I was looking at chain's implementation, instead of executing the commands with the visitor pattern, wouldn't a more powerful/flexible method be to implement it using a filter pattern?
See: http://java.sun.com/blueprints/corej2eepatterns/Patterns/InterceptingFilter. html Sample Chain: CommandServletStart CommandExceptionHandler CommandStoreState CommandExceptionThrower -- CommandStart.execute(Context ctx, Chain chain) throws Exception { ServletContext srvCtx = (ServletContext) ctx; String usrId = srvCtx.getRequestScope().getParameter("id")); String newName = srvCtx.getRequestScope().getParameter("name")); // get user and save state User user = SomeService.getUser(usrId); String oldName = user.getName(); // set new state and execute try { user.setName(newName); ctx.setAttribute("user", user); chain.executeNext(ctx); } catch (Exception e) { // on error, rollback change user.setName(oldName); throw e; } } -- CommandExceptionHandler.execute(Context ctx, Chain chain) throws Exception { try { chain.executeNext(ctx); } catch (SQLException e) { chain.executeNext(ctx); } // let unhandled exceptions go up } CommandExceptionThrower.execute(Context ctx, Chain chain) throws Exception { throw new SQLException(); } With this, you would not have to change your inheritance to add behavior (going from a command to a filter). The given command would already know best on how to handle itself and since it calls the next item in the chain, we can also wrap the context to take care of situations where we want to have listener type behavior. The chain could also allow for runtime manipulation by the commands, to insert additional commands to handle errors. chain.insert("SomeCommand").insert("AnotherCommand).executeNext(ctx); -------- Jacob Hookom Senior Programmer/Analyst McKesson Medical-Surgical Golden Valley, MN --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
