与 Eager 相比，使用延迟初始化有什么好处吗？

Question

When to use lazy initialization and when Eager as even if we do lazy then the variable which we will use will still contain some data associated with the following task. So how do we choose between both? Are there any performance and memory efficient differences between the both?

Please explain any best use case as some threads are supporting eager and some lazy.

Answer 1

In general, the advantages of lazy loading are:

• If you never need the value, you don't pay any speed or memory penalty in loading or storing it.
• Start-up is faster.

And the disadvantages are:

• The first time you need the value, you have to wait while it's loaded.
• There's often a small overhead associated with accessing the field in a thread-safe way. (This is needed because you don't usually want two different threads doing the loading twice; and in any case you don't want any thread to see partly-loaded data.)

In other languages another disadvantage is extra code to implement that last bit, but Kotlin's by lazy makes it ridiculously easy!

Obviously the overall benefit depends on how likely you are to need the value at all, and the impact of a delay at start-up compared to later on.

Answer 2

The first thing that we should discuss here is what lazy loading and eager loading are:

Eager Loading is a design pattern in which data initialization occurs on the spot Lazy Loading is a design pattern which is used to defer initialization of an object as long as it's possible Let's see how this actually works with some examples:

The UserLazy class:

@Entity
@Table(name = "USER")
public class UserLazy implements Serializable {
 
    @Id
    @GeneratedValue
    @Column(name = "USER_ID")
    private Long userId;
 
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "user")
    private Set<OrderDetail> orderDetail = new HashSet();
 
    // standard setters and getters
    // also override equals and hashcode
 
}

The OrderDetail class:

@Entity
@Table (name = "USER_ORDER")
public class OrderDetail implements Serializable {
    
    @Id
    @GeneratedValue
    @Column(name="ORDER_ID")
    private Long orderId;
    
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name="USER_ID")
    private UserLazy user;
 
    // standard setters and getters
    // also override equals and hashcode
 
}

One User can have multiple OrderDetails. In eager loading strategy, if we load the User data, it will also load up all orders associated with it and will store it in a memory.

But, when lazy loading is enabled, if we pull up a UserLazy, OrderDetail data won't be initialized and loaded into a memory until an explicit call is made to it.

In the next section we will see how the above example is implemented in Hibernate.

Find the article here: https://www.baeldung.com/hibernate-lazy-eager-loading

And read this also: https://www.imperva.com/learn/performance/lazy-loading/