How Does Spring Data Generate and Implement Repository Classes at Runtime?

Spring Data Repositories: Unveiling the Implementation Mystery

While working with Spring Data JPA repositories, you may have wondered about their inner workings. This article will shed light on the intriguing process of repository implementation at runtime.

How are Repository Classes Created and Methods Injected?

Contrary to common assumptions, there's no code generation or bytecode manipulation involved. Instead, Spring Data dynamically generates a JDK proxy instance that acts as the backing class for the repository interface. This proxy intercepts all method calls and redirects them to the appropriate locations:

• If there's a custom implementation part specified for the repository, it checks if the invoked method is implemented there.
• If the method is a query method, it executes the predetermined query specific to that method.
• Otherwise, the call is routed to a method from a repository base class (e.g., SimpleJpaRepository for JPA).

This routing is handled by the QueryExecutorMethodInterceptor, which acts as the proxy's method dispatcher.

Factory Pattern and DI Integration

The creation of repository proxy instances follows a Factory pattern, with the high-level proxy creation taking place in RepositoryFactorySupport. The store-specific implementations provide the necessary components to ease integration with frameworks like Spring.

In Spring, you can use the following code to create a repository:

EntityManager em = ...;
JpaRepositoryFactory factory = new JpaRepositoryFactory(em);
UserRepository repository = factory.getRepository(UserRepository.class);

This factory-based approach emphasizes that Spring Data can operate independently of a Spring container, as long as its libraries are on the classpath. However, for seamless integration, Spring Data offers Java config, XML namespace, and CDI extension support.

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