《Java ConcurrentHashMap 源码：线程安全的底层逻辑》（122）

# 《Java ConcurrentHashMap 源码：线程安全的底层逻辑》 🔍

在Java并发编程中，`ConcurrentHashMap`是线程安全哈希表的典范实现，它通过精妙的设计在性能与安全性之间取得了完美平衡。让我们深入源码，探索其底层逻辑！🚀

## 分段锁与CAS的智慧 🧠

JDK7采用**分段锁(Segment)**设计，将整个Map划分为多个Segment，每个Segment独立加锁。这种设计使不同Segment的操作可以并行，大幅提升并发性能！⚡

JDK8则更进一步，抛弃分段锁，采用更精细的**CAS+synchronized**组合：
- 对单个Node使用`synchronized`锁定
- 结合CAS实现无锁化的size计数等操作
- 利用volatile保证内存可见性

## 扩容机制的艺术 🎨

`ConcurrentHashMap`的扩容是渐进式的：
1. 线程发现需要扩容时，会协助完成数据迁移
2. 通过`ForwardingNode`标记已迁移的桶
3. 多线程并行迁移不同区段的数据

这种设计避免了传统HashMap扩容时的全局阻塞，实现了**无感知扩容**！🌈

## 迭代器的弱一致性 🔄

`ConcurrentHashMap`的迭代器采用**弱一致性**策略：
- 不保证反映所有最新修改
- 但保证不会抛出`ConcurrentModificationException`
- 适合高并发场景下的遍历需求

这种设计避免了迭代时加锁，保证了并发性能！💪

`ConcurrentHashMap`的精妙设计展现了Java并发大师们的智慧，是学习并发编程的绝佳范例！📚 理解其底层逻辑，能帮助我们写出更高效、更安全的并发代码。
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