如何实现树形目录的递归功能？

<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<!--
数据库结构：
库名：test
表名：tree
CREATE TABLE [dbo].[tree] (
[id] [int] IDENTITY (1, 1) NOT NULL ,
[parentid] [int] NOT NULL ,
[message] [varchar] (50) COLLATE Chinese_PRC_CI_AS NULL
)
为了达到比较好的效果，这里准备了五张小图片
加号：plus.gif
减号：minus.gif
打开的文件夹：openfold.gif
关闭的文件夹：closedfold.gif
白板：white.gif
-->
<%!//方便起见这里就不写成javabean了
class cn {//连接数据库，这里以MS-SQL为例
String jdbcDriver="com.microsoft.jdbc.sqlserver.SQLServerDriver";//jdbc驱动
String connectionString="jdbc:microsoft:sqlserver://localhost:1433;DatabaseName=test";//数据库连接字符串
String user="sa";//数据库用户名
String pass="";//数据库密码
Connection conn=null;
ResultSet rs=null;

public cn() {
try {
Class.forName(jdbcDriver);
} catch(ClassNotFoundException e) {
System.err.println(e.toString());
}
}

public ResultSet executeQuery(String sql) {
rs=null;
try {
conn=DriverManager.getConnection(connectionString,user,pass);
Statement stmt=conn.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
rs=stmt.executeQuery(sql);
}
catch(SQLException e) {
System.err.println(e.toString());
}
return rs;
}

public void executeUpdate(String sql) {
try {
conn=DriverManager.getConnection(connectionString,user,pass);
Statement stmt=conn.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
stmt.executeUpdate(sql);
}
catch(SQLException e) {
System.err.println(e.toString());
}
}
}

class tree {
cn conn=new cn();
public void init(javax.servlet.jsp.JspWriter out,javax.servlet.http.HttpServletRequest request) throws Exception {
out.println("<title>用jsp种树</title>");
dowith(request);
buildTree(out,0,0);//初始调用
}

private void dowith(javax.servlet.http.HttpServletRequest request) {
if(request.getParameter("parentid")==null||request.getParameter("parentid").equals(""))return;
String action=request.getParameter("action");
if(action.equals("add"))
conn.executeUpdate("insert into tree(parentid,message) values('"+request.getParameter("parentid")+"','"+request.getParameter("message")+"')");
else if(action.equals("delete"))
conn.executeUpdate("delete from tree where id="+request.getParameter("parentid")+" or parentid="+request.getParameter("parentid"));
}

public void buildTree(javax.servlet.jsp.JspWriter out,int parentid,int level) throws Exception {
level++;
ResultSet rs=conn.executeQuery("select * from tree where parentid="+parentid+" order by id");
while(rs.next()) {
out.println("<div>");
for(int i=0;i<level-1;i++)
out.print("<img src=/"white.gif/"> ");
if(has_child(rs.getInt("id"))) {
out.print("<img alt=/"展开/" style=/"cursor:hand;/" onclick=/"myClick('"+rs.getInt("id")+"');/" id=/"img"+rs.getInt("id")+"/" src=/"plus.gif/"> <img id=/"im"+rs.getInt("id")+"/" src=/"closedfold.gif/"> ");
out.print("<span onclick=/"myClick1('"+rs.getInt("id")+"');/" style=/"cursor:default;/" id=/"span"+rs.getInt("id")+"/">"+rs.getString("message")+" id="+rs.getInt("id")+"</span>");
out.println("<div style=/"display:none;/" id=/"div"+rs.getInt("id")+"/">");
buildTree(out,rs.getInt("id"),level);//递归调用
out.println("</div>");
} else
out.print("<img src=/"minus.gif/"> <img src=/"openfold.gif/"> <span onclick=/"myClick1('"+rs.getInt("id")+"');/" style=/"cursor:default;/" id=/"span"+rs.getInt("id")+"/">"+rs.getString("message")+" id="+rs.getInt("id")+"</span>");
out.println("</div>");
}
rs.close();
rs=null;
}

private boolean has_child(int parentid) throws Exception {
ResultSet rs=conn.executeQuery("select * from tree where parentid="+parentid+" order by id");
return rs.next();
}

public String getOption() throws Exception {
String option="";
ResultSet rs=conn.executeQuery("select * from tree order by id");
while(rs.next())
option+="<option value=/""+rs.getInt("id")+"/">"+rs.getInt("id")+"</option>/n";
return option;
}
}
%>
<!--以上代码可以写成javabean-->
<script language="JavaScript"><!--这段js为了实现树的展开和关闭的效果-->
<!--
function myClick(id) {
eval("var div=div"+id);
eval("var img=img"+id);
eval("var im=im"+id);
div.style.display=div.style.display!="none"?"none":"block";
img.src=div.style.display!="none"?"minus.gif":"plus.gif";
im.src=div.style.display!="none"?"openfold.gif":"closedfold.gif";
img.alt=div.style.display!="none"?"关闭":"展开";
}
function myClick1(id) {
document.form1.parentid.value=id;
}
//-->
</script>
<table>
<tr><td height="300" valign="top">
<%
tree myTree=new tree();
myTree.init(out,request);
%>
</td></tr>
<tr><td valign="top">
<%
if(myTree.getOption().equals("")) {
%>
<form name="form1" action="" method="get">
parentid:0-表示根节点
message:<input type="text" name="message">
<input type="hidden" name="parentid" value="0">
<input type="submit" name="action" value="add">
</form>
<%} else {%>
<form name="form1" action="" method="get">
parentid:<select name="parentid"><%=myTree.getOption()%></select>
message:<input type="text" name="message">
<input type="submit" name="action" value="add">&nbsp;<input type="submit" name="action" value="delete">
</form>
<%}%>
</td></tr></table>