Service calling repository in Spring Boot - Step-by-Step Execution

Concept Flow - Service calling repository

Client calls Service method

↓

Service method starts

↓

Service calls Repository method

↓

Repository accesses Database

↓

Repository returns data to Service

↓

Service processes data

↓

Service returns result to Client

This flow shows how a client calls a service method, which then calls the repository to get data from the database, processes it, and returns the result.

Execution Sample

Spring Boot

public class UserService {
  private final UserRepository repo;
  public UserService(UserRepository repo) { this.repo = repo; }
  public User getUserById(Long id) {
    return repo.findById(id).orElse(null);
  }
}

A service method calls the repository to find a user by ID and returns the user or null.

Execution Table

Step	Action	Service State	Repository Call	Repository Result	Service Return
1	Client calls getUserById(5)	Waiting	No call yet	No result	No return
2	Service calls repo.findById(5)	Calling repo	findById(5)	Searching DB	No return
3	Repository queries DB for ID=5	Waiting repo	DB query	User{id=5}	No return
4	Repository returns User to Service	Received User	Completed	User{id=5}	No return
5	Service returns User to Client	Done	No call	No result	User{id=5}
6	Client receives User	Idle	No call	No result	User{id=5}

💡 Execution stops after service returns the user to the client.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
id	null	5	5	5
repo.findById result	null	Pending	User{id=5}	User{id=5}
service return	null	null	User{id=5}	User{id=5}

Key Moments - 2 Insights

Why does the service call the repository instead of accessing the database directly?

The service calls the repository to separate concerns: the repository handles data access, while the service handles business logic. This is shown in steps 2 and 3 where the service calls repo.findById and the repository queries the database.

What happens if the repository does not find a user with the given ID?

If the repository returns empty, the service returns null as shown by orElse(null) in the code. This would be reflected in the execution table by repo.findById result being empty and service return being null.

Visual Quiz - 3 Questions

Look at the execution table, what is the repository result at step 3?

A User{id=5}

B Searching DB

C No result

D DB query

💡 Hint

Check the 'Repository Result' column at step 3 in the execution_table.

At which step does the service receive the user from the repository?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Service State' and 'Repository Result' columns to find when the service has the user.

If the repository returned empty, what would the service return?

A User object

B null

C Exception

D Empty list

💡 Hint

Refer to the code line with orElse(null) in the execution_sample.

Concept Snapshot

Service calls repository to get data.
Repository handles database access.
Service processes and returns data.
Use dependency injection for repository in service.
Service method calls repo method and returns result.
Keep responsibilities separate for clean code.

Full Transcript

In Spring Boot, a service class calls a repository class to get data from the database. The client calls a method on the service. The service method calls the repository method, which queries the database. The repository returns data to the service. The service processes or directly returns this data to the client. This separation keeps code organized and easier to maintain. The example shows a service method getUserById calling repo.findById and returning the user or null if not found.