Java development has supplanted IT security as the most difficult skill set for hiring managers to locate. Virtualization jobs, meanwhile, have become easier to fill, but hiring managers are increasingly having trouble finding talented software developers and C# programmers, according to a new Dice.com survey.The tech hiring site ranked the 10 most difficult skill sets and positions to fill in a survey of 600 human resources managers and recruiters.One year ago, security was the most difficult skill set to find in the tech talent market, with Java and J2EE development placing third. Now Java is No. 1, and security is second on the list. Virtualization, the second hardest skill to find one year ago, has dropped out of the top 10 entirely, suggesting that IT pros familiar with virtualization technologies have become more plentiful.