Ytukyg

I know what is DTO: An object that carries data between processes in order to reduce the number of method calls.
and I know what is lazy-loading in hibernate.

I read this sentences in "Full Stack Development with JHipster" book :
JHipster uses DTO (Data Transfer Object) and VM (View Model) on the server side. DTOs are for transferring data from the service layer to and from the resource layer. They break the Hibernate transactions and avoids further lazy loading from being triggered by the resource layer.

I don't understand the relationship between DTO and lazy loading.

If you pass Hibernate managed persistent entities to the resource layer with LazyLoading enabled, the resource layer will invoke get-methods in order to read their properties some of which are possibly not initialized at that time. Hibernate will then lazily load the data of that particular property from the persisntet store, sending an SQL statement (possibly each time starting and committing a new transaction) and waiting for response which is quite slow. If this happens hundred of times, each time having to initialize another property, each time triggering a lazy load, the user has to wait...

If you prepare a DTO in your service layer, containing all the data relevant to the resource layer, read access to that DTO is highly efficient without the risk of hitting the database for any get-method invokation as everything relevant is in memory already. No subsequent lazy load will be triggered.

Lazy loading is for entities, not DTOs.

A JPA entity can be represented as a POJO or a Proxy.

As I explained in this article, using EntityMnager.find gives you a POJO:

Post post = entityManager.find(Post.class, postId);

While the EtityManager.getReference method gives you a Proxy:

Post post = entityManager.getReference(Post.class, postId);

The POJO has its basic properties initialized because a SELECT statement was executed to fetch the entity. The Proxy does not hit the database upon creation. Only the id is set based on the provided entity identifier. Only you access the Proxy properties, a SELECT statement will be executed.

Proxies are also used for collections (e.g. @OneToMany or @ManyToMany) which are using the FetchType.LAZY strategy by default. Once you access the LAZY collection, a SELECT statement will be executed to fetch the associated collection.

Now, a DTO is based on a projection, hence a SELECT statement is executed prior to populating the DTO. For this purpose, you can say that the DTO is eagerly loaded every time.

DTOs are much more efficient than entities for read-only projections because you load just the table columns that you explicitly requested. For more details about the best way to use DTOs with JPA and Hibernate, check out this article.

In Simple terms: If you call getter of a lazy loading marked field, Hibernate will make a db query for that data.

This is one of the reasons you should not return Entities as backend response... any JSON conversion(Serialization)/getter call will trigger unwanted loading of the data.

whereas a DTO is made specifically to transfer data as per use and you create DTO from these Entities (Multiple DTO can be created from one Entity) and pick only required data fields

Eg.
User entity: has user details and list of friends
UserDetailsDTO: would only require the details and dont need 1000's of friends mapped to the user... and their friends...
UserFriendsDTO: we can selectively get the friends names or ids for this DTO