单例模式的双检锁的隐患和优化

public class Singleton {
    private static Singleton sInstance;
    public static Singleton getInstance() {
        if (sInstance == null) {//①　　
            synchronized (Singleton.class) {//②
                if (sInstance == null) {
                    sInstance = new Singleton();//③　　
                }
            }
        }
        return sInstance;
    }
    private Singleton() {}
}
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