This might sound complex but i will ask anyway:
I am running a service A which uses class X. I want to start another service B which uses classes A besides new classes.
Service A is already running. I do a hot deployment of Service B.
Here is the real question - Will Service B use the same instance of class X or a separate instance. How can singleton class help me here?
Each Service will run in it's own operating System (OS) Process space, and each process space has it's own class instances. A "singleton" class is normally coded using static fields in a class, which would be local to the process space the code was running in, so no, they will not share singletons. Each will get it's own instance.
You can do what you are trying to do, however, using some external shared synchronization process, (for example, exposing a singleton over whatever the java equivilent is to .Net Remoting (or WCF) - a network exposed endpoint which is coded to use a Singleton, and have both your services "connect to" and use that remotely accessible Singleton)
I'm not familiar with the details of how Java Web services are run, but if they are both running in the same VM then I think the classes would be shared across all applications in the VM and static fields would therefore be shared. Since the Singleton pattern is usually accomplished by attaching a single instance to a static member, the Singleton would be shared.
This based on: The Clean Code Talks - "Global State and Singletons"
You should be able to test it out by writing two simple Web services. One that does something to the singleton, such as set a flag, and another that checks for it.
来源：https://stackoverflow.com/questions/1494642/what-is-a-singleton-class-can-it-help-me-running-single-instance-of-a-class-for