How to use single JPA specification class and methods for multiple entities

Question

I am creating a Spring Boot application having entities like Product, Category, Machinery, UsageLocation etc.. Thing that is common in all these entities is that they all have a String attribute called name and can be filtered from UI using name. I have written a specification for product to filter using name and it is working. Below is the code

public final class ProductSpecifications 
{

    public static Specification<Product> whereNameContains(String name)
    {
        Specification<Product> finalSpec = (Root<Product> root, CriteriaQuery<?> query, CriteriaBuilder cb)
            -> cb.like(root.get(Product_.PRODUCT_NAME), "%"+name+"%");
        return finalSpec;
    }

    public static Specification<Product> whereNameEqauls(String name)
    {
        Specification<Product> finalSpec = (Root<Product> root, CriteriaQuery<?> query, CriteriaBuilder cb)
            -> cb.equal(root.get(Product_.PRODUCT_NAME), name);
        return finalSpec;
    }
}

Now problem is that I have to write same code again for filtering other entities with only difference being the class name(Product), field name(PRODUCT_NAME) and return type of method. Can I create a generic class and method to which I can pass class name and field name as parameters and it returns specification of respective return type.

Answer 1

I was able to solve this issue using Abinash's answer. Below is the working code for reusable specification class

import java.util.List;

import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;
import org.springframework.data.jpa.domain.Specification;
import org.springframework.stereotype.Service;

import com.ec.application.model.Product;
import com.ec.common.Filters.FilterAttributeData;
import com.ec.common.Filters.FilterDataList;

public class SpecificationsBuilder<T>
{

      //#######################################/#################//
     //      Level 0 - If the field that you want to query is parent entity               //
    //########################################################//    

    public Specification<T> whereDirectFieldContains(String key,List<String> names)
    {
        Specification<T> finalSpec = null;
        for(String name:names)
        {
            Specification<T> internalSpec = (Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb)
                    -> cb.like(root.get(key), "%"+ name  +"%");
            finalSpec  = specOrCondition(finalSpec,internalSpec); // to append specifications as I am filtering based on list of strings
        }
        return finalSpec;
    }

      //#######################################//
     //     Level 1 - If you want to query a child entity.          //                //
    //#######################################//

    public Specification<T> whereChildFieldContains(String childTable, String childFiledName,
            List<String> names) 
    {
        Specification<T> finalSpec = null;
        for(String name:names)
        {
            Specification<T> internalSpec = (Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb)
                    -> cb.like(root.get(childTable).get(childFiledName), "%"+ name  +"%"); 
            finalSpec  = specOrCondition(finalSpec,internalSpec);  // to append specifications as I am filtering based on list of strings
        }
        return finalSpec;
    }

      //#######################################//
     //     Reusable Spec Setter to handle  NULLs.               //
    //#######################################//

    public Specification<T> specAndCondition(Specification<T> finalSpec, Specification<T> internalSpec) 
    {
        if(finalSpec == null) return internalSpec;
        else return finalSpec.and(internalSpec);
    }

    public Specification<T> specOrCondition(Specification<T> finalSpec, Specification<T> internalSpec) 
    {
        if(finalSpec == null) return internalSpec;
        else return finalSpec.or(internalSpec);
    } 
}

And this is the code for entity specification class

public static Specification<Product> getSpecification(FilterDataList filterDataList)
    {
        List<String> productNames = specbldr.fetchValueFromFilterList(filterDataList,"product");
        List<String> categoryNames = specbldr.fetchValueFromFilterList(filterDataList,"category");
        Specification<Product> finalSpec = null;
        if(productNames != null && productNames.size()>0)
            finalSpec = specbldr.specAndCondition(finalSpec, specbldr.whereDirectFieldContains(Product_.PRODUCT_NAME, productNames));

        if(categoryNames != null && categoryNames.size()>0)
        {
            finalSpec = specbldr.specAndCondition(finalSpec,
                    specbldr.whereChildFieldContains(Product_.CATEGORY,Category_.CATEGORY_NAME, categoryNames));
        }   
        return finalSpec;   
    }

Answer 2

First make your SpecificationsBuilder generic

@Service
public final class SpecificationsBuilder<T>
{

    public static Specification<T> whereNameContains(String key,String name)
    {
        Specification<T> finalSpec = (Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb)
            -> cb.like(root.get(key), "%"+name+"%");
        return finalSpec;
    }
}

Then in controller @Autowire the SpecificationsBuilder

@Autowire
private final SpecificationBuilder<Product> specificationBuilder;

public List<Product> getAll(String name) {
    Specification<Product> specification =
        specificationBuilder.whereNameContains(Product_.PRODUCT_NAME, name);
    List<Product> products = productRepo.findAll(specification);// pass the specifications
    return products;
  }

You can create your own generic library for SpecificationsBuilder, I have one. You can find details here