To increase the memory efficiency in physical servers is a significant concern for increasing the number of virtual machines (VM) in them. When similar web application service runs in each guest VM, many string data with the same values are created in every guest VMs. These duplications of string data are redundant from the viewpoint of memory efficiency in the host OS. This paper proposes two approaches to reduce the duplication in Java string in a single Java VM (JVM) and across JVMs. The first approach is to share string objects across JVMs by using a read-only memory-mapped file. The other approach is to selectively unify string objects created at runtime in the web applications. This paper evaluates our approach by using the Apache DayTrader and the DaCapo benchmark suite. Our prototype implementation achieved 7% to 12% reduction in the total size of the objects allocated over the lifetime of the programs. In addition, we observed the performance of Day Trader was maintained even under a situation of high density guest VMs in a KVM host machine. Copyright © 2014 ACM.