报错：java.sql.SQLException: Listener refused the connection with the following error:
ORA-12505, TNS:listener does not currently know of SID given in connect descriptor
（有两个类，一个是登录类，一个是登录的处理类）
主要代码：（登录类）
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html; charset=utf-8");
PrintWriter out=response.getWriter();
out.print("

welcome

");
out.print("
" +
"ID:
" +
"Password:
" +
"
" +
"");

}

处理类的代码：
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub

request.setCharacterEncoding("utf-8");
response.getWriter().print("aaaa");
String id=request.getParameter("id");
String password=request.getParameter("password");

Connection ct=null;
ResultSet rs=null;
PreparedStatement ps=null;
try {
Class.forName("oracle.jdbc.driver.OracleDriver");
ct=DriverManager.getConnection("jdbc:oracle:thin:@127.0.0.1:1521:merlin","SCOTT","tiger");
ps=ct.prepareStatement("selcet * from U where id=? and password=?");
ps.setObject(1, id);
ps.setObject(2, password);
rs=ps.executeQuery();
if(rs.next())
{
request.getRequestDispatcher("/MainFrame").forward(request, response);
}else{
request.getRequestDispatcher("/Login").forward(request, response);
}

} catch (Exception e) {
e.printStackTrace();
// TODO: handle exception
}

}
网上有改listener.ora的说法，我改了还是不行啊

listener.ora Network Configuration File: D:\app\Merlin\product\11.2.0\dbhome_1\network\admin\listener.ora

Generated by Oracle configuration tools.

SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(SID_NAME = CLRExtProc)
(ORACLE_HOME = D:\app\Merlin\product\11.2.0\dbhome_1)
(PROGRAM = extproc)
(ENVS = "EXTPROC_DLLS=ONLY:D:\app\Merlin\product\11.2.0\dbhome_1\bin\oraclr11.dll")
)
)
(SID_DESC =
(GLOBAL_DBNAME = ORCL)
(ORACLE_HOME = D:\app\Merlin\product\11.2.0\dbhome_1)
(SID_NAME = ORCL)
)
)
LISTENER =
(DESCRIPTION_LIST =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
(ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
)
)

ADR_BASE_LISTENER = D:\app\Merlin

oracle账户的问题或者配置的文件，拒绝连接。

http://blog.csdn.net/shen_xiao_wei/article/details/5729463

把"jdbc:oracle:thin:@127.0.0.1:1521:merlin","SCOTT","tiger"里127.0.0.1改成localhost或者自己的IP试下

ORA-12505 是tns配置错误，你可以看一下

ct=DriverManager.getConnection("jdbc:oracle:thin:@127.0.0.1:1521:merlin","SCOTT","tiger");

怀疑你的实例应该是orcl，试试这个

ct=DriverManager.getConnection("jdbc:oracle:thin:@127.0.0.1:1521:orcl","SCOTT","tiger");