Filter过滤器

Filter：过滤器，用来过滤网站的数据。

处理中文乱码，登录验证等...

代码实现:

目录：

pom.xml:

<!-- servlet依赖 -->
<!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
    <scope>provided</scope>
</dependency>

<!-- jsp依赖 -->
<!-- https://mvnrepository.com/artifact/javax.servlet.jsp/jsp-api -->
<dependency>
    <groupId>javax.servlet.jsp</groupId>
    <artifactId>jsp-api</artifactId>
    <version>2.2</version>
    <scope>provided</scope>
</dependency>

CharacterEncodingFilter.java

package com.allen.filter;

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

public class CharacterEncodingFilter implements Filter {

    //初始化：web服务器启动，就已经初始化了，随时等待过滤对象出现！
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("CharacterEncodingFilter初始化");
    }

    //Chain：链
    /*
    1.过滤中的所有代码，在过滤特定请求的时候都会执行
    2.必须要让过滤器继续执行

    */
    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("CharacterEncodingFilter执行前");

        //让我们的请求继续走，如果不写，程序到这就会被拦截停止
        filterChain.doFilter(servletRequest, servletResponse);

        System.out.println("CharacterEncodingFilter执行后");
    }

    //web服务关闭的时候，过滤器会销毁
    public void destroy() {
        System.out.println("CharacterEncodingFilter销毁");
    }
}

ShowServlet.java

package com.allen.Servlet;

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

public class ShowServlet extends HttpServlet {

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

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

web.xml

<?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>ShowServlet</servlet-name>
        <servlet-class>com.allen.Servlet.ShowServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>ShowServlet</servlet-name>
        <url-pattern>/servlet/ShowServlet</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>ShowServlet</servlet-name>
        <url-pattern>/ShowServlet</url-pattern>
    </servlet-mapping>

    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.allen.filter.CharacterEncodingFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
        <url-pattern>/servlet/*</url-pattern>
    </filter-mapping>
</web-app>

访问/ShowServlet，出现乱码：

访问/servlet/ShowServlet，经过过滤器改变字符编码处理：

如果有多个过滤器，触发顺序按照web.xml配置的顺序，根据chain链式一个个加载运行。如下：