Handling not found exceptions in Spring Boot - Step-by-Step Execution

Concept Flow - Handling not found exceptions

Client sends request

↓

Controller receives request

↓

Service searches for resource

↓

Resource found?

No→Throw NotFoundException

↓

Exception Handler catches

↓

Return resource data

↓

Return 404 response

↓

Send response to client

The client requests a resource; if the resource is missing, a NotFoundException is thrown and caught by a handler that returns a 404 response.

Execution Sample

Spring Boot

public ResponseEntity<User> getUser(Long id) {
  User user = userService.findById(id)
    .orElseThrow(() -> new UserNotFoundException(id));
  return ResponseEntity.ok(user);
}

@ExceptionHandler(UserNotFoundException.class)
public ResponseEntity<String> handleNotFound(UserNotFoundException ex) {
  return ResponseEntity.status(HttpStatus.NOT_FOUND).body(ex.getMessage());
}

This code tries to find a user by ID; if not found, it throws an exception caught by a handler that returns a 404 error.

Execution Table

Step	Action	Resource Found?	Exception Thrown	Response Sent
1	Client requests user with id=5	N/A	No	No
2	Controller calls service to find user	No	No	No
3	Service returns empty Optional	No	No	No
4	orElseThrow triggers UserNotFoundException	No	Yes	No
5	ExceptionHandler catches UserNotFoundException	No	No	No
6	Handler returns 404 Not Found with message	No	No	Yes

💡 Execution stops after sending 404 response because resource was not found.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
user	null	null	null	Exception thrown	Exception handled

Key Moments - 2 Insights

Why does the exception get thrown at step 4?

Because the service returned an empty Optional, orElseThrow triggers the UserNotFoundException as shown in execution_table row 4.

What happens after the exception is thrown?

The exception is caught by the ExceptionHandler method, which sends a 404 response, as shown in rows 5 and 6.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the UserNotFoundException thrown?

A Step 2

B Step 4

C Step 6

D Step 1

💡 Hint

Check the 'Exception Thrown' column in the execution_table at step 4.

According to the variable tracker, what is the value of 'user' after step 3?

A null

B Optional.empty

C User object

D Exception thrown

💡 Hint

Look at the 'user' row in variable_tracker after step 3.

If the user was found at step 3, what would change in the execution table?

A Exception would still be thrown at step 4

B ExceptionHandler would catch an exception anyway

C No exception thrown and 200 OK response sent

D Response would be 404 Not Found

💡 Hint

Refer to the 'Resource Found?' and 'Response Sent' columns in execution_table.

Concept Snapshot

Handling Not Found Exceptions in Spring Boot:
- Use Optional.orElseThrow() to throw a custom exception if resource missing.
- Define @ExceptionHandler method to catch that exception.
- Return ResponseEntity with 404 status and message.
- This cleanly separates normal flow and error handling.
- Client receives clear 404 response when resource not found.

Full Transcript

In Spring Boot, when a client requests a resource, the controller calls the service to find it. If the resource is missing, the service returns an empty Optional. Using orElseThrow, the controller throws a custom NotFoundException. This exception is caught by an @ExceptionHandler method that returns a 404 Not Found response with a message. This flow ensures the client knows the resource was not found. The variable 'user' remains null since the exception is thrown before assignment. The exception handler sends the final response. This pattern cleanly handles missing data scenarios.