java - ConcurrentHashMap.Segment 是一个 ReentrantLock？

Question

关于ConcurrentHashMap.Segment的设计在 Java 1.6 集合库中:

  static final class Segment<K,V> extends ReentrantLock implements Serializable

恕我直言，给定 Segment不是 ReentrantLock , 那么为什么这个 extends ？它应该是一个组合:

static final class Segment<K,V> implements Serializable {
ReentrantLock lock = ...
}

Answer 1

javadoc 说:

Segments are specialized versions of hash tables. This subclasses from ReentrantLock opportunistically, just to simplify some locking and avoid separate construction.

鉴于它是一个非公共(public)内部类，我猜他们在性能、易用性和良好设计之间做出了权衡。