《Java ConcurrentHashMap 源码：线程安全的底层逻辑》

# 《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✨。它通过精妙的设计实现了高并发下的高性能，让我们深入探究其底层逻辑🔧。

## 分段锁与CAS的完美结合 🧩

不同于`Hashtable`的全局锁，`ConcurrentHashMap`在JDK7采用**分段锁(Segment)**设计，将数据分成多个段(默认16个)，每个段独立加锁🔒。这种设计使不同段的操作可以并行，大幅提升吞吐量🚀。

JDK8后改为更先进的**CAS+synchronized**方案：
- 使用`Node`数组+链表/红黑树结构🌳
- 对数组元素(`Node`)使用`synchronized`细粒度锁
- 结合`volatile`和CAS保证原子性⚡

## 关键源码解析 📖

```java
final V putVal(K key, V value, boolean onlyIfAbsent) {
  if (key == null || value == null) throw new NullPointerException();
  int hash = spread(key.hashCode());
  // ... 使用CAS处理并发插入
}
```

`spread()`方法通过再哈希减少碰撞，`putVal()`中：
1. 先尝试无锁CAS插入🔁
2. 冲突时对链表头节点加`synchronized`锁
3. 链表转红黑树时使用更严格的同步控制

## 并发扩容机制 🚧

扩容时采用多线程协同工作：
- 每个线程处理一定数量的桶(bucket)
- 通过`ForwardingNode`标记迁移状态
- 其他线程遇到迁移中的桶会协助迁移🤝

这种设计避免了传统扩容时的长时间停顿，实现平滑扩容。

`ConcurrentHashMap`的精妙设计展示了Java并发编程的艺术🎨，理解其原理对编写高性能并发代码至关重要！💡
https://www.nowcoder.com/discuss/822602139410186240
https://www.nowcoder.com/discuss/822602079117004800
https://www.nowcoder.com/discuss/822602029615837184
https://www.nowcoder.com/discuss/822601972451659776
https://www.nowcoder.com/discuss/822601920521981952
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https://www.nowcoder.com/discuss/822601716511072256
https://www.nowcoder.com/discuss/822601649788084224
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https://www.nowcoder.com/discuss/822602336206860288
https://www.nowcoder.com/discuss/822602263062446080
https://www.nowcoder.com/discuss/822602217558405120
https://www.nowcoder.com/discuss/822602139410186240
https://www.nowcoder.com/discuss/822602079117004800
https://www.nowcoder.com/discuss/822602029615837184
https://www.nowcoder.com/discuss/822601972451659776
https://www.nowcoder.com/discuss/822601920521981952
https://www.nowcoder.com/discuss/822601855384485888
https://www.nowcoder.com/discuss/822601788489494528
https://www.nowcoder.com/discuss/822601716511072256
https://www.nowcoder.com/discuss/822601649788084224
https://www.nowcoder.com/discuss/822601589880795136
https://www.nowcoder.com/discuss/822601521090068480
https://www.nowcoder.com/discuss/822601472650006528
https://www.nowcoder.com/discuss/822602336206860288
https://www.nowcoder.com/discuss/822602263062446080
https://www.nowcoder.com/discuss/822602217558405120
https://www.nowcoder.com/discuss/822602139410186240
https://www.nowcoder.com/discuss/822602079117004800
https://www.nowcoder.com/discuss/822602029615837184
https://www.nowcoder.com/discuss/822601972451659776
https://www.nowcoder.com/discuss/822601920521981952
https://www.nowcoder.com/discuss/822601855384485888
https://www.nowcoder.com/discuss/822601788489494528
https://www.nowcoder.com/discuss/822601716511072256
https://www.nowcoder.com/discuss/822601649788084224
https://www.nowcoder.com/discuss/822601589880795136
https://www.nowcoder.com/discuss/822601521090068480
https://www.nowcoder.com/discuss/822601472650006528
https://www.nowcoder.com/discuss/822602336206860288
https://www.nowcoder.com/discuss/822602263062446080
https://www.nowcoder.com/discuss/822602217558405120
https://www.nowcoder.com/discuss/822602139410186240
https://www.nowcoder.com/discuss/822602079117004800
https://www.nowcoder.com/discuss/822602029615837184
https://www.nowcoder.com/discuss/822601972451659776
https://www.nowcoder.com/discuss/822601920521981952
https://www.nowcoder.com/discuss/822601855384485888
https://www.nowcoder.com/discuss/822601788489494528
https://www.nowcoder.com/discuss/822601716511072256
https://www.nowcoder.com/discuss/822601649788084224
https://www.nowcoder.com/discuss/822601589880795136
https://www.nowcoder.com/discuss/822601521090068480
https://www.nowcoder.com/discuss/822601472650006528
https://www.nowcoder.com/discuss/822602336206860288
https://www.nowcoder.com/discuss/822602263062446080
https://www.nowcoder.com/discuss/822602217558405120
https://www.nowcoder.com/discuss/822602139410186240
https://www.nowcoder.com/discuss/822602079117004800
https://www.nowcoder.com/discuss/822602029615837184
https://www.nowcoder.com/discuss/822601972451659776
https://www.nowcoder.com/discuss/822601920521981952
https://www.nowcoder.com/discuss/822601855384485888
https://www.nowcoder.com/discuss/822601788489494528
https://www.nowcoder.com/discuss/822601716511072256
https://www.nowcoder.com/discuss/822601649788084224
https://www.nowcoder.com/discuss/822601589880795136
https://www.nowcoder.com/discuss/822601521090068480
https://www.nowcoder.com/discuss/822601472650006528
https://www.nowcoder.com/discuss/822602336206860288
https://www.nowcoder.com/discuss/822602263062446080
https://www.nowcoder.com/discuss/822602217558405120
https://www.nowcoder.com/discuss/822602139410186240
https://www.nowcoder.com/discuss/822602079117004800
https://www.nowcoder.com/discuss/822602029615837184
https://www.nowcoder.com/discuss/822601972451659776
https://www.nowcoder.com/discuss/822601920521981952