ResultSet.getString（1）throws java.sql.SQLException：在当前光标位置的无效操作

当我运行下面的servlet时：

// package projectcodes; public void doPost(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException { String UserID = request.getParameter("UserID"); String UserPassword = request.getParameter("UserPassword"); String userName = null; String Email = null; Encrypter encrypter = new Encrypter(); String hashedPassword = null; try { hashedPassword = encrypter.hashPassword(UserPassword); Context context = new InitialContext(); DataSource ds = (DataSource)context.lookup("java:comp/env/jdbc/photog"); Connection connection = ds.getConnection(); String sqlStatement = "SELECT email,firstname FROM registrationinformation WHERE password='" + hashedPassword + "'"; PreparedStatement statement = connection.prepareStatement(sqlStatement); ResultSet set = statement.executeQuery(); userName = set.getString(1); // <<---------- Line number 28 response.sendRedirect("portfolio_one.jsp"); // userName = set.getString("FirstName"); Email = set.getString(3); if(set.wasNull() || Email.compareTo(UserID) != 0) { // turn to the error page response.sendRedirect("LoginFailure.jsp"); } else { // start the session and take to his homepage HttpSession session = request.getSession(); session.setAttribute("UserName", userName); session.setMaxInactiveInterval(900); // If the request doesn't come withing 900 seconds the server will invalidate the session RequestDispatcher rd = request.getRequestDispatcher("portfolio_one.jsp"); rd.forward(request, response); // forward to the user home-page } }catch(Exception exc) { System.out.println(exc); } 

我得到以下例外：

 INFO: java.sql.SQLException: Invalid operation at current cursor position. at org.apache.derby.client.am.SQLExceptionFactory40.getSQLException(Unknown Source) at org.apache.derby.client.am.SqlException.getSQLException(Unknown Source) at org.apache.derby.client.am.ResultSet.getString(Unknown Source) at com.sun.gjc.spi.base.ResultSetWrapper.getString(ResultSetWrapper.java:155) -----> at projectcodes.ValidateDataForSignIn.doPost(ValidateDataForSignIn.java:28 at javax.servlet.http.HttpServlet.service(HttpServlet.java:754) at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1539) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175) at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595) at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:98) at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:91) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:162) at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:330) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231) at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:174) at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:828) at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:725) at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1019) at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225) at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137) at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104) at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90) at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79) at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54) at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59) at com.sun.grizzly.ContextTask.run(ContextTask.java:71) at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532) at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513) at java.lang.Thread.run(Thread.java:722) Caused by: org.apache.derby.client.am.SqlException: Invalid operation at current cursor position. at org.apache.derby.client.am.ResultSet.checkForValidCursorPosition(Unknown Source) at org.apache.derby.client.am.ResultSet.checkGetterPreconditions(Unknown Source) ... 30 more 

从服务器上面的日志显示行号28是exception的原因。 但我无法得到例外的理由。 表中的所有列都有一个varchar数据types。

我在servlet代码中突出显示了行号28 （根据服务器日志的exception原因） 。

Answer 1

你应该先使用next语句。

 ResultSet set = statement.executeQuery(); if (set.next()) { userName = set.getString(1); //your logic... } 

UPDATE

正如Java 6文档所述

结果集游标最初位于第一行之前; 下一个方法的第一个调用使第一行成为当前行; 第二次调用使第二行成为当前行，依此类推。

这意味着当你执行这个句子

 ResultSet set = statement.executeQuery(); 

ResultSet set将被创build并指向数据的第一个结果之前的一行。 你可以这样看：

select电子邮件，名字从注册信息

  email | firstname ____________________________________ 0 <= set points to here 1 [e-mail] | Email1 Person 2 [e-mail] | Foo Bar 

所以，在打开你的ResulSet之后，你执行next方法把它移到第一行。

 if(set.next()) 

现在看起来像这样。

  email | firstname ____________________________________ 0 1 [e-mail] | Email1 Person <= set points to here 2 [e-mail] | Foo Bar 

如果您需要读取ResultSet中的所有数据，则应该使用while而不是if：

 while(set.next()) { //read data from the actual row //automatically will try to forward 1 row } 

如果set.next()返回false，则意味着没有行可读，所以while循环将会结束。

更多信息在这里 。

Answer 2

您必须将指针设置到正确的位置：

 while(set.hasNext()){ set.next(); String a = set.getString(1); String b = set.getString(2); } 

Answer 3

ResultSet set = statement.executeQuery（）;

迭代集合，然后得到string。

  while(set.next()) { me = set.getString(1); // <<---------- Line number 28 }