UML中关联、聚合和合成

摘自: sangmado - UML中关联（Association）、聚合（Aggregation）和合成（Composition）之间的区别

元素名称	符号图例	含义
Association		A 和 B 相互调用和访问对方的元素。 A and B call and access each other’s elements.
Aggregation		A 中拥有一个 B，但 B 脱离于 A 仍然可以独立存活。 A has a B, and B can outlive A. A "uses" B = Aggregation : B exists independently (conceptually) from A.
Composition		A 中拥有一个 B，B 脱离 A 后在系统中没有任何存活的意义。 A has a B, and B depends on A. A "owns" B = Composition : B has no meaning or purpose in the system without A.

我们可以从不同的角度来理解和区分这三种关系：

	Association	Aggregation	Composition
Owner	No owner	Single owner	Single owner
Lifetime	Have their own lifetime	Have their own lifetime	Owner's lifetime
Child Object	Child objects all are independent	Child objects belong to a single parent	Child objects belong to single parent

所以，总结来说，聚合（Aggregation）是一种特殊的关联（Association），合成（Composition）是一种特殊的聚合（Aggregation）。

Association->Aggregation->Composition