Java过滤器的基本概念

概述

Java 过滤器是 Java EE (Jakarta EE) 中的一种组件，用于在请求到达 Servlet 或 JSP 之前对其进行预处理，或者在响应返回客户端之前对其进行后处理。过滤器主要应用于以下场景：

请求参数过滤和转换
字符编码处理
身份验证和授权
日志记录
压缩响应数据
缓存控制

核心接口和方法

init(FilterConfig config) - 初始化过滤器
doFilter(ServletRequest request, ServletResponse response, FilterChain chain) - 执行过滤逻辑
destroy() - 销毁过滤器

public class CharacterEncodingFilter implements Filter {@Overridepublic void init(FilterConfig filterConfig) throws ServletException {Filter.super.init(filterConfig);}@Overridepublic void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {}@Overridepublic void destroy() {Filter.super.destroy();}
}

配置方式

过滤器可以通过以下方式配置：

1.注解配置（Servlet 3.0+）

使用@WebFilter注解可以直接在过滤器类上配置过滤器，无需在 web.xml 中配置。别忘了在 Spring Boot 应用主类上添加@ServletComponentScan注解以启用 Servlet 组件扫描：

package com.cni23.filter;import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;/*** urlPatterns 匹配所有请求，/*拦截所有请求，可以填写具体请求路径*/
@WebFilter(urlPatterns = "/*",filterName = "CharacterEncodingFilter")
public class CharacterEncodingFilter implements Filter {private String encoding;/*** 初始化过滤器*/@Overridepublic void init(FilterConfig filterConfig) throws ServletException {String encoding = filterConfig.getInitParameter("encoding");if (encoding == null){encoding = "UTF-8";}}/*** 执行过滤器逻辑*/@Overridepublic void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {//获取请求HttpServletRequest request =(HttpServletRequest) servletRequest;request.setCharacterEncoding(encoding);servletResponse.setCharacterEncoding(encoding);servletResponse.setContentType("text/html;charset=utf-8");//filterChain:链//放行，继续执行下一个过滤器，如果没有过滤器，则执行目标资源filterChain.doFilter(servletRequest,servletResponse);}/*** 销毁过滤器*/@Overridepublic void destroy() {}
}

2.web.xml 配置（传统方式）

<filter><filter-name>CharacterEncodingFilter</filter-name><filter-class>com.cni23.filter.CharacterEncodingFilter</filter-class></filter><filter-mapping><filter-name>CharacterEncodingFilter</filter-name><url-pattern>/*</url-pattern></filter-mapping>

3.Java 代码配置（Spring 等框架中常用，）

1）编写过滤规则

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;
import java.util.Enumeration;public class RequestLoggingFilter implements Filter {@Overridepublic void init(FilterConfig filterConfig) throws ServletException {// 初始化逻辑}@Overridepublic void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {HttpServletRequest httpRequest = (HttpServletRequest) request;// 记录请求信息System.out.println("Request URL: " + httpRequest.getRequestURL());System.out.println("Method: " + httpRequest.getMethod());// 记录请求头Enumeration<String> headerNames = httpRequest.getHeaderNames();while (headerNames.hasMoreElements()) {String headerName = headerNames.nextElement();System.out.println(headerName + ": " + httpRequest.getHeader(headerName));}// 继续请求处理链chain.doFilter(request, response);}@Overridepublic void destroy() {// 清理资源}
}

2）配置我们的过滤器

import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;@Configuration
public class FilterConfig {@Beanpublic FilterRegistrationBean<RequestLoggingFilter> loggingFilter() {FilterRegistrationBean<RequestLoggingFilter> registrationBean = new FilterRegistrationBean<>();// 设置过滤器实例registrationBean.setFilter(new RequestLoggingFilter());// 设置过滤器应用的URL模式registrationBean.addUrlPatterns("/api/*", "/secure/*");// 设置过滤器名称registrationBean.setName("requestLoggingFilter");// 设置过滤器执行顺序（值越小，优先级越高）registrationBean.setOrder(1);// 添加初始化参数registrationBean.addInitParameter("enabled", "true");return registrationBean;}
}