Filter（过滤器）

Filter（过滤器）

Filter

功能：

1.用来拦截传入和传出的响应。

2.修改或以某种方式处理正在客户端和服务端之间交换的数据流。

使用方法：

与使用Servlet相似，Filter是JavaWeb提供的一个接口，开发者只要自定义一个类并且实现该接口即可。

注意事项：

1.接口实现时一定要引用 Filter(Javax.servlet)

2.doFilter()方法处理完业务逻辑之后，必须使用 filterChain对象调用 doFilter()方法，将请求/响应向后边传递，否则该请求/响应会一直滞留在过滤器中。

package com.wildfire.servlet.Filter;

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

/*一定要引用 Filter(javax.servlet)*/
public class CharacterFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("UTF-8");
        /*doFilter方法中处理完业务逻辑之后，必须添加filterChain.doFilter(servletRequest,servletResponse);
        否则请求/响应无法向后传递，会一直停留在过滤器中*/
        servletResponse.setCharacterEncoding("UTF-8");
        filterChain.doFilter(servletRequest,servletResponse);
    }
}

在web.xml中配置文件

<filter>
    <filter-name>CharacterFilter</filter-name>
    <filter-class>com.wildfire.servlet.Filter.CharacterFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>CharacterFilter</filter-name>
    <url-pattern>/Filter1Servlet</url-pattern>
    <url-pattern>/Filter2Servlet</url-pattern>
</filter-mapping>

Filter的生命周期

当Tomcat启动是，通过反射机制调用Filter的无参构造函数实例化对象，同时调用init方法来实现初始化，doFilter方法会执行多次，当Tomcat服务关闭时，调用destory来销毁Filter对象。

⽆参构造函数：只调⽤⼀次，当 Tomcat 启动时调⽤（Filter ⼀定要进⾏配置，否则不会加载）。

init ()⽅法：只调⽤⼀次，Tomcat启动后，当 Filter 的实例化对象创建完成之后调⽤。

doFilter()：调⽤多次，访问 Filter 的业务逻辑都写在 Filter 中。

destory()：只调⽤⼀次，Tomcat 关闭时调⽤。

同时配置多个Filter，Filter的调用顺序是由web.xml中的配置顺序来决定的，配置写在前面的先调用，配置写在后面的后调用，因为web.xml是从上到下顺序读取的。

<filter>
    <filter-name>CharacterFilter</filter-name>
    <filter-class>com.wildfire.servlet.Filter.CharacterFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>CharacterFilter</filter-name>
    <url-pattern>/Filter1Servlet</url-pattern>
    <url-pattern>/Filter2Servlet</url-pattern>
</filter-mapping>

<filter>
    <filter-name>CharacterFilter2</filter-name>
    <filter-class>com.wildfire.servlet.Filter.CharacterFilter2</filter-class>
</filter>

<filter-mapping>
    <filter-name>CharacterFilter2</filter-name>
    <url-pattern>/Filter1Servlet</url-pattern>
    <url-pattern>/Filter2Servlet</url-pattern>
</filter-mapping>

也可以通过注解的方式来简化过滤器的配置，但是使用注解的方法无法设定过滤器的执行顺序。

@WebServlet("/CharacterFilter2")
public class CharacterFilter2 implements Filter {
  
}

以上代码等同于在web.xml中配置，在web.xml中配置文件的代码如下：

<filter>
    <filter-name>CharacterFilter2</filter-name>
    <filter-class>com.wildfire.servlet.Filter.CharacterFilter2</filter-class>
</filter>

<filter-mapping>
    <filter-name>CharacterFilter2</filter-name>
    <url-pattern>/Filter1Servlet</url-pattern>
    <url-pattern>/Filter2Servlet</url-pattern>
</filter-mapping>

实际开发中用到Filter的场景：

1.统一处理中文乱码。

2.屏蔽敏感词

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Filter3</title>
</head>
<body>
    <form action="/WordServlet" method="get">
        文字：<input type="text" name="text">
        <input type="submit" value="提交">
    </form>
</body>
</html>

package com.wildfire.servlet.Filter;

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

@WebFilter("/WordServlet")
public class WordFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("UTF-8");
        String text=servletRequest.getParameter("text");
        /*这是个简单的例子，并没有词库，就单纯的将输入的 敏感词 三个字，替换为*** */
        text=text.replaceAll("敏感词","***");
        /*上面text我们取出了，并对其进行了修改，但是Parameter中的text并没有修改，如果继续往下传递，传递的依然是未进行修改的text
        同时系统没有提供setParameter()方法，可以用setAttribute() 将修改后的值传递下去*/
        servletRequest.setAttribute("text",text);
        filterChain.doFilter(servletRequest,servletResponse);
    }
}

package com.wildfire.servlet.Filter;

import com.wildfire.servlet.HttpServlet;

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

@WebServlet("/WordServlet")
public class WordServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       /* 使用setAttribute()方法存的，就要用对应的方法取出来*/
        String text=(String)req.getAttribute("text");
        String text1=req.getParameter("text");
        System.out.println("屏蔽前的文字:"+text1);
        System.out.println("屏蔽后的文字:"+text);
    }
}

3.控制资源访问权限

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>下载页面</title>
</head>
<body>
    <h1>下载</h1>
    <a href="">资源一</a>
    <a href="">资源二</a>
    <a href="">资源三</a>

</body>
</html>

package com.wildfire.servlet.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("/Filter4_downloading.jsp")
public class DowdingFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request=(HttpServletRequest)servletRequest;
        HttpServletResponse response=(HttpServletResponse)servletResponse;
        HttpSession session =request.getSession();
        String name=(String)session.getAttribute("name");
        if(name==null){
            //不是登录状态，不允许下载，跳转到登录界面
            response.sendRedirect("Filter5_login.jsp");
        }else{
            filterChain.doFilter(servletRequest, servletResponse);
        }
    }
}

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <h1>登录</h1>
    <form action="/DownloadingServlet" method="post">
        账号：<input type="text" name="name"><br/>
        密码：<input type="password" name="password"><br/>
        <input type="submit" value="提交">
    </form>
</body>
</html>

package com.wildfire.servlet.Filter;

import com.wildfire.servlet.HttpServlet;

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

@WebServlet("/DownloadingServlet")
public class DownLoadingServlet 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);
            resp.sendRedirect("Filter4_downloading.jsp");
        }
    }
}

package com.wildfire.servlet.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("/Filter4_downloading.jsp")
public class DowdingFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request=(HttpServletRequest)servletRequest;
        HttpServletResponse response=(HttpServletResponse)servletResponse;
        HttpSession session =request.getSession();
        String name=(String)session.getAttribute("name");
        if(name==null){
            //不是登录状态，不允许下载，跳转到登录界面
            response.sendRedirect("Filter5_login.jsp");
        }else{
            filterChain.doFilter(servletRequest, servletResponse);
        }
    }
}

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>下载页面</title>
</head>
<body>
    <h1>下载</h1>
    <a href="">资源一</a>
    <a href="">资源二</a>
    <a href="">资源三</a>

</body>
</html>
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原文地址：https://www.cnblogs.com/wind-and-sky/p/13961094.html