16-filter过滤器

怎么都理解不了的东西，只不过是需要点拨而已，愿所有人都找到适合自己的方法

概述

本文主要讲解过滤器，过滤器其实是在servlet前面增加了一层，用来处理统一的请求或者响应的过滤处理
常见应用：

1. web服务器有一些垃圾请求，后台不应该处理，或者应该报错
2. 处理乱码问题

重点

写一个过滤器只要实现javax.servlet包下的Filter接口，重写里面的init,destroy,doFilter几个接口即可
过滤器的初始化是在tomacat启动的时候完成的
销毁是在web服务器关闭的时候完成的
另外过滤器是可以传递的，从一个filter传到其他filter,也必须通过FilterChain传递

实践

Servlet1.java

package com.kuang.servlet;

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

/**
 * 功能描述
 *
 * @since 2022-08-23
 */
public class Servlet1 extends HttpServlet {
    private static final long serialVersionUID = 7727991466501115729L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().write("你好世界");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

CharacterEncodingFilter.java

package com.kuang.filter;

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 java.io.IOException;

/**
 * 功能描述
 *
 * @since 2022-08-23
 */
public class CharacterEncodingFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("过滤器初始化");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
        throws IOException, ServletException {
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        servletResponse.setContentType("text/html;charset=utf-8");
        System.out.println("chain 传递前");
        filterChain.doFilter(servletRequest, servletResponse);
        System.out.println("chain 传递后");
    }

    @Override
    public void destroy() {
        System.out.println("过滤器销毁");
    }
}

<?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_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>s1</servlet-name>
        <servlet-class>com.kuang.servlet.Servlet1</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>s1</servlet-name>
        <url-pattern>/s1</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>s1</servlet-name>
        <url-pattern>/servlet/s1</url-pattern>
    </servlet-mapping>

    <filter>
        <filter-name>f1</filter-name>
        <filter-class>com.kuang.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>f1</filter-name>
        <url-pattern>/servlet/*</url-pattern>
    </filter-mapping>
</web-app>

子pom

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>JavaWeb-Study</artifactId>
        <groupId>org.example</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>08-Filter</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

</project>

父pom

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>JavaWeb-Study</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>
    <modules>
        <module>01-HelloServlet</module>
        <module>02-ServletContext</module>
        <module>03-Download</module>
        <module>04-VerificationCode</module>
        <module>05-Login</module>
        <module>06-Cookie</module>
        <module>07-Session</module>
        <module>08-Filter</module>
    </modules>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/javax.servlet.jsp/javax.servlet.jsp-api -->
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>javax.servlet.jsp-api</artifactId>
            <version>2.3.3</version>
        </dependency>
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>jsp-api</artifactId>
            <version>2.2</version>
        </dependency>
        <dependency>
            <groupId>org.apache.tomcat</groupId>
            <artifactId>jasper</artifactId>
            <version>6.0.53</version>
        </dependency>
    </dependencies>

</project>

标签：ServletException,16,filter,过滤器,import,servlet,javax
来源： https://www.cnblogs.com/Oh-mydream/p/16618571.html