Hi You have to annotate you controller class with @Validated annotation see example below:

For testing purpose please try commenting @Validated annotation you won't notice javax.validation.ConstraintViolationException: hello.name: size must be between 4 and 10 but once you place it back its works again. More technical info here Difference between @Valid and @Validated in Spring

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @Validated
    @RestController
    @RequestMapping("/hello")
    class HelloController {
        @GetMapping
        public String hello(@Valid
                            @NotNull(message = "Name cannot be empty")
                            @Size(min = 4, max = 10) @RequestParam("name") String name) {
            return "Hello, " + name + "!";
        }
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.0.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-validation</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

Validation starter not included in web starters anymore.

The spring-boot-starter-validation is not a transitive dependency of spring-boot-starter-web and spring-boot-starter-webflux anymore.

Add this dependency for validations work.

<dependency>
 <groupId>org.springframework.boot</groupId>
 <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Actually there error was in the unit tests. The validation was working well.

For those who came here with the same issue, it's very likely that you are missing to add the following dependency to the pom.xml as Braian Silva suggested above:

<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Thanks for your help guys!

I've got a similar issue here. But in my case, the validation is warning me that a sequence is missed in oracle schema, but it is there. I think that is a bug.. I will follow with the 2.4.0 version for while..

According to spring boot 2.3.1 release there is no longer contains spring-boot-starter-validation with spring starter

how to add starter validation on

maven

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Gradle

dependencies {
  ...
  implementation 'org.springframework.boot:spring-boot-starter-validation'
}

referee the release note

https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.3-Release-Notes#validation-starter-no-longer-included-in-web-starters

If your experiencing the issue of for example: not being able to see the validation errors (default-messages) returned back to the client, this is what you could do:

Top Solution 1: Simply add devtools. This should solve the issue. After I did this, all my binding-results were returned back to the client. I recommend you to test this out first:

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
</dependency>

Solution 2:

I found out that this is due to using Spring Boot 2.3+ So if youre using Spring Boot 2.3 or higher, add this dependency in your pom.xml file as its no longer included within the 'web'-dependency itself.

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Now its necessary to set 'include binding errors' in java/resources/application.properties to "always". Same goes for 'message' as well although I think this is optional.

server.error.include-message=always
server.error.include-binding-errors=always

Solution 3: (before I discovered solution 2 which could be helpful as well)

So I found out that this is due to having Spring boot 2.3+. But I could not find caution-messages on the new updated usage of @Valid in Spring Boot v2.3+.

So I ended up switching back to Spring boot v2.2.10 (latest version of 2.2) by adjusting the release version in the pom.xml file like so:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.10.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

This worked perfectly for me by rolling back to an older version. Although id like to update my Spring Boot version some day. (Revisit solution 1 & 2)

A simple solution to the above issue is to add:

<repositories>
        <repository>
            <id>Central Maven repository</id>
            <name>Central Maven repository https</name>
            <url>https://repo.maven.apache.org/maven2</url>
            <layout>default</layout>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
        </repository>
    </repositories>

to your POM.xml file. This will fix the issue with this version

Vipul.