JAVA单例模式（线程安全）

静态内部类实现：私有静态内部类，getInstance 获取内部类的静态实例化变量

public class Configurator {

    public static Configurator getInstance(){
        return Holder.INSTANCE;
    }

    private static class Holder{
       private static final Configurator INSTANCE = new Configurator();
    }

}

双重检查加锁机制实现： 私有构造方法，私有实例，通过 getInstance 获取实例

public class Core {

	private volatile static Core sInstance = null;
	private Core(){}
	
	public static Core getInstance() {
	    if (sInstance == null) {
	        synchronized (Core.class) {
	            if (sInstance == null) {
	                sInstance = new Core();
	            }
	        }
	    }
	    return sInstance;
	}
}