Custom Json Validation in anypoint studio

I am trying to validate JSon using the json schema validator. But it returns a generic message. "Json content is not compliant with schema" . I have a HTTP POST REQUEST which sends a payload as follows:

{ "key1" : "value1", "key2" : "value2" ,"key3": "value3" }

if key1 and key2 is missing . I want it to give error messages as follows:

{{
  errorCode :1001,
  errorMessage : key1 is missing",
},
  errorCode :1002,
  errorMessage : key2 is missing"
}
}

I tried writing the errors to a file(json file containing all the warnings and messages} looks something like this:

{
  "level" : "error",
  "domain" : "validation",
  "keyword" : "required",
  "message" : "object has missing required properties ([\"key1\",\"key2\",\"key3\"])",
  "required" : [ "key1", "key2", "key3"],
  "missing" : [ "key1", "key2"]
}

This is just a small part of this file. I'll have to loop through the file to reach this information. Is there any other way , I can perform custom validations and return proper error messages to the user.

EDIT 1:

I have created the following RequestObj class:

public class RequestObj {

@Valid
@NotBlank
@NotNull
private String key1;

@Valid
@NotBlank
private String key2;

@Valid
@NotBlank
private String key3;

@Override
public String toString() {
    return "RequestObj [key1=" + key1 + ", key2=" + key2 + ", key3=" + key3 + "]";
}

It is not validating key1 as not null.

postman request :

POST /validate HTTP/1.1 Host: localhost:8081 Content-Type: application/json

{ "key2" :"gg", "key3" : "hh" }

EDIT 2:

when I implement the validator interface. I dont get access to the mule Event. How will I access the json that I need to validate in this case ?

This is how my result looks after performing custom validations on the json input. I used JSR-303 annotations for validating the data.

class Example{ 
   @NotBlank
        @Size(min = 3, max = 5)
        private String key1;

        @Pattern(regexp=".+@.+\\.[a-z]+") // email
        private String key2;

        private String key3;
}

then I wrote a custom validator and I invoked the static function validate by passing all the values:

public class ValidationServiceImpl {

    public static HashMap<String,String> validate(String key1 , String key2 , String key3)  {
        HashMap<String,String> result = new HashMap();
        Example req = new Example(key1 , key2, key3);
    Validator validator;
    ValidatorFactory validatorFactory = Validation.buildDefaultValidatorFactory();
    validator = validatorFactory.getValidator();
    Set<ConstraintViolation<Example>> violations = validator.validate(req);
    if(!CollectionUtils.isEmpty(violations)) {
        for (ConstraintViolation<Example> violation : violations) 
        {
            String propertyPath = violation.getPropertyPath().toString();
            String message = violation.getMessage();
            result.put(propertyPath, message);
        }
    }
    return result;
    }

}

The result is the hashmap which returns all the violations.Logging it will give you the result.

POM dependencies required are:

<!-- https://mvnrepository.com/artifact/org.hibernate.validator/hibernate-validator -->
<dependency>
    <groupId>org.hibernate.validator</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>6.1.0.Final</version>
</dependency>
   <!-- https://mvnrepository.com/artifact/org.hibernate.validator/hibernate-validator-annotation-processor -->
<dependency>
    <groupId>org.hibernate.validator</groupId>
    <artifactId>hibernate-validator-annotation-processor</artifactId>
    <version>6.1.0.Final</version>
</dependency>


    <!-- https://mvnrepository.com/artifact/javax.validation/validation-api -->
<dependency>
    <groupId>javax.validation</groupId>
    <artifactId>validation-api</artifactId>
    <version>2.0.1.Final</version>
</dependency>
<!-- https://mvnrepository.com/artifact/javax.el/javax.el-api -->
<dependency>
    <groupId>javax.el</groupId>
    <artifactId>javax.el-api</artifactId>
    <version>3.0.0</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.glassfish/javax.el -->
<dependency>
    <groupId>org.glassfish</groupId>
    <artifactId>javax.el</artifactId>
    <version>3.0.0</version>
</dependency>