We know that Spring is by default singleton.
Now the question is How it will behave when more than two request comes at same time on the same method.How that will be handled in the by Spring.
I suppose same instance will be shared but what is the normal approach to handle that.I mean the JVM behavior for this also.
"How to handle it" depends on what it's actually doing. If there's no shared state and you're just calling methods with only parameters and local variables, there's nothing to do. If you're reading shared state that might change, or mutating shared state, something somewhere along the line would need to be synchronized, whether it's a simple collection, and entire method, etc. Totally depends.