Spring Web服务器异步调用共享资源同步失效问题排查

咱们先梳理下你遇到的情况：你有个SpringBoot Web服务，里面有个操作单例状态的路由，为了保证多调用方访问时的线程安全，你给set()方法加了Kotlin的@Synchronized注解，同时之前还尝试过用ReentrantLock，相关代码还没移除。但现在日志里出现了多次连续的“锁定”日志，之后才批量出现“释放”日志，和你期望的“锁定-释放”交替出现的情况不符。

你的核心代码 #

@Synchronized public fun <T> set(t: T) { 
    val incomingState = State.from(t) 
    if(incomingState == innerState){ return } 
    change(incomingState) 
} 

@Synchronized private fun change(newState: State){ 
    this.stateAccessLock.lock() 
    SystemLog.logResourceLocking("HomeState", isLocked = true) 
    SystemLog.logHomeEvent("Changing HomeState from '${innerState.name}' to '${newState.name}'") 
    try { 
        Home.INSTANCE.EXTENSION.broadcastStateChange( innerState?.name, newState.name ) 
    } finally { 
        innerState = newState 
        Home.save() 
        stateAccessLock.unlock() 
        SystemLog.logResourceLocking("HomeState", isLocked = false) 
    } 
}

// ReentrantLock相关代码
private var stateAccessLock = ReentrantLock() 
@Autowired fun HomeState(@Qualifier("stateAccessLock") stateAccessLock: ReentrantLock) { 
    this.stateAccessLock = stateAccessLock 
}

日志异常情况 #

实际日志（不符合预期）：

[2020-10-09T16:07:01.709] LOCK: Resource locked
[2020-10-09T16:07:01.710] LOCK: Resource locked
[2020-10-09T16:07:01.725] LOCK: Resource locked
[2020-10-09T16:07:01.727] LOCK: Resource locked
[2020-10-09T16:07:01.818] LOCK: Resource unlocked
[2020-10-09T16:07:01.828] LOCK: Resource unlocked
[2020-10-09T16:07:01.829] LOCK: Resource unlocked
[2020-10-09T16:07:01.839] LOCK: Resource unlocked

期望日志：

[2020-10-09T16:07:01.709] LOCK: Resource locked
[2020-10-09T16:07:01.710] LOCK: Resource unlocked
[2020-10-09T16:07:01.725] LOCK: Resource locked
[2020-10-09T16:07:01.727] LOCK: Resource unlocked
...

问题根源：同步机制混用 + 锁实例不唯一 #

你忽略了两个关键问题：

1. 同时混用两种独立的同步机制
   Kotlin的@Synchronized注解本质是给方法加了对象级别的内置锁（和Java的synchronized方法一致），而你又在change()里手动加了ReentrantLock的独占锁。这两种锁是完全独立的，互相不会感知对方的状态。

   • 如果HomeState是多实例的，每个实例的@Synchronized锁是各自独立的，多个线程可以同时进入不同实例的set()方法，然后各自去抢自己的ReentrantLock（如果锁实例不唯一的话），导致多个线程同时成功锁定，日志连续输出“locked”。

2. ReentrantLock实例可能不是单例
   从你的注入代码来看，如果Spring容器中stateAccessLock这个bean没有被定义为单例（比如误设为原型scope），那么每个HomeState实例都会拿到一个独立的ReentrantLock对象。这种情况下，多个线程操作不同HomeState实例时，各自的锁互不干扰，自然能同时锁定，产生连续的锁定日志。

解决方案：二选一，统一同步机制 #

你只需要选择一种同步机制即可，不需要同时使用：

方案1：只用@Synchronized注解（移除ReentrantLock）

既然@Synchronized已经能保证同一时间只有一个线程进入set()和change()方法（只要HomeState是单例），就不需要额外的ReentrantLock了。修改后代码如下：

@Synchronized public fun <T> set(t: T) { 
    val incomingState = State.from(t) 
    if(incomingState == innerState){ return } 
    change(incomingState) 
} 

@Synchronized private fun change(newState: State){ 
    SystemLog.logResourceLocking("HomeState", isLocked = true) 
    SystemLog.logHomeEvent("Changing HomeState from '${innerState.name}' to '${newState.name}'") 
    try { 
        Home.INSTANCE.EXTENSION.broadcastStateChange( innerState?.name, newState.name ) 
    } finally { 
        innerState = newState 
        Home.save() 
        SystemLog.logResourceLocking("HomeState", isLocked = false) 
    } 
}

方案2：只用ReentrantLock（移除@Synchronized）

确保stateAccessLock是单例bean，然后在set()方法中统一加锁，避免重复加锁：

// 确保stateAccessLock是单例bean
@Bean("stateAccessLock")
fun stateAccessLock(): ReentrantLock {
    return ReentrantLock()
}

// HomeState类代码
private lateinit var stateAccessLock: ReentrantLock

@Autowired fun HomeState(@Qualifier("stateAccessLock") stateAccessLock: ReentrantLock) { 
    this.stateAccessLock = stateAccessLock 
}

public fun <T> set(t: T) { 
    stateAccessLock.lock()
    try {
        val incomingState = State.from(t) 
        if(incomingState == innerState){ return } 
        change(incomingState) 
    } finally {
        stateAccessLock.unlock()
    }
} 

private fun change(newState: State){ 
    SystemLog.logResourceLocking("HomeState", isLocked = true) 
    SystemLog.logHomeEvent("Changing HomeState from '${innerState.name}' to '${newState.name}'") 
    try { 
        Home.INSTANCE.EXTENSION.broadcastStateChange( innerState?.name, newState.name ) 
    } finally { 
        innerState = newState 
        Home.save() 
        SystemLog.logResourceLocking("HomeState", isLocked = false) 
    } 
}

内容的提问来源于stack exchange，提问作者Blatzheim