The main goal of this project is to show how to use Spring Data Specification.

Reference: https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#specifications

JPA 2 introduces a criteria API that you can use to build queries programmatically. By writing a criteria, you define the where clause of a query for a domain class. Taking another step back, these criteria can be regarded as a predicate over the entity that is described by the JPA criteria API constraints.

For Criteria API please refer my other project: https://github.com/mtumilowicz/jpa-criteria-api

1. To support specifications, you can extend your repository interface with the JpaSpecificationExecutor interface, as follows:

   @Repository
   public interface EmployeeRepository extends JpaRepository<Employee, Integer>, JpaSpecificationExecutor<Employee> {
   
   }
   

   The additional interface has methods that let you execute specifications in a variety of ways:

   public interface JpaSpecificationExecutor<T> {
       Optional<T> findOne(@Nullable Specification<T> var1);
   
       List<T> findAll(@Nullable Specification<T> var1);
   
       Page<T> findAll(@Nullable Specification<T> var1, Pageable var2);
   
       List<T> findAll(@Nullable Specification<T> var1, Sort var2);
   
       long count(@Nullable Specification<T> var1);
   }
   

2. The Specification interface is defined as follows:

   public interface Specification<T> {
     Predicate toPredicate(Root<T> root, CriteriaQuery<?> query,
               CriteriaBuilder builder);
   }
   

   so exemplary implementation (using Java 8):

   public final class EmployeeSpecifications {
       public static Specification<Employee> hasName(@NonNull String name) {
           return (Root<Employee> root, CriteriaQuery<?> query, CriteriaBuilder cb)
                   -> cb.equal(root.get(Employee_.name), name);
       }
   
       public static Specification<Employee> hasAge(int age) {
           return (Root<Employee> root, CriteriaQuery<?> query, CriteriaBuilder cb)
                   -> cb.equal(root.get(Employee_.age), age);
       }
   }
   

3. The power of specifications really shines when you combine them to create new Specification objects:

   • or

   @Test
   public void or() {
       List<Employee> employees = repository.findAll(hasName("Tumilowicz").or(hasName("Mrozek")));
   
       assertThat(employees, hasSize(2));
   }    
   

   • and

   @Test
   public void and() {
       List<Employee> employees = repository.findAll(hasName("Tumilowicz").and(hasAge(29)));
   
       assertThat(employees, hasSize(1));
   }