public class SingleInstance {
    //volatile防止JVM指令重排
    private static volatile SingleInstance singleInstance;

    private SingleInstance() {
    }
    public static SingleInstance getSingleInstance() {

        if (singleInstance == null) {
            synchronized (SingleInstance.class) {
                //doubleCheck
                if (singleInstance == null) {
                    singleInstance = new SingleInstance();
                }
            }
        }
        return singleInstance;
    }

}