首先是建立一個filter.java檔案，注意implements的是Filter的servlet的那個包：

public class filter11 implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }//開啟web伺服器，開始初始化

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws
 
 IOException, ServletException {
        //解決中文亂碼問題
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        servletResponse.setContentType("text/html;charset=utf-8");

        System.out.println("過濾器執行前：");
        filterChain.doFilter(servletRequest,servletResponse);
 
//為了讓程式繼續執行，需要新增這句話，否則就會終止
        System.out.println("過濾器執行後：");
    }

    @Override
    public void destroy() {
        System.out.println("過濾器銷燬！");
    }//關閉web伺服器，銷燬
}

然後再web.xml檔案中進行註冊：

<filter>
    <filter-name>filter11</filter-name>
    <filter-class>filter11</filter-class
 
>
  </filter>
  <filter-mapping>
    <filter-name>filter11</filter-name>
    <url-pattern>/servlet/*</url-pattern>
  </filter-mapping>

然後裡面的路徑：<url-pattern>/servlet/*</url-pattern> 表示在servlet下的java檔案都可以經過這個過濾器，嗯，方便了一丟丟。避免重複的寫utf-8這幾句話了。