Dynamic software updating is critical for many systems that must provide continuous service. And Java language is gaining increasing popularity in developing distributed system. Most of previous works on updating concerns on safely update one class every time. It occur many limitations on updating classes, such as not allowing deleting method which is invoked in other class. In this paper, update transaction is purposed to dynamically update class set, and we also discuss some properties about it, such as ACID. Then the type-safety property is proven formally. This paper also proposes an implementation method to updating classes without changing JVM and Java program language. The method makes use of Java class loading mechanism and reflection mechanism. At the end of the paper, an experiment is made for analyzing.