The difference between fetchtype lazy and eagle in JPA

Suppose there is 2 Entity , One is University, The other is Student, There are many students in the University , So their relationship is 1 For more than .

University Entities may have some basic attributes , for example id、name、address etc. , And one called students Set properties of , This attribute returns a list of students from a given University ：

University class

public class University {
    
   private String id;
   private String name;
   private String address;
   private List<Student> students;

   // setters and getters
}

Now? , When you load universities from the database ,JPA Will load it for you id、name and address Field . But for how to load students , You have two options ：

1. Load it with the rest of the fields （ That is, eagerly eager）, or
2. When you call the University getStudents() Methods on demand （ That is, lazily lazy） Load it .

When a university has many students , It is inefficient to load all students together , Especially when they don't need it , under these circumstances , You can declare that you want students to load when they actually need . This is called Delay loading .

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This is an example , among students Clearly marked as urgent eager load ：

@Entity
public class University {
    

    @Id
    private String id;

    private String name;

    private String address;

    @OneToMany(fetch = FetchType.EAGER)
    private List<Student> students;

    // etc. 
}

This is a students Clearly marked as delayed lazy Loaded examples ：

@Entity
public class University {
    

    @Id
    private String id;

    private String name;

    private String address;

    @OneToMany(fetch = FetchType.LAZY)
    private List<Student> students;

    // etc.
}

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source ：
hibernate - Difference between FetchType LAZY and EAGER in Java Persistence API? - Stack Overflow