如何在一个url-pattern中使用俩个过滤器

当不同的过滤器之间的url-pattern范围相同时，第一个过滤器执行doFilter拦截后，第二个过滤器再次执行doFilter是就会报空指针异常。

在调用hello页面时需要CharaterEncodingFilter过滤器解决乱码问题，但是调用时会被上一个过滤器拦截，报出空指针异常。

其他过滤器若设置为/*同样会对当前页面拦截，或者无法显示内容，或者报出空指针。

慕粉1712150383 2023-01-06

源自：过滤器 1-1 初识过滤器Filter

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1回答

好帮手慕小蓝 2023-01-07 10:33:32

同学你好，多个过滤器是按照web.xml中配置url-pattern的顺序自上而下执行的，建议同学将字符过滤器配置在最上面。

另外，仅从描述，老师无法确定问题的原因，建议同学重构一下代码，放弃业务逻辑，仅能演示错误即可。

然后将代码提供到问答区，这样老师可以测试代码，更效率的解决同学的疑问。

祝学习愉快~

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提问者慕粉1712150383 #1

按照课程代码，我的项目在运行后有时500空指针，有时200但无内容显示

<?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">
    <filter>
        <filter-name>firstFilter</filter-name>
        <filter-class>com.imooc.filter.FirstFilter</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>UTF-8</param-value>
        </init-param>
    </filter>
    <filter>
        <filter-name>urlPatternFilter</filter-name>
        <filter-class>com.imooc.filter.UrlPatternFilter</filter-class>
    </filter>
    <filter>
        <filter-name>filterA</filter-name>
        <filter-class>com.imooc.filter.chain.FilterA</filter-class>
    </filter>
    <filter>
        <filter-name>filterB</filter-name>
        <filter-class>com.imooc.filter.chain.FilterB</filter-class>
    </filter>
    <filter>
        <filter-name>filterC</filter-name>
        <filter-class>com.imooc.filter.chain.FilterC</filter-class>
    </filter>

    <filter>
        <filter-name>deviceAdapterFilter</filter-name>
        <filter-class>com.imooc.filter.DeviceAdapterFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>firstFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>characterEncodingFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>urlPatternFilter</filter-name>
        <url-pattern>/v1/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>filterA</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>filterB</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>filterC</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>deviceAdapterFilter</filter-name>
        <url-pattern>*.html</url-pattern>
    </filter-mapping>
    <listener>
        <listener-class>com.imooc.WebListener</listener-class>
    </listener>
</web-app>

package com.imooc.filter;

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

public class FirstFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("FirstFilter已被初始化");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("firstFilter过滤器已生效");
        //将请求响应向后传递，被后续的filter或servlet进行处理
        chain.doFilter(request,response);
    }

    @Override
    public void destroy() {
        System.out.println("FirstFilter已被销毁");
    }
}

package com.imooc.filter;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class CharacterEncodingFilter implements Filter {
    private String encoding;
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String encoding = filterConfig.getInitParameter("encoding");
        System.out.println("Encoding:" + encoding);
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;
        req.setCharacterEncoding(encoding);
        resp.setContentType("text/html;charset=" + encoding);;
        chain.doFilter(req,resp);
    }

    @Override
    public void destroy() {

    }
}

package com.imooc.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("/v1/hello")
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().println("<h1>你好 世界</h1>");
    }
}

HTTP状态 500 - 内部服务器错误
类型 异常报告
描述 服务器遇到一个意外的情况，阻止它完成请求。
例外情况
java.lang.NullPointerException
	org.apache.tomcat.util.buf.B2CConverter.getCharset(B2CConverter.java:60)
	org.apache.catalina.connector.Request.setCharacterEncoding(Request.java:1705)
	org.apache.catalina.connector.RequestFacade.setCharacterEncoding(RequestFacade.java:329)
	com.imooc.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:20)
	com.imooc.filter.FirstFilter.doFilter(FirstFilter.java:16)
):注意 主要问题的全部 stack 信息可以在 server logs 里查看
Apache Tomcat/8.5.66

2023-01-07 10:45:16