Understanding Spring Bean Scopes: Singleton vs Prototype

Hello, I am the "Architecture Jun", a developer who also writes poetry.

Spring provides five bean scopes—singleton, prototype, request, session, and global session—but this article focuses on the first two, which were the original scopes offered by Spring.

When a bean is defined as a singleton, Spring creates only one instance and stores it in a map; every subsequent request retrieves the same instance from the cache, avoiding repeated instantiation.

In contrast, a prototype bean is instantiated anew for each request, with no caching involved.

Diagram analysis shows the lifecycle of singleton versus prototype beans, illustrating that singletons are created once and then reused, while prototypes are created on demand.

Source‑code analysis reveals that during bean creation Spring first checks the scope: if it is singleton, it attempts to fetch the bean from the cache and creates it only if absent; if it is prototype, it directly creates a new instance.

Advantages of singleton beans include reduced instance‑creation overhead, lower JVM garbage‑collection pressure, and faster bean retrieval because most accesses hit the cache.

The main disadvantage is that a singleton bean is not thread‑safe when it holds mutable state, which can cause concurrency issues in high‑traffic scenarios; prototype beans do not share this problem.

In summary, Spring defaults to singleton beans to improve performance by minimizing object creation, reducing garbage collection, and enabling quick cache retrieval, while developers must be aware of potential thread‑safety concerns for stateful singletons.