单例模式的一种实现

rongyun 的sdk单例模式

private static ConcurrentHashMap<String, RongCloud> rongCloud = new ConcurrentHashMap<String,RongCloud>();

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1. public static RongCloud getInstance(String appKey, String appSecret) {  
2.     if (null == rongCloud.get(appKey)) {  
3.         rongCloud.putIfAbsent(appKey, new RongCloud(appKey, appSecret));  
4.     }  
5.     return rongCloud.get(appKey);  
6. }  

putIfAbsent见：

http://blog.youkuaiyun.com/silyvin/article/details/79094642