过滤器Filter-蒲公英云

文章目录

- 介绍
- 使用
- Filter生命周期
- Filter的顺序
- Filter的应用场景

介绍

过滤器是用来拦截传入的请求和传出的响应，可以处理客户端和服务端之间传输的数据流。

使用

过滤器和Servlet类似，是java web提供的一个接口，开发者需要自定义一个类并实现该接口即可。

public class MyFilter implements Filter { 
    @Override
    public void init(FilterConfig filterConfig) throws ServletException { 
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { 
        servletRequest.setCharacterEncoding("UTF-8");
        filterChain.doFilter(servletRequest,servletResponse);
    }
    @Override
    public void destroy() { 
    }
}

然后需要在web.xml对服务端的请求映射到filter上：

<filter>
        <filter-name>encoding</filter-name>
        <filter-class>com.julian.filter.MyFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>encoding</filter-name>
        <url-pattern>/login</url-pattern>
        <url-pattern>/test</url-pattern>
    </filter-mapping>

另外我们可以通过注解的方式简化Filter的xml配置：

@WebFilter("/login")
public class MyFilter implements Filter { 
...
}
@WebFilter(urlPatterns={ "/test","/login"}) //1个filter配置多个servlet
public class MyFilter implements Filter { 
...
}

Filter生命周期

当tomcat启动是，通过反射机制调用Filter的无参构造创建实例化对象，同时调用init方法完成初始化，doFilter方法可重复调用，当tomcat关闭的时候，调用destroy方法销毁Filter对象。

无参构造函数：调用一次，当tomcat启动是调用（需要在web.xml里配置）
init()：创建实例化对象后调用一次
doFilter()：调用多次
destroy()：调用一次，tomcat关闭时调用

Filter的顺序

如果配置多个Filter，Filter的调用顺序由web.xml的配置顺序决定的，web.xml按顺序读取，由上到下顺序执行。

<filter>
        <filter-name>encoding</filter-name>
        <filter-class>com.julian.filter.MyFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>encoding</filter-name>
        <url-pattern>/login</url-pattern>
        <url-pattern>/test</url-pattern>
    </filter-mapping>
    <filter>
        <filter-name>test</filter-name>
        <filter-class>com.julian.filter.YourFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>test</filter-name>
        <url-pattern>/login</url-pattern>
    </filter-mapping>

如上面的配置，配置两个Filter映射了login，那么顺序是这样的：
在这里插入图片描述

注意：如果配置多个filter，有严格的执行顺序，那么就不能通过注解的方式进行配置，只能通过xml的方式配置

Filter的应用场景

实际开发中需要用到Filter的场景：

统一处理中文乱码

屏蔽敏感词

package com.julian.filter;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import java.io.IOException;

@WebFilter(“/login”)
public class WordFilter implements Filter {

@Override
public void init(FilterConfig filterConfig) throws ServletException { 
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { 
    servletRequest.setCharacterEncoding("UTF-8");
    String name = servletRequest.getParameter("name");
    name = name.replaceAll("敏感词","***");
    servletRequest.setAttribute("name",name);
    filterChain.doFilter(servletRequest, servletResponse);
}
@Override
public void destroy() { 
}

}

package com.julian.servlet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(“/login”)
public class MyServlet extends HttpServlet {

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
    String name = (String) req.getAttribute("name");
    System.out.println(name);
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
}

}

控制资源的访问权限：
访问download.jsp，如果是登录状态，则进下载页面，如果不是登录状态，返回登录页面

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<form action="/login" method="post">
    用户名：<input type="text" name="name"><br>
    密码：<input type="text" name="password"><br>
    <input type="submit" value="确定">
</form>
</body>
</html>

LoginServlet.java

package com.julian.servlet;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
@WebServlet("/login")
public class LoginServlet extends HttpServlet { 
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
        String name = req.getParameter("name");
        String password = req.getParameter("password");
        if(name.equals("admin")&& password.equals("123"))
        { 
            HttpSession session = req.getSession();
            session.setAttribute("name", name);
            session.setAttribute("password", password);
            resp.sendRedirect("/download.jsp");
        }
    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
        super.doGet(req, resp);
    }
}

DownloadFilter.java

package com.julian.filter;
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
@WebFilter("/download.jsp")
public class DownloadFilter implements Filter { 
    @Override
    public void init(FilterConfig filterConfig) throws ServletException { 
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { 
        HttpServletRequest req = (HttpServletRequest) servletRequest;
        HttpServletResponse resp = (HttpServletResponse) servletResponse;
        HttpSession session = req.getSession();
        String name = (String) session.getAttribute("name");
        if(name == null)
        { 
            resp.sendRedirect("/login.jsp");
        }
        else
        { 
            filterChain.doFilter(servletRequest, servletResponse);
        }
    }
    @Override
    public void destroy() { 
    }
}

download.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<a href="">资源1</a>
<a href="">资源2</a>
<a href="">资源3</a>
</body>
</html>