Fliter过滤器

骑猪看日落 2023-07-12 05:16 54阅读 0赞

**Filter(过滤器)** 是JavaWeb的三大组件之一，用于在Servlet之外对Request或者Response进行修改。

**1.基本流程:**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pYW5nb3UwOTE1_size_16_color_FFFFFF_t_70]  
**2.实现方式**  
  方法一：实现`javax.servlet.Filter`接口,在web.xml中配置:

<web-app>
    	<display-name>Archetype Created Web Application</display-name>
    	<filter>
    		<filter-name>filter1</filter-name>
    		<filter-class>com.wantao.filter.Filter1</filter-class>
    		<init-param>
    		<param-name>name</param-name>
    		<param-value>Selenium</param-value>
    		</init-param>
    	</filter>
    	<filter>
    		<filter-name>filter2</filter-name>
    		<filter-class>com.wantao.filter.Filter2</filter-class>
    	</filter>
    	<filter-mapping>
    		<filter-name>filter1</filter-name>
    		<url-pattern>/*</url-pattern>
    	</filter-mapping>
    	<filter-mapping>
    		<filter-name>filter2</filter-name>
    		<url-pattern>/*</url-pattern>
    	</filter-mapping>
    </web-app>

方法二：实现`javax.servlet.Filter`接口,在Filter类上使用注解`@WebFilter`(servlet3.0及以上版本才可以)

@Slf4j
    @WebFilter(filterName="filter3",urlPatterns="/*")
    public class Filter3 implements Filter{ 
        @Override
    	public void init(FilterConfig filterConfig) throws ServletException { 
    		log.info("filter3 init");
    	}
    
    	@Override
    	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
    			throws IOException, ServletException { 
    		log.info("filter3 doFilter");
    		chain.doFilter(request, response);}
    
    	@Override
    	public void destroy() { log.info("filter3 destory");}
    
    }

方法三:实现`javax.servlet.Filter`接口,将Filter在配置类中进行注册(SpringBoot)

@Bean
        public FilterRegistrationBean myFilter(){ 
              FilterRegistrationBean filterRegistrationBean=new FilterRegistrationBean();
              filterRegistrationBean.setFilter(new MyFilter());//设置Filter
              filterRegistrationBean.addUrlPatterns("/threadLocal/*");//设置url
            return filterRegistrationBean;
       }

**3.Filter详解**

package com.wantao.filter;
    import java.io.IOException;
    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 lombok.extern.slf4j.Slf4j;
    /** * @author wantao * @date 2019-05-21 5:32:30 PM * @description talk is cheap,show me your code */
    @Slf4j
    public class Filter1 implements Filter { 
        private FilterConfig config;
    	@Override
    	public void init(FilterConfig filterConfig) throws ServletException { 
    		this.config=filterConfig;//在web.xml中的filter中设置好init-param参数，可通过FilterCOnfig对象在init方法中调用
    		log.info("filter1 init");
    		log.info("filter init-param:"+config.getInitParameter("name"));
    	}
    @Override
    	public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)
    			throws IOException, ServletException { 
    		log.info("filter1 doFilter");
    		filterChain.doFilter(request, response);//不调用此方法，则请求会停止
    	}
    @Override
    	public void destroy() { 
    		log.info("filter1 destory");
    		}
    }

1.以Filter1为例子，可以看到它重写了init(),doFilter(),destory()方法  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pYW5nb3UwOTE1_size_16_color_FFFFFF_t_70 1]  
在下图中可以看出来init只被执行了一次,而刷新页面(重新发出请求)则是反复的调用doFilter方法.

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pYW5nb3UwOTE1_size_16_color_FFFFFF_t_70 2]

2.在doFilter方法中，必须调用`filterChain.doFilter(request, response);`,以调用下一个Filter,否则请求会停留在此filter，而无法传递过去,你就无法访问web服务器中的资源.

3 .在web.xml中，filter可以配置一些属性,如:  
    ①设置初始化参数

<init-param>
    		<param-name>name</param-name>
    		<param-value>Selenium</param-value>
    		</init-param>

并通过FilterConfig对象在init()方法中调用

private FilterConfig config;
    	@Override
    	public void init(FilterConfig filterConfig) throws ServletException { 
    		this.config=filterConfig;//在web.xml中的filter中设置好init-param参数，可通过FilterCOnfig对象在init方法中调用
    		log.info("filter1 init");
    		log.info("filter init-param:"+config.getInitParameter("name"));
    	}

②可以在`<filter-mapping></filter-mapping>`中在`<url-pattern>/*</url-pattern>`后面通过`<dispatcher></dispatcher>`设置请求过滤的级别(默认是REQUEST)

REQUEST：表示仅当直接请求Servlet时才生效。
    FORWARD：表示仅当某Servlet通过FORWARD到该Servlet时才生效。
    INCLUDE：JSP中可以通过<jsp:include>标签请求某Servlet或调用RequestDispatcher的include()方法请求某Servlet，仅这种情况下有效。
    ERROR：JSP中可以通过<%@ page errorPage=”error.jsp”>标签指定错误处理页面，仅这种情况下有效。

4.在web.xml中filter按照定义的顺序由上到下执行(所以我们在使用`CharacterEncodingFilter`时必须定义在所有的filter之前)  
在`@WebFilter`中,好像是按照filter name 的顺序执行(看到有人说有时这个顺序也不一定,我建议还是在web.xml中配置好一点),如:filterA比filterB先执行

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