严重: Servlet.service() for servlet [TEXT_Login] in context with path [/Online_Examination] threw exception
java.lang.NullPointerException
at javabean.LoginJavabean.login(LoginJavabean.java:13)
at servlet.TEXT_Login.doPost(TEXT_Login.java:80)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2441)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2430)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
但是项目报异常：上面的是异常代码

这是连接数据库的java类
package javabean;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Statement;
public class conn {
private Connection conn = null;
private Statement stmt = null;
private ResultSet rs = null;
// ****************建立连接**********************
public conn() {
try {
Class.forName("com.microsoft.jdbc.sqlserver.SQLServerDriver");
} catch (Exception e) {
}
try {
String path = "jdbc:microsoft:sqlserver://127.0.0.1:1433;DatabaseName=user1";
String name = "sa";
String word = "sa";
conn = DriverManager.getConnection(path, name, word);
} catch (Exception e) {
}
}
// ********************查询*********************
public ResultSet query(String sql) {
try {
stmt = conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_UPDATABLE);
rs = stmt.executeQuery(sql);
System.out.println("数据库查询成功!");
return rs;
}
catch (Exception e) {
}
return rs;
}
// ******************添加/修改/删除*************************
public int update(String sql){
try {
stmt = conn.createStatement();
System.out.println("更新成功");
return stmt.executeUpdate(sql);
} catch (Exception e) {
System.out.println(e.getMessage());
}
return 0;
}
public void close() throws Exception{
if(rs != null) rs.close();
if(stmt != null) stmt.close();
if(conn != null) conn.close();
}
}

这是处理post 的方法
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=gb2312");
PrintWriter out = response.getWriter();
request.setCharacterEncoding("gb2312");
String username=request.getParameter("username");
String password=request.getParameter("password");
LoginJavabean aa = new LoginJavabean();//业务处理Bean的对象
String temp=aa.login(username,password);
HttpSession session = request.getSession();
if(temp.equals("1")){
session.setAttribute("user1", username);
response.sendRedirect("main.jsp");
}
else if(temp.equals("2")){
session.setAttribute("user1", username);
response.sendRedirect("main1.jsp");
}
else if(temp.equals("3")){
session.setAttribute("user1", username);
response.sendRedirect("main2.jsp");
}
else{
out.println("<script>");
out.println("alert('对不起，用户名或密码错误！');");
out.println("window.location='login.jsp'");
out.println("</script>");
}
}
/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occure
*/
public void init() throws ServletException {
// Put your code here
}
}

这是登陆的javabean
package javabean;
import java.sql.ResultSet;
import java.sql.SQLException;
public class LoginJavabean {
public String login(String username,String password){
conn nn = new conn();
String temp="";
String sql="select * from Online_Examination where username='"+username+"' and password='"+password+"'";
ResultSet rs=nn.query(sql);
try {
if(rs.next()){
temp=rs.getString("quanxian");
return temp;
}
} catch (SQLException e) {
e.printStackTrace();
}
return temp;
}
}

有没有大神在啊