Hibernate注解多对多关系理解
我已经尝试了两种不同的休眠多对多关系的方法，但我真的找不到它们之间的确切区别。
第一种情况：
@Entity
public class Entity1 implements Serializable {
@ManyToMany
List<Entity2> entitiy2;
第二种情况：
@Entity
public class Entity1 implements Serializable {
@ManyToMany
List<Entity2> entitiy2 = new ArrayList<Entity2>();
I have tried 2 different things with hibernate many to many relationship, but i really couldn't find the exact difference between them.
First Scenario :
@Entity
public class Entity1 implements Serializable {
@ManyToMany
List<Entity2> entitiy2;
Second Scenario :
@Entity
public class Entity1 implements Serializable {
@ManyToMany
List<Entity2> entitiy2 = new ArrayList<Entity2>();
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评论（1）
对于 Hibernate 来说，没有区别。每次 Hibernate 加载 Entity1 时，其 Entity2 实例列表都将使用自定义非空 Hibernate 列表进行初始化（或在第二种情况下重新初始化）。
对于您自己的代码来说，它确实有所不同。对于第一种情况，每次创建 Entity1 实例时，它都会处于错误状态，Entity2 实例列表为 null 而不是空。每次您想要将新元素添加到列表中并迭代列表时（即使在单元测试中），您都必须先检查列表是否为空。第二种情况更好，因为它以良好的状态初始化对象，其中列表已准备好使用。
For Hibernate, there is no difference. Each time an Entity1 is loaded by Hibernate, its list of Entity2 instances will be initialized (or re-initialized in the second case) with a custom non-null Hibernate list.
For your own code, it does make a difference. With the first case, each time you create an Entity1 instance, it will be in a bad state, with a list of Entity2 instances that is null instead of being empty. Each time you'll want to add a new element to the list of iterate through the list (even in unit tests), you'll have to check before if the list is null or not. The second case is better, since it initializes the object in a good state, where the list is ready to be used.