How to I get Spring-Data-MongoDB to validate my objects?

First make sure that you have JSR-303 validator on classpath, for example:

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>4.2.0.Final</version>
</dependency>

If you use Java config, the way to go is to create 2 beans:

@Bean
public ValidatingMongoEventListener validatingMongoEventListener() {
    return new ValidatingMongoEventListener(validator());
}

@Bean
public LocalValidatorFactoryBean validator() {
    return new LocalValidatorFactoryBean();
}

Voilà! Validation is working now.

Starting with Spring Boot 2.3 the spring-boot-starter-validation dependency has to be added in pom.xml (for Maven):

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

Declaring a validator bean is not necessary.