Difference between FetchType LAZY and EAGER in Java Persistence API?

In the Java Persistence API (JPA), the FetchType enum is used to specify the strategy for fetching data from the database. There are two values of FetchType: LAZY and EAGER.

The LAZY fetch type specifies that the data should be fetched lazily, which means that the data is fetched when it is needed. This can be more efficient in cases where the data is not needed immediately, because it avoids the overhead of fetching the data upfront.

The EAGER fetch type specifies that the data should be fetched eagerly, which means that the data is fetched when the parent entity is fetched. This can be more efficient in cases where the data is needed immediately, because it avoids the overhead of fetching the data later.

You can use the FetchType enum with the @OneToOne, @OneToMany, and @ManyToMany annotations to specify the fetching strategy for relationships between entities. For example:

@Entity
public class Post {
  @OneToMany(fetch = FetchType.LAZY)
  private List<Comment> comments;
}

In this example, the comments field is a List of Comment entities that is fetched lazily. This means that the Comment entities will be fetched when they are needed, rather than when the Post entity is fetched.

It's important to note that the FetchType only affects the fetching strategy, and does not guarantee when the data is actually fetched. The JPA implementation may choose to ignore the fetch type and fetch the data at a different time, based on its own optimization strategies.

I hope this helps! Let me know if you have any questions.