Designing a generic validation tool in Java

The task is to design a generic validation tool . It should be

• able to validate n different type of entities.
• Each entity has its own set of validation constraints.
• each Entity can be composed of zero or more other entities
• The validation operation will validate n or less than n entities every time, each validation will return a Validation result containing details of failure (if any),The final response should be a combined result of all the validations
• The sequence of validations might differ.
• The entity classes are already existing and cant be modified.

Is visitor design pattern a good choice here? I will have to create classes corresponding to each entity which will have "has-a" relationship with each entity and this class implements an interface with accept method. Is this too much convoluted logic?

I am using Java 8 , so is function composition a good choice here. Another option is having a Collection of validate functions for each entity and then composing them on the fly , and using it. Is this the right direction? Any other options that you guys can point me to?