Why Spring Beans Default to Singleton: Benefits and Drawbacks Explained

Difference Between Singleton and Prototype Beans

If a bean is declared as a singleton, Spring creates only one instance in the container and reuses it for all subsequent requests, storing it in a map. For prototype beans, a new instance is created for each request without caching.

1. Diagram Analysis

2. Source Code Analysis

When generating a bean, Spring first determines whether it is singleton or prototype.

If it is a singleton, Spring attempts to retrieve it from the cache; if not found, it creates a new instance.

Conclusion

Singleton beans are created only once and then reused, reducing object creation overhead.

Prototype beans are created anew for each request.

Advantages of Singleton Beans

Because they are not created for each request, singleton beans offer several performance benefits:

1. Reduced Instance Creation Overhead

Creating instances involves reflection or CGLIB, both of which are performance-intensive, and memory allocation algorithms add further cost.

2. Reduced JVM Garbage Collection

Fewer bean instances mean fewer objects for the garbage collector to reclaim.

3. Faster Bean Retrieval

After the initial creation, beans are fetched from the cache, making access very fast.

https://juejin.im/post/5ca42bfa6fb9a05e17799e07

Disadvantages of Singleton Beans

A major drawback is the lack of thread safety for stateful beans; since all requests share the same instance, concurrent access can cause issues, unlike prototype beans which are isolated per request.

Summary

Why Does Spring Default Beans to Singleton?

To improve performance by reducing instance creation, minimizing garbage collection, and enabling fast cache-based retrieval.

Fewer instances created.

Less garbage collection.

Quick cache access.

What Are the Drawbacks?

Stateful singleton beans can be thread-unsafe in concurrent environments.