Start with the resource and its contract
A REST API is a contract between clients and a server. Design routes around resources such as tasks, courses or orders, then use HTTP methods to express actions. A collection endpoint might list records, while a resource endpoint addresses one record by identifier. Keep route names and behavior predictable so a client does not need to guess how the service works.
Document each operation’s required fields, response shape and possible status codes. Use request and response DTOs to define what crosses the API boundary. Returning persistence entities directly can expose internal fields and make database changes unexpectedly break clients.
Keep responsibilities clear in Spring Boot
A controller should translate HTTP input into an application call and translate the result into a response. Business rules belong in a service that can be tested without an HTTP server. A repository handles data access. This separation is useful when each layer has a real responsibility; avoid adding layers that only forward every method unchanged.
Constructor injection makes dependencies explicit. It also makes it straightforward to supply a test double when testing a service. Keep controllers small enough that request handling, persistence and business decisions are not all mixed together.
Validate at the boundary
Requests should be treated as untrusted. Validate required values, allowed ranges and formats before processing them. Bean Validation annotations can express common constraints on request DTOs, while service-level checks handle rules that depend on application state. For example, a valid identifier format does not prove that a referenced record exists or that the current user may modify it.
Return field-level feedback that helps a client correct a request, but do not expose stack traces, SQL details or secrets. Never build a database query by concatenating user-supplied text; use parameterized framework APIs.
Use status codes and errors consistently
Choose a status code that describes the result: successful reads and updates, creation of a resource, invalid input, an absent resource or an unexpected server failure. A frontend can then respond appropriately without parsing a vague message. A consistent error structure may contain a general message and safe field errors.
Centralized exception handling can map known application exceptions to the API contract. Avoid broad catches that convert every failure into a success-shaped response. Unexpected errors should be logged using the application’s standard mechanism and returned as a generic server error without private details.
Test behavior, not just annotations
Write service tests for business rules, including boundary cases. Add web tests that exercise routes, validation and response codes. Test requests with missing fields, invalid values and unknown identifiers. A test that only checks a controller method was called may miss a broken public contract.
Finally, write down examples of requests and responses. A React client or API consumer should be able to integrate using the documented contract. For broader Java full-stack practice, connect an endpoint to a small React flow and verify the complete behavior.