How Does Spring Data JPA Dynamically Implement Repository Interfaces?

How Spring Data Repositories are Implemented

Spring Data JPA repositories are essential for simplifying database access in Java applications. While the repository interfaces define the methods that interact with the database, the actual implementation is dynamically generated by Spring.

Generation of Repository Implementation

Contrary to popular belief, Spring Data JPA does not use code generation or bytecode manipulation libraries like CGLib. Instead, it dynamically creates a JDK proxy that implements the repository interface.

Interception and Method Routing

When a method is invoked on the repository proxy, a method interceptor, QueryExecutorMethodInterceptor, intercepts the call and routes it to the appropriate place based on the following criteria:

• If a custom implementation has been provided and the method is defined within it, the call is directed to the custom implementation.
• If the method is a query method (determined by examining annotations or the method name), the query execution mechanism creates and executes the appropriate query.
• If neither of the above applies, the method is a standard CRUD operation and is delegated to the repository base class (e.g., SimpleJpaRepository for JPA).

Proxy Creation

The repository proxy creation is handled by a factory pattern implemented by RepositoryFactorySupport. Store-specific implementations, such as JpaRepositoryFactory, add the necessary infrastructure to support the dynamic proxy creation.

Container Agnostic Nature

It's important to note that the underlying implementation of Spring Data repositories is container agnostic. It can be used in Spring-managed applications or within plain DI containers like CDI. Integration with Spring and CDI is achieved through configuration and extensions that simplify the setup process.

Supported Documentation

• Spring Data JPA Reference Documentation: https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#jpa.query-methods

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