How does Hibernate implement lazy loading and greedy loading?

Hibernate provides lazy loading and greedy loading strategies for managing object and database interactions. Lazy loading loads associated objects lazily, while greedy loading loads them immediately. When choosing a strategy, consider performance and usage scenarios. Lazy loading reduces database queries and improves performance; greedy loading increases initial loading time but avoids additional queries.

Hibernate's lazy loading and greedy loading

Introduction

Hibernate is a An object-oriented persistence framework that implements database access by mapping objects to database tables. Lazy loading and greedy loading are two mechanisms used by Hibernate to manage the interaction between objects and the database.

Lazy loading

Lazy loading is a lazy loading strategy. Under this strategy, Hibernate sends a query to the database to load the associated object only when it is needed. Therefore, in most cases, unnecessary database queries can be avoided, thus improving performance.

Code example:

// 假设 User 类有 Set<Order> orders 属性
public User {
    // 延迟加载关联列表，仅在访问时加载
    @OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
    private Set<Order> orders;
}

Greedy loading

Greedy loading is an immediate loading strategy. Under this strategy, Hibernate loads all associated objects immediately when loading the parent object. While this increases initial load time, it avoids sending extra database queries when using related objects.

Code example:

// 假设 User 类有 Set<Order> orders 属性
public User {
    // 立即加载关联列表
    @OneToMany(mappedBy = "user", fetch = FetchType.EAGER)
    private Set<Order> orders;
}

Practical case

Suppose there is a user interface that needs to display the user’s details information and its order information.

• Use lazy loading: If you use the lazy loading strategy, the order will not be retrieved when the user object is loaded. Only when the user clicks on the order details, a query is sent to load the order. This avoids unnecessary database queries when the page loads.
• Use greedy loading: If you use the greedy loading strategy, the order will be loaded immediately when the user object is loaded. While this increases page load time, it avoids sending additional database queries when the user clicks on order details.

Choose the right strategy

When choosing a lazy loading and greedy loading strategy, you need to consider the following factors:

• Performance: Lazy loading reduces database queries, thereby improving performance, while greedy loading increases initial load time.
• Usage scenarios: If the associated object is rarely used, lazy loading is appropriate. Greedy loading can improve performance if the associated object is used frequently.
• eager attribute: In the hibernate.cfg.xml configuration file, you can use the EagerLoading attribute to set the global loading strategy. The default is lazy loading.

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