连接oracle数据库，出现异常：

连接oracle数据库，出现异常：java.sql.SQLException:   Io   异常:   Connection   refused(DESCRIPTION=(TMP=)(VSNNUM=153092352)(ERR=12504)(ERROR_STACK=(ERROR=(CODE=12504)(EMFI=4))))     
  请问是什么出现的问题？谢谢 
import java.sql.*;
public class GetResultExample1{
    public static void main(String args[]){
try{
    new oracle.jdbc.driver.OracleDriver();
    String url="jdbc:oracle:thin:@localhost:1521:o9i";
    Connection conn=DriverManager.getConnection(url,"scott","tiger");
    Statement stmt=conn.createStatement();
    //执行查询语句
    ResultSet rs=stmt.executeQuery(
"select * from student order by student_id");
    while(rs.next()){
     System.out.println("----------------------------------");
System.out.println("student id: "+rs.getString(1)
+"      student name: "+rs.getString(2));
    }
stmt.close();
conn.close();
}catch(SQLException e){
    e.printStackTrace();
}
    }
}