目标资源            return;        }        String user=(String)request.getSession().getAttribute("username");        if(user!=null){            chain.doFilter(req,resp); //执行目标资源        }else{            RequestDispatcher rd=request.getRequestDispatcher("/index.jsp"); //转到用户登录页面            rd.forward(request,response);        }    }    public void init(FilterConfig config) throws ServletException {        System.out.println("-----过滤器初始化------");        //读取不需要过滤的url        String prefix=config.getServletContext().getContextPath();        String ignorePath=config.getInitParameter("ignorePath");        String[] ignoreArray=ignorePath.split(";");        for(String s:ignoreArray){            prefixIgnores.add(prefix+s);        }    }    private boolean canIgnore(HttpServletRequest request){        String requestUrl=request.getRequestURI();        for(String s:prefixIgnores){            if(requestUrl.startsWith(s))                return true;        }        return false;    }}

LoginServlet

public class LoginServlet extends javax.servlet.http.HttpServlet {    protected void doPost(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {        response.setHeader("contentType","text/html;charset=UTF-8");        String user= request.getParameter("username");        String pwd=request.getParameter("password");        if(user.equals("admin") && pwd.equals("123456")){            //success            request.getSession().setAttribute("user",user);            RequestDispatcher rd=request.getRequestDispatcher("/WEB-INF/views/a.jsp");            rd.forward(request,response);        }else{            //fail            RequestDispatcher rd=request.getRequestDispatcher("/WEB-INF/views/b.jsp");            rd.forward(request,response);        }    }    protected void doGet(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {    }}

web配置如下

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"         version="3.1">    <servlet>        <servlet-name>LoginServlet</servlet-name>        <servlet-class>com.lpq.LoginServlet</servlet-class>    </servlet>    <servlet-mapping>        <servlet-name>LoginServlet</servlet-name>        <url-pattern>/login</url-pattern>    </servlet-mapping>    <!--注册Filter-->    <filter>        <filter-name>LoginFilter</filter-name>        <filter-class>com.lpq.LoginFilter</filter-class>        <init-param>            <param-name>ignorePath</param-name>            <param-value>/login</param-value>        </init-param>    </filter>    <!--映射Filter-->    <filter-mapping>        <filter-name>LoginFilter</filter-name>        <url-pattern>/*</url-pattern>    </filter-mapping></web-app>