How does Spring @Transactional work?

Direct answer

What a strong answer covers

How it works

Spring wraps a bean that has @Transactional methods in a proxy. The proxy intercepts external calls, starts or joins a transaction according to the propagation setting, binds the connection to the thread, and commits when the method returns or rolls back when a RuntimeException escapes; checked exceptions do not trigger rollback unless rollbackFor says so. Because the interception lives in the proxy, a method calling another @Transactional method on this bypasses it entirely, and private or final methods cannot be intercepted at all.

Example

A service method annotated @Transactional that calls this.saveAudit(), also annotated with REQUIRES_NEW, runs the audit inside the outer transaction and loses it on rollback because the self-call never passed through the proxy. Moving saveAudit into a separate bean restores the intended independent transaction.

Common mistakes

• Annotating an interface method and expecting it to apply with class-based proxies.
• Catching an exception inside the transactional method and expecting the transaction to roll back anyway.
• Holding a transaction open across a remote HTTP call and exhausting the connection pool under load.

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