ConcurrentHashMap的putIfAbsent总结

最近在设计一个可缓存的类，发现putIfAbsent使用还是有些坑要注意的，总结一下。之前的代码如下

private static ConcurrentHashMap<String, Pattern> compliedPattern = new ConcurrentHashMap<String, Pattern>();public static Pattern getPattern(String pattern) {return compliedPattern.putIfAbsent(pattern, Pattern.compile(pattern));}

?之前以为这样一行代码就完美了，结果测试每次都返回null，查看api看到文档：

public static Pattern getPattern(String pattern) {Pattern p = Pattern.compile(pattern);Pattern ret = compliedPattern.putIfAbsent(pattern, p);if (ret == null) {return ret;} else {return p;}}

?但这样还搞个毛的缓存啊，每次都会compile了。正确的解法是：

public static Pattern getPattern(String pattern) {Pattern ret = compliedPattern.get(pattern);if (ret == null) {Pattern newPattern = Pattern.compile(pattern);ret = compliedPattern.putIfAbsent(pattern, Pattern.compile(pattern));if (ret == null) {ret = newPattern;}}return ret;}

?参考:

http://stackoverflow.com/questions/3752194/best-practice-to-use-concurrentmaps-putifabsent

http://stackoverflow.com/questions/10743622/concurrenthashmap-avoid-extra-object-creation-with-putifabsent

http://en.wikipedia.org/wiki/Double-checked_locking

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