java过滤器

Myth丶恋晨 2022-10-01 03:47 146阅读 0赞

在开发javaWeb项目的时候会经常使用到过滤器，那过滤器怎么去使用呢？  
写一个实现javax.servlet.Filter接口的类，里面有三个需要去实现的方法—init(初始化)，destroy（销毁），doFilter（执行），写好这个实现类之后在web.xml文件中进行配置。  
代码如下：

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 javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    /** * 编码过滤器 * @author chenjingbin * */
    public class EncodingFilter implements Filter {
    
        public void destroy() {
            // TODO Auto-generated method stub
            System.out.println("销毁");
        }
    
        public void doFilter(ServletRequest req, ServletResponse resp,
                FilterChain chain) throws IOException, ServletException {
            // TODO Auto-generated method stub
            HttpServletRequest request = null;
            HttpServletResponse response = null;
            try {
                request = (HttpServletRequest) req;
                response = (HttpServletResponse) resp;
            } catch (Exception e) {
                // TODO: handle exception
            }
            //修改编码方式
            request.setCharacterEncoding("utf-8");
            response.setContentType("text/html;charset=utf-8");
            //放行
            chain.doFilter(request, response);
        }
    
        public void init(FilterConfig arg0) throws ServletException {
            // TODO Auto-generated method stub
            System.out.println("初始化");
        }
    
    }
    
    //-------------------------------------------------------
    web.xml文件
    <?xml version="1.0" encoding="UTF-8"?>
    <web-app version="2.5" 
        xmlns="http://java.sun.com/xml/ns/javaee" 
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
        xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
      <display-name></display-name> 
      <filter>
            <filter-name>demo</filter-name>
            <filter-class> com.demo.filter.FilterDemo</filter-class>    
            <init-param>
                <param-name>encoding</param-name>
                <param-value>utf-8</param-value>
            </init-param>
        </filter>
        <filter-mapping>
            <filter-name>demo</filter-name>
            <url-pattern>/*</url-pattern> </filter-mapping> <welcome-file-list> <welcome-file>index.jsp</welcome-file> </welcome-file-list> </web-app>

通过上面步骤那就写好过滤器了，不过底层的原理还是需要去了解的。  
在设计模式里面就有一个责任链模式，单纯的责任链模式是比较少见到的，比较多的是不纯的责任链模式。  
直接看UML图那比较清楚一点，下面是UML图：![责任链模式UML][UML]

看了UML图我们比较容易去理解这个模式。然后我们在去看JavaEE里面的文档对Filter的解释——A filter is an object that performs filtering tasks on either the request to a resource (a servlet or static content), or on the response from a resource, or both.  
Filters perform filtering in the doFilter method. Every Filter has access to a FilterConfig object from which it can obtain its initialization parameters, and a reference to the ServletContext which it can use, for example, to load resources needed for filtering tasks.

Filters are configured in the deployment descriptor of a web application. 上面就是对过滤器的功能的解释，里面还有一个FilterChain类放行，FilterConfig配置参数。

[UML]: /images/20220705/7e38f57dba21417084f4d20f915ba312.png