ConcurrentHashMap:我们可以信任它吗? [英] ConcurrentHashMap: Can we trust on it?

问题描述

来自 ConcurrentHashMap 的文档:

From the document of ConcurrentHashMap:

一个哈希表，支持检索的完全并发性和可调整的更新预期并发性.

A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.

我们可以完全相信ConcurrentHashMap 是线程安全的操作吗?

Can we fully believe that ConcurrentHashMap does thread safe operation?

我使用 ConcurrentHashMap 来映射键和它们的值.我的键值对是:

I am using ConcurrentHashMap for mapping key with their values.My key-value pair is:

Map<Integer,ArrayList<Double>> map1 = new ConcurrentHashMap();

键的大小范围为 [0,1000000].我有 20 个线程，可以一次访问/修改与一个键对应的值.这不是那么频繁，但这种情况是可能的.我是从以下方法获得无穷大:

The size of key ranges from [0,1000000]. I have 20 threads which can access/modify value corresponding to a key at a time. This not so frequent but that condition is possible. I am getting an infinity from following method:

Double sum =0.0; 
sum = sum + Math.exp(getScore(contextFeatureVector,entry.getValue())+constant);

contextFeatureVector 和 entry.getValue() 是与键关联的数组列表.

contextFeatureVector and entry.getValue()are arraylist associated with a key.

 constant =0.0001

private double getScore(List<Double> featureVector,List<Double>weightVector) throws NullPointerException    
{
    double score =0.0;
    int length = featureVector.size();
    for (int i =0 ; i< length ; i++){
    score = score + (featureVector.get(i)*weightVector.get(i)); 
    }

    return score;
}

featureVector<> 和 weightVector 看起来都像

[ - 0.005554038592516575，0.0048966974158881175，-0.05315976588195846，-0.030837804373964654，0.014483064988148562，-0.018962129117649，-0.015221386014208877，0.015825702365331477，-0.11363620479662287，0.00802609847263844，-0.062106636476812194，0.0081088544712​​93185，-0.03193255218671684，0.04949650992670292，-0.0545583154094599，-0.04873314092706468，0.013534731656877033，0.08433117163682455，0.050310355477044114，-0.002420513353516017，-0.02708299928442614，-0.023489187394176294，-0.1277699782685597，-0.10071004855129333，0.08649040730064464，-0.04940329664431305，-0.027481729446035053，-0.0571846057609884，-0.036738550618481455，-0.035608113682344365]

[-0.005554038592516575, 0.0048966974158881175, -0.05315976588195846, -0.030837804373964654, 0.014483064988148562, -0.018962129117649, -0.015221386014208877, 0.015825702365331477, -0.11363620479662287, 0.00802609847263844, -0.062106636476812194, 0.008108854471293185, -0.03193255218671684, 0.04949650992670292, -0.0545583154094599, -0.04873314092706468, 0.013534731656877033, 0.08433117163682455, 0.050310355477044114, -0.002420513353516017, -0.02708299928442614, -0.023489187394176294, -0.1277699782685597, -0.10071004855129333, 0.08649040730064464, -0.04940329664431305, -0.027481729446035053, -0.0571846057609884, -0.036738550618481455, -0.035608113682344365]

因此从 getScore 返回的值不会特别大.它将在几千.

thus the value returned from getScore does not go exceptionally too large. it will be in some thousands.

推荐答案

您使用的数据结构让我相信您的代码中一定存在一些错误.您很可能正在从地图中获取列表并对其进行更新:

The data structure you use makes me believe there must some bug in your code. Most likely you are fetching the list from map and updating it:

map1.get(42).add(5);

请注意，add(5) 是不是线程安全的，因为它对普通的 ArrayList 进行操作.您需要线程安全的 ArrayList 或 replace(K key, V oldValue, V newValue) 方法.

Note that add(5) is not thread-safe as it operates on ordinary ArrayList. You either need thread safe ArrayList or replace(K key, V oldValue, V newValue) method.

如果您仔细阅读ConcurrentHashMap 提供的保证，您可以有效地使用它.

If you read carefully through the guarantees ConcurrentHashMap is giving, you can use it effectively.

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