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参考：http://blog.csdn.net/kufeiyun/article/details/6166673
private static Single instance = null public synchronized static Single getInstance(){ if(instance==null){ instance = new Single(); } }
private static Single instance = new Single(); public static Single getInstance(){ return instance; }
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原文地址：http://www.cnblogs.com/wjw334/p/4442340.html
