关于JSP中select option的值传递，然后通过判断查询不同的表来实现登录的问题

Question

JSP<select name="status">
   <option value=""></option>
         <option value="teacher">老师</option>
         <option value="student">学生</option>
         </select>dao.java部分代码
if(status=="student"){
    sql = "select * from student where state='1' and student_password='"
+ password + "'and student_id='" + username + "'";
}else if(status=="teacher"){
sql = "select * from manager where state='1' and man_password='"
+ password + "'and man_id='" + username + "'";
}
rs = stmt.executeQuery(sql);调试后出现的问题是登录不成功myeclipse提示
java.sql.SQLException: [Microsoft][SQLServer JDBC Driver]Invalid SQL specified.
at com.microsoft.jdbc.base.BaseExceptions.getException(Unknown Source)
at com.microsoft.jdbc.base.BaseStatement.commonValidateSQL(Unknown Source)
at com.microsoft.jdbc.base.BaseStatement.preProcessSQL(Unknown Source)
at com.microsoft.jdbc.base.BaseStatement.executeQueryInternal(Unknown Source)
at com.microsoft.jdbc.base.BaseStatement.executeQuery(Unknown Source)
at cn.hmj.dao.Dao.login(Dao.java:35)
at cn.hmj.servlet.Servlet.doPost(Servlet.java:45)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:763)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
java.lang.NullPointerException
at cn.hmj.dao.Dao.login(Dao.java:45)
at cn.hmj.servlet.Servlet.doPost(Servlet.java:45)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:763)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
我想应该是值没有传递成功，导致判断失败查询数据库失败。小弟是菜鸟，不知道上面写的是不是正确的。请各位老大指点，分不多，请不吝赐教。

Answer 1

你把这后台得到的status打印出来看下，有没有传过来？

Answer 2

问题如下：
1、String是Object型的，在if条件中，应该使用equals，如："student".equals(status);
2、在java中，做equals比较时应该把常量放在前面，避免变量为null时出现异常；
3、当status不是"student"也不是"teacher"时，sql没有付值，导致stmt.executeQuery(sql);出现异常