JSP2自定义标签报cannot be resolved to a type问题

学习JSP2自定义标签的内容，没有使用开发工具，编写了标签类HelloWorldTag.java,标签库定义文件mytaglib.tld,以及使用自定义标签的jsp页面HelloWorldTag.jsp。启动tomcat后，访问HelloWorldTag.jsp页面，报HelloWorldTag cannot be resolved to a type错误，但我之前已经使用javac命令手动编译了标签类HelloWorldTag.java,并将生成的class文件放到了WEB-INF/classes路径下了。
代码：①HelloWorldTag.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.SimpleTagSupport;
public class HelloWorldTag extends SimpleTagSupport {
    public void doTag() throws JspException,IOException {
        getJspContext().getOut().write("Hello World"+new java.util.Date());
    }
}

②mytaglib.tld

<?xml version="1.0" encoding="UTF-8" ?>
<taglib xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
    version="2.0">
    <description>A tag library exercising SimpleTag handlers.</description>
    <tlib-version>1.0</tlib-version>
    <short-name>mytaglib</short-name>
    <uri>http://www.jjp.org/mytaglib</uri>
    <tag>
        <name>helloWorld</name>
        <tag-class>HelloWorldTag</tag-class>
        <body-content>empty</body-content>
    </tag>  
</taglib>

③HelloWorldTag.jsp

<%@ page contentType="text/html;charset=GBK" language="java" errorPage="" %>
<%@ taglib uri="http://www.jjp.org/mytaglib" prefix="jjp" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <title>自定义标签示例</title>
    <head>
    <body>
        <h3>以下是自定义标签中的内容</h3>
        <jjp:helloWorld/>
    </body>
</html>