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Servlet中的过滤器Filter是实现了javax.servlet.Filter接口的服务器端程序，主要的用途是过滤字符编码、做一些业务逻辑判断等。其工作原理是，只要你在web.xml文件配置好要拦截的客户端请求，它都会帮你拦截到请求，此时你就可以对请求或响应(Request、Response)统一设置编码，简化操作；同时还可进行逻辑判断，如用户是否已经登陆、有没有权限访问该页面等等工作。它是随你的web应用启动而启动的，只初始化一次，以后就可以拦截相关请求，只有当你的web应用停止或重新部署的时候才销毁

package ...;   
import ...;   
// 主要目的：过滤字符编码；其次，做一些应用逻辑判断等.   
// Filter跟web应用一起启动   
// 当web应用重新启动或销毁时，Filter也被销毁   
public class MyCharsetFilter implements Filter {   
     private FilterConfig config = null;   
     public void destroy() {   
         System.out.println("MyCharsetFilter准备销毁...");   
     }   
    
     public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain chain) throws IOException, ServletException {   
         // 强制类型转换   
         HttpServletRequest request = (HttpServletRequest)arg0;   
         HttpServletResponse response = (HttpServletResponse)arg1;   
         // 获取web.xm设置的编码集，设置到Request、Response中         
         request.setCharacterEncoding(config.getInitParameter("charset"));          
         response.setContentType(config.getInitParameter("contentType"));          
         response.setCharacterEncoding(config.getInitParameter("charset"));            
        // 将请求转发到目的地   
         chain.doFilter(request, response);   
     }   
    
     public void init(FilterConfig arg0) throws ServletException {   
         this.config = arg0;   
         System.out.println("MyCharsetFilter初始化...");   
     }   
 }

<filter>   
      <filter-name>filter</filter-name>   
      <filter-class>dc.gz.filters.MyCharsetFilter</filter-class>   
      <init-param>   
          <param-name>charset</param-name>   
          <param-value>UTF-8</param-value>   
      </init-param>   
      <init-param>   
          <param-name>contentType</param-name>   
          <param-value>text/html;charset=UTF-8</param-value>   
      </init-param>   
  </filter>   
  <filter-mapping>   
      <filter-name>filter</filter-name>   
      <!-- * 代表截获所有的请求  或指定请求/test.do  /xxx.do -->   
      <url-pattern>/*</url-pattern>   
  </filter-mapping>

在spring boot中加入filter 

@Component
@WebFilter(urlPatterns = "/webapi/*", filterName = "authFilter")
public class AuthFilter implements Filter {
    ......
}

 或者 

@Bean    
public FilterRegistrationBean testFilterRegistration() {        
    FilterRegistrationBean registration = new FilterRegistrationBean(new TestFilter());     
    registration.addUrlPatterns("/webapi/*"); //        
    registration.addInitParameter("paramName", "paramValue"); //        
    registration.setName("testFilter");     
    return registration;    
}