@NotNull, @NotBlank, @NotEmpty in Spring Boot - Step-by-Step Execution

Concept Flow - @NotNull, @NotBlank, @NotEmpty

Start Validation

↓

Check @NotNull

Yes / No↓

Fail: Value is null

Yes↓

Check @NotEmpty

Yes / No↓

Fail: Collection/String is empty

Yes↓

Check @NotBlank

Yes / No↓

Fail: String is blank

Yes↓

Validation Passes

Validation checks run in order: first @NotNull ensures value is not null, then @NotEmpty checks collections or strings are not empty, finally @NotBlank ensures strings have non-whitespace content.

Execution Sample

Spring Boot

import javax.validation.constraints.NotBlank;
import javax.validation.constraints.NotEmpty;
import javax.validation.constraints.NotNull;
import java.util.List;

public class User {
  @NotNull
  private String id;

  @NotEmpty
  private List<String> roles;

  @NotBlank
  private String name;
}

This code defines a User class with fields validated by @NotNull, @NotEmpty, and @NotBlank annotations.

Execution Table

Step	Field	Annotation	Value	Validation Result	Action
1	id	@NotNull	null	Fail	Reject: id cannot be null
2	id	@NotNull	"123"	Pass	Continue
3	roles	@NotEmpty	[]	Fail	Reject: roles cannot be empty
4	roles	@NotEmpty	["admin"]	Pass	Continue
5	name	@NotBlank	" "	Fail	Reject: name cannot be blank
6	name	@NotBlank	"Alice"	Pass	Validation passes

💡 Validation stops at first failure or passes if all checks succeed

Variable Tracker

Field	Initial Value	After Validation
id	null	Validation failed if null
id	"123"	Validation passed
roles	[]	Validation failed if empty
roles	["admin"]	Validation passed
name	" "	Validation failed if blank
name	"Alice"	Validation passed

Key Moments - 3 Insights

Why does @NotNull fail but @NotEmpty might not?

@NotNull fails if the value is null (no object), while @NotEmpty fails if the value exists but is empty (like an empty list or string). See execution_table rows 1 and 3.

Can @NotBlank be used on a collection?

@NotBlank only works on strings to check for non-whitespace content. Collections need @NotEmpty. See execution_table row 5 for @NotBlank on a string.

What happens if a string is " " (spaces) with @NotEmpty?

@NotEmpty passes because the string is not empty, but @NotBlank fails because it only has whitespace. See execution_table rows 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the validation result for 'roles' when the value is []?

A Pass

B Fail

C Error

D Skipped

💡 Hint

Check row 3 in the execution_table where roles is []

At which step does the validation pass for the 'name' field?

A Step 4

B Step 5

C Step 6

D Step 3

💡 Hint

Look at the execution_table rows for 'name' field validation

If the 'id' field is set to "123", what happens at step 2?

A Validation passes and continues

B Validation fails because id is empty

C Validation fails because id is null

D Validation is skipped

💡 Hint

See execution_table row 2 for id with value "123"

Concept Snapshot

@NotNull: Fails if value is null
@NotEmpty: Fails if collection or string is empty
@NotBlank: Fails if string is blank (only whitespace)
Use @NotNull for any object
Use @NotEmpty for collections or strings
Use @NotBlank only for strings with content

Full Transcript

This visual execution shows how Spring Boot validation annotations @NotNull, @NotEmpty, and @NotBlank work step-by-step. First, @NotNull checks if a value is null and fails if so. Then, @NotEmpty checks if collections or strings are empty and fails if empty. Finally, @NotBlank checks if strings contain only whitespace and fails if blank. The execution table traces example values through these checks, showing when validation passes or fails. Key moments clarify common confusions, like the difference between null and empty, and when to use each annotation. The quiz tests understanding by referencing the execution steps and variable states.