问题
if I understand EJB correctly, @Singleton is actually the same as Singleton in plain Java and also singleton in spring -> one instance, every call goes through the same instance concurrently. @Stateless declares a bean, that could (but must not) have multiple instance, with the limitation that only one call can be in an instance at the same time. Right sofar? This remains me on the servlet programming model: in theory servlet containers are allowed to make multiple copies of the servlet, in practice I haven't seen any servlet container to do so. So assuming I do not have REALLY LIMITED resources like doors, windows or printers in my code (and if I did I could still solve it with queues and stuff), what is the REAL example, where usage of @Stateless is advantageous over usage of @Singleton.
regards Leon
回答1:
You can have multiple instances of a stateless bean to increase throughput.
On the other hand there is only one instance of a singleton. The reason for this is normally to share state in application scope, serializes access to resources etc., and this implies locking or synchronization.
So if you are not really having a singleton, then use a stateless bean.
If you have a "stateless singleton", there is no difference. But if you read "singleton", it has a special meaning by convention (= there must be a reason for using the singleton pattern).
回答2:
Stateless implies that the bean is thread safe. This is because there is no code in the bean that relies on state. This means that running any of its methods will not affect future running of said methods.
An example of a stateless class would a class that does addition and subtraction. All the necessary parameters are passed into the method. Doing an addition or subtraction does not alter the way these methods work at a later call. This implies that you do not need to worry about concurrency with the class.
A singleton is usually used for a class that is very expensive to create such as a Database connection. You do not want every class creating a new Database connection every time they need to use the database so you have it instantiated once at program start up. Being a singleton does not necessarily mean that the class is thread safe (although it absolutely should be).
So Stateless means the class is threadsafe.
Singleton refers to the fact that the class is only created once. While this heavily implies that the class is (AND IT SHOULD BE) thread safe, it does not directly imply it like stateless does.
来源：https://stackoverflow.com/questions/17115533/what-is-a-real-use-case-for-stateless-over-singleton-in-ejb