单例模式

静态内部类方式 JVM保证单例 加载外部类时不会加载内部类，这样可以实现懒加载

public class SingleObject {
    private SingleObject() {
    }
    private static class SingleObjectHolder {
        private final static SingleObject INSTANCE = new SingleObject();
    }
    public static SingleObject getInstance() {
        return SingleObjectHolder.INSTANCE;
    }
}