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为什么我们需要Scala的ParHashMap，而Java却有ConcurrentHashMap

[英]Why do we need for ParHashMap from Scala while there is ConcurrentHashMap from Java

原文 2017-01-09 22:58:08 4 1 java/ scala/ data-structures/ trie/ concurrenthashmap

I considered two collections with a similar concept - ParHashMap from Scala and ConcurrentHashMap from Java. 我考虑了两个具有类似概念的集合ParHashMap和Java的ConcurrentHashMap。 Both of them have the same time complexity and both of them are thread safe and lock-free, but they only are based on different concepts under the hood - trie and hash table accordingly. 它们都具有相同的时间复杂度，并且都是线程安全且无锁的，但是它们仅基于不同的概念-特里和哈希表。 And this reasoning leads to question: why do we need for ParHashMap from Scala while there is ConcurrentHashMap from Java? 这种推理导致了一个问题：为什么Java中有ConcurrentHashMap时，我们为什么需要Scala中的ParHashMap？

1 个解决方案

ConcurrentHashMap is a thread safe Map<> implementation. ConcurrentHashMap是线程安全的Map<>实现。 If you have multiple threads accessing it at the same time they will be in sync. 如果您有多个线程同时访问它，则它们将同步。

ParHashMap is a parallel collection. ParHashMap是一个并行集合。 If you execute operations here (like map() , filter() , aggregate() ) Scala will parallelize it for you (similar to Spark but only within a single JVM). 如果您在此处执行操作（例如map() ， filter() ， aggregate() ），Scala会为您并行化它（类似于Spark，但仅在单个JVM中）。

To summarize, ConcurrentHashMap gives the primitive to synchronize threads for concurrency, ParHashMap takes care of both sync and execution. 总而言之， ConcurrentHashMap提供了用于并发同步线程的原语， ParHashMap负责同步和执行。

Edit : Note that ParHashMap is not itself necessarily thread-safe. 编辑：请注意， ParHashMap本身不一定是线程安全的。 The idea is to call its methods from a single thread and let the parallelism be handled by the parallel data structure itself. 想法是从单个线程调用其方法，并让并行性由并行数据结构本身处理。