什么情况下用ConcurrentHashMap

很多同事都了解了HashMap和ConcurrentHashMap的原理，并且也看了两个类的源码，但是还是不知道在什么情况下使用ConcurrentHashMap。

1，在多线程并发向HashMap中put数据时，就需要把HashMap换成ConcurrentHashMap。

（原因为并发向HashMap中put数据会出现死循环，导致CPU使用率暴增。参考参考：http://mailinator.blogspot.com/2009/06/beautiful-race-condition.html）

2，在业务中可以使用ConcurrentMap接口中定义的方法解决并发问题时。

public interface ConcurrentMap<K, V> extends Map<K, V> {
    V putIfAbsent(K key, V value);
    boolean remove(Object key, Object value);
    boolean replace(K key, V oldValue, V newValue);
    V replace(K key, V value);
}

举个例子

private final Map<String, Long> wordCounts = new ConcurrentHashMap<>();
 
public long increase(String word) {
    Long oldValue = wordCounts.get(word);
    Long newValue = (oldValue == null) ? 1L : oldValue + 1;
    wordCounts.put(word, newValue);
    return newValue;
}

 替换为

private final ConcurrentMap<String, Long> wordCounts = new ConcurrentHashMap<>();
 
public long increase(String word) {
    Long oldValue, newValue;
    while (true) {
        oldValue = wordCounts.get(word);
        if (oldValue == null) {
            // Add the word firstly, initial the value as 1
            newValue = 1L;
            if (wordCounts.putIfAbsent(word, newValue) == null) {
                break;
            }
        } else {
            newValue = oldValue + 1;
            if (wordCounts.replace(word, oldValue, newValue)) {
                break;
            }
        }
    }
    return newValue;
}