Lazy Loading vs. Eager Loading

Lazy Loading and eager loading are both strategies for managing how data is fetched from a database or API in application development, particularly in System design. Lazy Loading delays loading related data until it's actually accessed, while Eager Loading retrieves all the necessary data in a single query.

Lazy Loading

Lazy Loading is a strategy where related data is loaded only when it is accessed for the first time.

• Default for @OneToMany and @ManyToMany relationships
• Uses proxy objects internally
• Reduces initial database load

Example:

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    private String name;

    @OneToMany(mappedBy = "department", fetch = FetchType.LAZY)
    private List<Employee> employees;
}

Explanation: fetch = FetchType.LAZY tells Hibernate not to load employees until getEmployees() is called.

Eager Loading

Eager Loading is a strategy where related data is fetched immediately along with the parent entity.

• Default for @ManyToOne and @OneToOne
• Data is always available after loading the entity

Example:

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    private String name;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "department_id")
    private Department department;
}

Explanation: fetch = FetchType.EAGER ensures that department is loaded immediately with the Employee object.

Difference Between Lazy Loading and Eager Loading