我在休眠时看不到 JPA 中的日志

Question

I have already been setting property in persistence.xml

<property name="hibernate.show_sql" value="true" /> 

But! when i execute a source which has method to insert SQL and select SQL, I can't see logs about select SQL on console. Just I can see logs about insert SQL.

Why? this is right?

public class JpaMain
{

    public static void main(String[] args) {

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("jpabook");
        EntityManager em = emf.createEntityManager();
        EntityTransaction tx = em.getTransaction();

        try{
            tx.begin();
            System.out.println("###########save start#################");
            save(em);
            System.out.println("###########save end#################");
            System.out.println("###########find start#################");
            find(em);
            System.out.println("###########find end#################");

            tx.commit();

        }

        catch(Exception e) {
            e.printStackTrace();
            tx.rollback();
        }
        finally{
            em.close();
        }

        emf.close();
    }

    public static void save(EntityManager em) {
        Product productA = new Product();
        productA.setId("productA");
        productA.setName("pA");
        em.persist(productA);

        Member member1 = new Member();
        member1.setId("member1");
        member1.setUsername("m1");
        member1.getProducts().add(productA);
        em.persist(member1);
    }

    public static void find(EntityManager em) {
        for (Product product : em.find(Member.class, "member1").getProducts()) {
            System.out.println(product.getName());
        }
    }
}

Answer 1

When you execute find , there are different layers of caching that it will need to go through:

Persistence Context > Shared Cache > SQL query to the Database

Persistence Context can be considered a cache because it keeps references to all the managed entities.

If the entity exists in the persistence context , the managed instance is returned. If it does not exist or no persistence context had yet been associated with the entity manager, the entity manager goes to the factory to see if the shared cache has the entity instance. If it does, a new entity is created from the shared one and inserted into the persistence context, and the new managed instance is returned to the caller. If it is not in the shared cache, an SQL query is generated to select the entity from the database.

In your case, both persist and find operations have been invoked in the same transaction. So by persisting, your entities will become managed and they will be stored in the Persistence Context. After which, you executed find method and since your entities are already in the Persistence Context, it will no longer need to execute any SQL queries to the database. Entity object located in the Persistence Context will be returned.

This should explain why you're not getting any SELECT SQL logged.