Custom validation in Mule ESB - mule-component

Question

I am trying to add custom validation against a generated request object for incoming JSON payload using the javax.validation framework and openapi-generator-maven-plugin in Mule ESB.
I am following the below mulesoft documentation :
https://docs.mulesoft.com/mule-runtime/3.9/building-a-custom-validator
I have added the below custom validation :
<validation:custom-validator doc:name="Validation" class="com.poltu.validation.CustomValidator" />
But when I amtrying to run the application ,it fails with the below message :
Invalid content was found starting with element 'validation:custom-validator'. One of '{"http://www.mulesoft.org/schema/mule/core":abstract-message-processor, "http://www.mulesoft.org/schema/mule/core":abstract-mixed-content-message-processor}' is expected.
Edit :
below is the xml configuration that I am now using :
<?xml version="1.0" encoding="UTF-8"?>
<mule xmlns:validation="http://www.mulesoft.org/schema/mule/validation"
xmlns:xml-module="http://www.mulesoft.org/schema/mule/xml-module"
xmlns:json="http://www.mulesoft.org/schema/mule/json"
xmlns:java="http://www.mulesoft.org/schema/mule/java"
xmlns:ee="http://www.mulesoft.org/schema/mule/ee/core"
xmlns:db="http://www.mulesoft.org/schema/mule/db"
xmlns:http="http://www.mulesoft.org/schema/mule/http"
xmlns="http://www.mulesoft.org/schema/mule/core"
xmlns:doc="http://www.mulesoft.org/schema/mule/documentation"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
http://www.mulesoft.org/schema/mule/json
http://www.mulesoft.org/schema/mule/json/current/mule-json.xsd
http://www.mulesoft.org/schema/mule/core
http://www.mulesoft.org/schema/mule/core/current/mule.xsd
http://www.mulesoft.org/schema/mule/http
http://www.mulesoft.org/schema/mule/http/current/mule-http.xsd
http://www.mulesoft.org/schema/mule/db
http://www.mulesoft.org/schema/mule/db/current/mule-db.xsd
http://www.mulesoft.org/schema/mule/ee/core
http://www.mulesoft.org/schema/mule/ee/core/current/mule-ee.xsd
http://www.mulesoft.org/schema/mule/java
http://www.mulesoft.org/schema/mule/java/current/mule-java.xsd
http://www.mulesoft.org/schema/mule/xml-module
http://www.mulesoft.org/schema/mule/xml-module/current/mule-xml-module.xsd
http://www.mulesoft.org/schema/mule/validation
http://www.mulesoft.org/schema/mule/validation/current/mule-validation.xsd">
<db:config name="Database_Config" doc:name="Database Config"
doc:id="d31ea473-7b58-4d28-83b3-bed7e0bebf2c" >
<db:oracle-connection host="localhost" user="system" password="system"
instance="xe" />
</db:config>
<http:listener-config name="HTTP_Listener_config" doc:name="HTTP Listener
config" doc:id="78fddb86-0942-4ccf-a300-2d721291c964" basePath="/emp_service"
>
<http:listener-connection host="0.0.0.0" port="8081" />
</http:listener-config>
<flow name="mule1Flow" doc:id="246fb555-ea4b-4b9f-bf2c-a332dc3a0ea1" >
<http:listener doc:name="Add_emp_Listener" doc:id="a9d8ff2a-00a7-4759-
8c0f-c53b9211e0e9" config-ref="HTTP_Listener_config" path="add_emp"
allowedMethods="POST"/>
<validation:custom-validator doc:name="validation"
class="com.poltu.validation.CustomValidator" />
<db:insert doc:name="Insert" doc:id="d44b5680-ae0a-4e97-8c91-
9baba4039e7e" config-ref="Database_Config">
<db:sql ><![CDATA[insert into emp_details
values(:emp_id,:emp_name,:emp_status)]]></db:sql>
<db:input-parameters ><![CDATA[#[{
"emp_id": payload.emp_id,
"emp_name" : payload.emp_name,
"emp_status" : payload.emp_status
}]]]></db:input-parameters>
</db:insert>
<ee:transform doc:name="Transform Message" doc:id="7e7be7f4-c2fc-43f9-
8b76-bf63bf8ac51b" >
<ee:message >
<ee:set-payload ><![CDATA[%dw 2.0
output application/json
---
{
"status":"OK",
"response":"Created re bhaii...."
}]]></ee:set-payload>
</ee:message>
</ee:transform>
<logger level="INFO" doc:name="Logger" doc:id="647043ee-5a28-414e-898e-
dce291114c2c" message="response payload is -- #[payload.response]"/>
</flow>
</mule>

Answer 1

The documentation that you are using is for Mule 3.9. That method is not compatible with Mule 4. To create a custom validator in Mule 4 you need to create a Mule Extension with the Mule SDK for Java. Follow the instructions to create the module. Once your module builds, add a validator operation as described at https://docs.mulesoft.com/mule-runtime/4.3/migration-module-validation#custom_validator