struts开发图书系统问题

javax.servlet.ServletException
    org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:535)
    org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:433)
    org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
    org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
    org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)


root cause

java.lang.NullPointerException
    com.action.Manager.managerLogin(Manager.java:53)
    com.action.Manager.execute(Manager.java:20)
    org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
    org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
    org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
    org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)