Are Spring singleton beans thread-safe?

Separates container instance reuse from concurrency guarantees and designs singleton services around stateless or safely coordinated state.

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No. Spring's singleton scope means one instance is shared in the application context, not that Spring synchronizes its methods. In a web service, many request threads can call that instance concurrently. A singleton service is usually safe when it is stateless and its collaborators are thread-safe. Mutable request-specific values must stay in method-local data, request scope, or another correctly owned object rather than fields on the singleton.

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Shared-instance reasoning, request concurrency, mutable fields, safe publication, and correct state ownership.

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Spring shares a singleton instance; the bean's code and state determine whether concurrent use is safe.
No. Spring's singleton scope means one instance is shared in the application context, not that Spring synchronizes its methods. In a web service, many request threads can call that instance concurrently. A singleton service is usually safe when it is stateless and its collaborators are thread-safe. Mutable request-specific values must stay in method-local data, request scope, or another correctly owned object rather than fields on the singleton.
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A field such as currentUser, a reusable mutable builder, or an unsynchronized HashMap can mix requests or lose updates. Making a reference final only prevents reassignment; it does not make the referenced object immutable. If shared mutable state is truly required, the full invariant needs a concurrency design using immutability, atomics, locks, concurrent structures, or an external transactional store. The same review applies to caches and third-party clients injected into the singleton.

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Assuming Spring serializes singleton method calls.
Storing request state in instance fields.
Equating a final reference with immutable state.

Are Spring singleton beans thread-safe?

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