Kotlin读写分离CopyOnWriteArrayList

以你之姓@ 2023-10-14 15:58 7阅读 0赞

### Kotlin读写分离CopyOnWriteArrayList ###

基于读写分离思想Copy-On-Write(COW)设计的线程安全ArrayList变体，读读共享、写写互斥、读写互斥、写读互斥。读时直接读，不用加锁同步，线程安全。写/删/修改数据时复制一个副本，在新的List副本中写/删/改操作，写/删/改操作完成后，再把副本替换成原来的List。一般情况可当做普通ArrayList使用，规避List常见的ConcurrentModificationException异常问题。

![5e225e733a454d31a7f62049eae17840.png][]

优点：适合读多写少的场景，线程安全，读时候无须加锁（因为内部实现已经线程同步）。读写时候不会被阻塞。

缺点：（1）**内存占用问题。**占用内存高。因为在写操作时候，会复制一条相同的List，相当于每次写操作都会产生两倍内存占用需求。

（2）**数据一致性问题。**CopyOnWriteArrayList只能保证数据的最终一致性，不能保证实时一致性。

（3）**数写速度问题。**读时候快，写/删/改慢。

用普通ArrayList，多线程环境下抛错ConcurrentModificationException：

import java.util.Collections
    import java.util.concurrent.Executors
    
    fun main() {
        val nThreads = 200
        val mExecutorService = Executors.newFixedThreadPool(nThreads)
    
        var NUM = 0
        var list = Collections.synchronizedList(ArrayList<Int>())
        for (i in 1..nThreads) {
            mExecutorService.execute {
                while (true) {
                    Thread.sleep(10)
                    list.add(NUM++)
    
                    println("@tid:${Thread.currentThread().threadId()}")
                    list.forEachIndexed { index, j ->
                        //println("${list}")
                    }
                }
            }
        }
    
        mExecutorService.shutdown()
    }

![16866883ffbb493f9c66864c8d6ab910.png][]

改用CopyOnWriteArrayList，运行正常：

import java.util.concurrent.CopyOnWriteArrayList
    import java.util.concurrent.Executors
    
    fun main() {
        val nThreads = 200
        val mExecutorService = Executors.newFixedThreadPool(nThreads)
    
        var NUM = 0
        var list = CopyOnWriteArrayList<Int>()
        for (i in 1..nThreads) {
            mExecutorService.execute {
                while (true) {
                    Thread.sleep(10)
                    list.add(NUM++)
    
                    println("@tid:${Thread.currentThread().threadId()}")
                    list.forEachIndexed { index, j ->
                        //println("${list}")
                    }
                }
            }
        }
    
        mExecutorService.shutdown()
    }

避免了多线程环境读写ArrayList不安全的操作。

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