Filter

Filter开发步骤：
1.导包

2.编写过滤器
实现Filter接口，实现对应的方法：

public class CharacterEncodingFilter implements Filter {
    @Override
    //web服务器启动就已经初始化了，随时等待监听
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("CharacterEncodingFilter初始化");
    }

    @Override
    //Chain：链
    /*
    1.过滤中的所有代码，在过滤特定的请求时候都会执行
    2.必须要让过滤器继续同行
    filterChain.doFilter(servletRequest,servletResponse);
     */
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletResponse.setContentType("text/html;charset=UTF-8");
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        System.out.println("CharacterEncodingFilter执行前......");

        filterChain.doFilter(servletRequest,servletResponse);//让我们的请求继续走，如果不写，程序到这就停止

        System.out.println("CharacterEncodingFilter执行后......");
    }

    @Override
    //web服务器关闭的时候，过滤器会销毁
    public void destroy() {
        System.out.println("CharacterEncodingFilter已销毁");
    }
}

3. 在web.xml中配置Filter

<filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.dai.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
<!--        只要是/servlet下面的任何请求都会经过这个过滤器-->
        <url-pattern>/servlet/*</url-pattern>
    </filter-mapping>