I have a question about getting the default WebApplication in the farm - how do I do it?
I need to, because my timer job must be completed only for one WebApplication.
PS: I'm using SharePoint 2010
There is no such thing as a default WebApplication in a SharePoint farm.
The closest thing would be the WebApplication for Central Administration, but that's usually not the one you would like to get.
If you want to get that then you can Access the WebApplications property of the AdministrationService static property of SPWebService (here in PowerShell syntax)
As mentioned, there is no default Web Application.
You can install a timer job using a Feature. You need to use the FeatureActivated method in a Feature Receiver.
Your Feature needs to have the Web Application scope. This will let you activate the Feature in a chosen Web Application, using Central Administration.
The Web Application then runs with the context of the current Web Application.
Read this tutorial on MSDN for more information on how to do this: http://msdn.microsoft.com/en-us/library/ff798313.aspx
To be clear many of the videos from MS Technet will refer to a Default Webapp. However, they are just referring to the default Webapp that they built after installing SharePoint 2010 for demoing reasons. Central Adminastration is not a Default WebApp but, the main WebApp tool much like PowerShell, IIS, FeatureAdmin(Codeplex) and so on. So, don't think anything of it when those videos mention it or any other source.
To answer your question: 1. On App Server - Start - Central Administration 2. In Central Admin Click Monitoring 3. In monitoring find the Timer Job 4. Note the WebApp it is running on and disable it on all WebApps you don't want the timer to run on.