synchronized 可重入机制
一、一句话核心
同一个线程可以多次获取同一把锁，不会死锁。
- 原理：锁自带计数器，加锁+1，释放-1，归0才真正释放
- 类比：一把家门钥匙能开家里所有房间门
二、为什么必须有可重入？
没有可重入，下面代码会直接死锁：
java
public synchronized void a() { b(); } // 拿了锁调用b()
public synchronized void b() {} // 再拿同一把锁，自己等自己三、三种锁的可重入实现对比
| 锁状态 | 计数器在哪 | 怎么判断是同一个线程 |
|---|---|---|
| 偏向锁 | 对象头Mark Word | 检查里面存的线程ID |
| 轻量级锁 | 线程栈的Lock Record链表 | 检查锁记录的所有者 |
| 重量级锁 | Monitor对象的count字段 |
检查Monitor的owner |
四、重量级锁（最常考）实现
Monitor核心结构
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owner: 谁拿着锁
count: 重入次数
执行流程
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否
是
否
是
否
线程拿锁
owner是自己?
count+1
owner是空?
owner=自己, count=1
排队等
执行代码
释放: count-1
count=0?
owner清空, 唤醒排队线程
锁还在自己手里
五、轻量级锁实现
用栈里多个锁记录代替计数器：
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锁记录3
锁记录2
锁记录1 ← 对象头指向这里
- 重入1次，栈里多1个锁记录
- 释放1次，栈里弹出1个锁记录
- 全弹完，锁真正释放
六、常见误区
| 误区 | 正确结论 |
|---|---|
| 不同线程也能重入 | 只允许同一个线程 |
| 静态和普通方法能互相重入 | 锁的对象不同，不能 |
| 重入次数无限 | 最多2^31-1次，超了报错 |
七、关键总结
| 特性 | 说明 |
|---|---|
| 核心目的 | 避免自己把自己卡死 |
| 性能影响 | 几乎为0，只是计数器加减 |
| 释放规则 | 加锁多少次，必须释放多少次 |