Error 500: Internal Servlet Error

Question

I am getting the following error message testing the deployment of a mult-tiered applications, which currently has two EJBs and one web component. I am using container managed persistance and I have checked the SQL for the finder methods etc in the delpoyment settings. I am passing a username and password from a static HTML page to a servlet which invokes a finder method of a EJB which it looks up. The finder method is findByLogin( String username, password ). It's SQL implementation with the deployer is - SELECT username FROM warehouse_clerk WHERE username = ?1 AND password = ?2. From the database the table is defined as: CREATE TABLE warehouse_clerk (username VARCHAR(50) , password VARCHAR(50), CONSTRAINT pk_username_ware PRIMARY KEY (username) ).
java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:
java.rmi.RemoteException: Transaction aborted (possibly due to transaction time out).; nested exception is:
javax.transaction.RollbackException
java.rmi.RemoteException: Transaction aborted (possibly due to transaction time out).; nested exception is:
javax.transaction.RollbackException
javax.transaction.RollbackException
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Answer 1

I'm a newbie to Enterprise Java, I haven't tested the EJBs in isolation. The code in the servlet to invoke the EJB is:
InitialContext context = new InitialContext();
// Look up WarehouseClerk EJB
Object object = context.lookup( "java:comp/env/ejb/WarehouseClerk" );
WarehouseClerkHome warehouseClerkHome = ( WarehouseClerkHome )
PortableRemoteObject.narrow( object, WarehouseClerkHome.class );
// Find WarehouseClerk with given username and password
WarehouseClerk warehouseClerk = warehouseClerkHome.findByLogin
                    ( username, password );
// Get WarehouseClerkModel for WarehouseClerk
WarehouseClerkModel warehouseClerkModel = warehouseClerk.getWarehouseClerkModel();
// Set username in WarehouseClerk's session
request.getSession().setAttribute ( "username", warehouseClerkModel.getUsername() );
The EJB has been deployed with the JNDI name of ejb/WarehouseClerk. Any help would be most appreciated, I'm lost.