simplejparepository.md

Great question! 🔍 You're right to ask:

"If Repository is just a marker interface and we're extending it in MyBaseRepository, then where is the actual implementation of the methods like save() or findById()?"

Let's break it down clearly 👇

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✅ Short Answer:

The implementation comes from Spring Data itself — specifically from classes like SimpleJpaRepository — which dynamically implements your repository interface at runtime using Spring Data proxies.

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💡 How Spring Does It Internally:

1. You define this:

@NoRepositoryBean
public interface MyBaseRepository<T, ID> extends Repository<T, ID> {
    Optional<T> findById(ID id);
    <S extends T> S save(S entity);
}

public interface UserRepository extends MyBaseRepository<User, Long> {
    User findByEmailAddress(String email);
}

2. Spring Boot + Spring Data JPA (or Mongo) scans all interfaces that extend Repository, CrudRepository, or JpaRepository.

3. For each such interface (like UserRepository), Spring:

   • Detects the domain class (User)

   • Detects the ID type (Long)

   • Finds a matching implementation class like SimpleJpaRepository<User, Long>

   • Creates a proxy at runtime that:

     • Implements your UserRepository interface
     • Delegates method calls like save() and findById() to internal logic in SimpleJpaRepository

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🔄 How save() and findById() get implemented:

Even if you didn’t extend CrudRepository, Spring knows how to handle methods like save() and findById() because the method signatures match the known ones in the internal Spring Data API.

So Spring internally does something like:

if method signature == known CRUD method (e.g., save(), findById())
   then delegate to built-in implementation (e.g., SimpleJpaRepository)

That’s how it works without you having to write any code.

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🎯 Why This Works

• Repository is a marker interface — just used to tell Spring:

  “This is a repository interface — apply your magic here.”

• @EnableJpaRepositories in Spring Boot auto-configuration triggers the scanning and proxy creation logic.

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🧪 Want Proof?

You can debug your app and put a breakpoint inside your service class where you call userRepository.save(user). You’ll see that it hits a Spring-generated proxy backed by SimpleJpaRepository.

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✅ Summary:

Concept	Meaning
Repository interface	Marker only — tells Spring this is a data repo
MyBaseRepository	Your own abstraction, defines only the methods you want
No actual implementation	Spring Data provides it at runtime via proxies
Matches method signatures	If your method signature matches known ones (like save), Spring wires it automatically

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🔄 No, SimpleJpaRepository is not a proxy — it's a real class that contains the actual logic.

✅ What happens is:

1. Spring creates a dynamic proxy for your repository interface (e.g., UserRepository)
2. That proxy delegates method calls to an internal instance of SimpleJpaRepository<User, Long> (or another store-specific implementation)

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🎯 Key Roles:

Component	Role
SimpleJpaRepository	The actual implementation class with real code (like how to do save(), findById(), etc.)
Proxy	The dynamic Spring-generated object that implements your interface (UserRepository) and delegates to SimpleJpaRepository
Your UserRepository interface	What you use in your service layer — defines the methods you want

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🧠 So when you call this:

userRepository.save(user);

This is what happens under the hood:

YourService -> UserRepository proxy -> SimpleJpaRepository.save() logic

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🔍 Regarding the Methods:

"Will SimpleJpaRepository only have the methods that we define?"

No. SimpleJpaRepository has all CRUD methods like:

• save
• findById
• findAll
• deleteById
• etc.

But if your interface (like MyBaseRepository) only declares save() and findById(), then only those two will be visible and usable in your code — even though the underlying SimpleJpaRepository knows how to do much more.

So:

• Your interface defines what's exposed
• Spring handles what's actually implemented behind the scenes

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✅ Summary

• SimpleJpaRepository is a concrete class with full implementation.
• Spring creates a proxy object for your custom repository (e.g., UserRepository) that delegates to SimpleJpaRepository.
• You control what methods are exposed by defining them in your base interface (like MyBaseRepository).
• Even though SimpleJpaRepository has all CRUD logic, you can hide unused/dangerous methods by not including them in your interface.