线程并发时，防止单例出现多个实例。

package cn.sigangjun.project;/** * @author sigangjun * */public class SingtonTest {private static SingtonTest instance = null;private SingtonTest(){};public static SingtonTest getInstance(){if(instance==null){synchronized (SingtonTest.class) {if(instance==null){instance=new SingtonTest();}}}return instance;}}