oracle数据库联接成功，但不能执行SQL语句

本人刚学习ORACLE，请高手指点一下，万分感谢！！！
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class DBUtil {
//oracle联接
private static String user="manager";
private static String password="admin";
private static String DRIVER="oracle.jdbc.driver.OracleDriver";
private static String url="jdbc:oracl:thin:@127.0.0.1:1521:bookshop";
private Connection con=null;
private PreparedStatement ps=null;
private ResultSet rs=null;

public DBUtil(){
con=DBUtil.getConnection();
}

/**
* 取得数据库联接对象
* @return 数据库联接对象
*/
public  static Connection getConnection(){
Connection conn=null;

try {
Class.forName(DRIVER);
conn=DriverManager.getConnection(url, user, password);
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
return conn;
}

/**
* 数据的更新，添加，删除
* @param sql Statement
*/
public void update(String sql){
try {
Statement st=this.con.createStatem