如何在LDAP Java中返回所有用户的特定属性？

import javax.naming.*;
import javax.naming.directory.*;

public class LDAPExample {
    public static void main(String[] args) {
        // LDAP服务器连接参数
        String ldapUrl = "ldap://ldap.example.com:389";
        String username = "cn=admin,dc=example,dc=com";
        String password = "adminpassword";

        // LDAP查询语句和属性设置
        String baseDn = "dc=example,dc=com";
        String filter = "(objectClass=person)";
        String[] returningAttributes = { "cn" };

        try {
            // 建立与LDAP服务器的连接
            Hashtable<String, String> env = new Hashtable<>();
            env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory");
            env.put(Context.PROVIDER_URL, ldapUrl);
            env.put(Context.SECURITY_AUTHENTICATION, "simple");
            env.put(Context.SECURITY_PRINCIPAL, username);
            env.put(Context.SECURITY_CREDENTIALS, password);
            DirContext ctx = new InitialDirContext(env);

            // 执行LDAP查询
            SearchControls searchControls = new SearchControls();
            searchControls.setSearchScope(SearchControls.SUBTREE_SCOPE);
            searchControls.setReturningAttributes(returningAttributes);
            NamingEnumeration<SearchResult> results = ctx.search(baseDn, filter, searchControls);

            // 遍历结果集
            while (results.hasMore()) {
                SearchResult result = results.next();
                Attributes attributes = result.getAttributes();
                Attribute cnAttribute = attributes.get("cn");

                // 获取特定属性的值
                if (cnAttribute != null) {
                    for (int i = 0; i < cnAttribute.size(); i++) {
                        String cnValue = (String) cnAttribute.get(i);
                        System.out.println("cn: " + cnValue);
                    }
                }
            }

            // 关闭LDAP连接
            ctx.close();
        } catch (NamingException e) {
            e.printStackTrace();
        }
    }
}