Google gson fromJson 问题

public static void main(String args[]){
	Book book = new Book("中国");
	book.setPrice(3.2f);
	String json = Tools.toJson(book);
		
	Gson gson = new Gson();
	Book bb = gson.fromJson(json, Book.class);  
	System.out.println(bb.getName());
	}

上面代码运行 抛出如下异常

No-args constructor for class com.Lighten.test.Book does not exist. Register an InstanceCreator with Gson for this type to fix this problem.

Gson 的 fromJson 的对象如果没有不带参数的构造函数就报这个错，是我的code问题， 还是Gson 本身的问题呢？