Migrating to Hexagonal Architecture (Ports and Adapters) promises decoupled code and domain-centric design. However, it introduces a specific friction point for Spring Boot developers: Transaction Management. In a traditional Layered Architecture, placing @Transactional on the Service layer is muscle memory. In Hexagonal, the "Service" is split into Use Cases, Input Ports, and Application Services. If you misplace the transaction boundary, you face two critical failures: the dreaded LazyInitializationException during DTO mapping, or worse, data corruption due to partial commits when a business operation spans multiple driven adapters. This guide defines strictly where @Transactional belongs in a robust Hexagonal setup, why it belongs there, and how to implement it using Java 21 and Spring Boot 3. The Root Cause: Why Boundaries Break To understand where to put the annotation, we must understand what Spring and Hibernate are doing under th...
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