Criteria Api Vs QueryDsl Vs JPA 元模型

Question

I am a bit confused about these three concepts.

• Criteria API
• Query Dsl
• Jpa 2.0 meta model

From what I have read, One of the major benifits of using QueryDsl or JPA metamodel is type safety.
But I can achieve type safety even with Criteria API's. (I am using JPA with eclipselink)

javax.persistence.EntityManager has two variants

public Query createQuery(String sqlString);   
public <T> TypedQuery<T> createQuery(CriteriaQuery<T> criteriaQuery); 

I agree with first version where I pass sql as string I don't get type safety. But with the second version I get type safety. Or am I missing something here? Can someone explain with an example how using criteria is not type safe.

What is the difference between QueryDsl and JPA static meta model ?

Answer 1

The syntax you provided is JPQL not a Criteria Api. JPQL has query types @Query, @TypedQuery, @NamedQuery and they are written in plain JPQL which is error prone. We have also @NativeQuery which should be avoided if possible.
There is other more safe options like Criteria Api or QueryDSL for querying only.

Criteria Api can be used with string based attributes

CriteriaQuery<Employee> query = cb.createQuery(Employee.class);
Root<Employee> employee = query.from(Employee.class);
 query.select(employee)
      .where(cb.equal(employee.get("dept"), "Admin"));

Or we can reference attributes using metamodel class generated by Jpa metamodel

Root<Employee> employee = query.from(Employee.class);
query.select(employee)
     .where(cb.equal(employee.get(Employee_.dept), "Admin"));

And there is QueryDSL which is more intuitive(IMHO) alternative to Criteria API.

QEmployee employee = QEmployee.employee;
query.from(employee).where(employee.dept.eq("Admin"))

Answer 2

您可以在 Criteria API 中使用 JPA 元模型来确保类型安全，但与 QueryDSL 相比，criteria api 相当复杂