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Understanding the Spring Framework’s Declarative Transaction Implementation

It is not sufficient merely to tell you to annotate your classes with the @Transactional annotation, add @EnableTransactionManagement to your configuration, and expect you to understand how it all works. To provide a deeper understanding, this section explains the inner workings of the Spring Framework’s declarative transaction infrastructure in the context of transaction-related issues.

The most important concepts to grasp with regard to the Spring Framework’s declarative transaction support are that this support is enabled via AOP proxies and that the transactional advice is driven by metadata (currently XML- or annotation-based). The combination of AOP with transactional metadata yields an AOP proxy that uses a TransactionInterceptor in conjunction with an appropriate TransactionManager implementation to drive transactions around method invocations.

Spring AOP is covered in the AOP section.

Spring Framework’s TransactionInterceptor provides transaction management for imperative and reactive programming models. The interceptor detects the desired flavor of transaction management by inspecting the method return type. Methods returning a reactive type such as Publisher or Kotlin Flow (or a subtype of those) qualify for reactive transaction management. All other return types including void use the code path for imperative transaction management.

Transaction management flavors impact which transaction manager is required. Imperative transactions require a PlatformTransactionManager, while reactive transactions use ReactiveTransactionManager implementations.

@Transactional commonly works with thread-bound transactions managed by PlatformTransactionManager, exposing a transaction to all data access operations within the current execution thread. Note: This does not propagate to newly started threads within the method.

A reactive transaction managed by ReactiveTransactionManager uses the Reactor context instead of thread-local attributes. As a consequence, all participating data access operations need to execute within the same Reactor context in the same reactive pipeline.

When configured with a ReactiveTransactionManager, all transaction-demarcated methods are expected to return a reactive pipeline. Void methods or regular return types need to be associated with a regular PlatformTransactionManager, e.g. through the transactionManager attribute of the corresponding @Transactional declarations.

The following image shows a conceptual view of calling a method on a transactional proxy: