JDBC:从数据库中取数据的一个bug

时间：2021-04-09 18:13:34

先看错误信息：

 java.sql.SQLException: Before start of result set

     at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:910)

     at com.mysql.jdbc.ResultSet.checkRowPos(ResultSet.java:720)

     at com.mysql.jdbc.ResultSet.getStringInternal(ResultSet.java:5624)

     at com.mysql.jdbc.ResultSet.getString(ResultSet.java:5544)

     at hw5servlet.doGet(hw5servlet.java:38)

     at hw5servlet.doPost(hw5servlet.java:62)

     at javax.servlet.http.HttpServlet.service(HttpServlet.java:643)

     at javax.servlet.http.HttpServlet.service(HttpServlet.java:723)

     at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)

     at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)

     at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)

     at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)

     at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)

     at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)

     at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)

     at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)

     at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)

     at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:617)

     at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1760)

     at java.lang.Thread.run(Thread.java:745)

再看编写的JDBC查看代码（如下）：

 Class.forName("com.mysql.jdbc.Driver");

             Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/day36","root","root");

             String insertString="select password from register where email="+email+";";

             //String insertString="insert into  register values(?,?,?)";

             //System.out.println("hw5servlet.doGet()"+insertString);

             Statement ps = connection.createStatement();

             boolean execute = ps.execute(insertString);

             //ps.executeQuery(insertString);

             if(execute){

                 ResultSet rs = ps.getResultSet();

                 //rs.beforeFirst();

                 //rs.next();
　　　　　　　　　　　　//本以为从数据库中去代码只有一行，默认应该是在第一行的，然后直接rs.getString(1)就可以。
                  //但事实不然，getString()时一定要有rs.next().

                 String password1 = rs.getString(1);

                 System.out.println("hw5servlet.doGet()"+password1);

                 if(password.equals(password1)){

                     response.getWriter().write("登陆成功");

                 }else {

                     response.getWriter().write("密码错误，请重新登录");

                     response.sendRedirect("/Day36_JDBC_hw4/index.jsp");

                 }

             }

找了许久没找到，然后百度：http://blog.csdn.net/killua_hzl/article/details/6073618

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