各位大神，我在myeclipse里编写服务器，报如下错误：
url = select * from user where username = 'null'
stmt = com.mysql.jdbc.Statement@39c51b0e
java.sql.SQLException: Illegal operation on empty result set.
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:910)
at com.mysql.jdbc.ResultSet.checkRowPos(ResultSet.java:685)
at com.mysql.jdbc.ResultSet.getStringInternal(ResultSet.java:5476)
at com.mysql.jdbc.ResultSet.getString(ResultSet.java:5420)
at com.mysql.jdbc.ResultSet.getString(ResultSet.java:5436)
at com.test.servlet.loadMessage.doLogin(loadMessage.java:81)
at com.test.servlet.loadMessage.doGet(loadMessage.java:61)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
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.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
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)
result = {"result_code":2}
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
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:2442)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2431)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)
菜鸟刚开始学，求各位大神指点啊啊啊
错误代码可能在这里，但是没看出来哪里错了。。。。
String doLogin(String username,
String password) {
HashMap<String, Object> resultMap = new HashMap<String, Object>();
String sql = "select * from " + DBManager.TABLE_NAME + " where " + DBManager.COLUMN_USERNAME + " = " + "'" + username + "'" ;
System.out.println("url = " + sql);
。。。。。。
}
username这个变量是个参数啊，它的使用在这里：感觉也没错啊：
String username = request.getParameter("username");
String password = request.getParameter("password");
String result = doLogin(username, password);
不懂，求教！！！
