[过滤器filter]

过滤器

Filter是servlet中的特殊类，用于拦截servlet请求并处理

定义一个过滤器的步骤

1.定义一个java类，实现Filter接口

2.重写其中的抽象方法

3.在web.xml配置文件中，配置过滤器拦截请求

例如：exampleFilter.java 

public class exampleFilter implements Filter{
    @Override
	public void init(FilterConfig filterConfig) throws ServletException { } 

    @Override
    public void doFilter
            (ServletRequest request, ServletResponse response, FilterChain chain)  
                    throws IOException,ServletException{
        //使用子接口
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;
        
        response.setContentType("text/html;charset=utf-8");
        request.setCharacterEncoding("utf-8");
        PrintWriter out = response.getWriter();

        if(example!=true){
            //需要被拦截
            //进行拦截操作
        }else{
            chain.doFilter(req,resp);
            //使用了doFilter方法之后才会执行后续的servlet组件
        }        
    }
    
    @Override
    public void destroy(){
    }
}

含有初始化参数的 exampleFilter2.java

public class exampleFilter2{
    @Override
    public void init(FilterConfig filterConfig) throws ServletException{
        this.filterConfig = filterConfig;
    }
    
    @Override
    public void doFilter
            (ServletRequest request, ServletResponse response, FilterChain chain)
                throws IOException,ServletException{
     //获取参数
    filterConfig.getInitParameter("length");
    }

    @Override
    public void destroy(){
    }
}

web.xml配置文件

<filter>
    <filter-name>exampleFilter</filter-name>
    <filter-class>filter.exampleFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>exampleFilter</filter-name>
    <url-pattern>/example</url-pattern>
</filter-mapping>