@Valid annotation on request body in Spring Boot - Step-by-Step Execution

Concept Flow - @Valid annotation on request body

Client sends HTTP request with JSON body

↓

Spring Boot Controller receives request

↓

@Valid triggers validation on request body object

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Validation passes

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Response sent back to client

When a client sends data, Spring Boot uses @Valid to check the data. If valid, the controller runs. If not, an error response is sent.

Execution Sample

Spring Boot

public ResponseEntity<String> createUser(@Valid @RequestBody User user) {
    return ResponseEntity.ok("User created");
}

This code checks the User object sent in the request body. If valid, it returns success.

Execution Table

Step	Action	Input Data	Validation Result	Controller Behavior	Response
1	Receive HTTP POST with JSON body	{"name":"Alice","age":25}	Not yet validated	Waiting for validation	No response yet
2	@Valid triggers validation on User object	{"name":"Alice","age":25}	Passes all constraints	Controller method executes	No response yet
3	Controller returns success message	User object valid	N/A	Returns ResponseEntity.ok	HTTP 200 OK with 'User created'
4	Receive HTTP POST with invalid JSON body	{"name":"","age":-5}	Not yet validated	Waiting for validation	No response yet
5	@Valid triggers validation on User object	{"name":"","age":-5}	Fails: name blank, age negative	Controller method skipped	Spring returns HTTP 400 Bad Request with error details

💡 Execution stops after sending HTTP response based on validation result

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
user.name	null	"Alice"	""	""
user.age	null	25	-5	-5
validationResult	null	valid	invalid	invalid
response.status	null	200 OK	400 Bad Request	400 Bad Request

Key Moments - 2 Insights

Why does the controller method not run when validation fails?

Because @Valid triggers validation before the method runs. If validation fails (see step 5 in execution_table), Spring stops and sends an error response instead.

What happens if the request body is missing required fields?

Validation fails since required constraints are not met. The execution_table row 5 shows validation failure and Spring returns 400 Bad Request.

Visual Quiz - 3 Questions

Look at the execution_table, what is the validation result at step 2?

A Not yet validated

B Fails due to missing fields

C Passes all constraints

D Controller skipped

💡 Hint

Check the 'Validation Result' column at step 2 in execution_table

At which step does Spring return a 400 Bad Request response?

A Step 5

B Step 3

C Step 1

D Step 2

💡 Hint

Look at the 'Response' column in execution_table for HTTP 400 Bad Request

If the user.age was 30 instead of -5 in step 5, what would change?

A Validation would still fail

B Validation would pass and controller runs

C Response would be 400 Bad Request

D No change in validation result

💡 Hint

Refer to variable_tracker for user.age values and validation results

Concept Snapshot

@Valid on @RequestBody triggers automatic validation of incoming data.
If data is valid, controller method runs normally.
If invalid, Spring returns 400 Bad Request with errors.
Use validation annotations on model fields (e.g., @NotBlank, @Min).
This helps catch bad input early and keeps code clean.

Full Transcript

When a client sends a request with data, Spring Boot uses the @Valid annotation on the request body to check if the data meets rules like not empty or minimum values. If the data is good, the controller method runs and sends a success response. If the data is bad, Spring stops the method and sends back an error message with status 400. This process helps keep the app safe and user-friendly by catching mistakes early.