GSON-JSON解析字符串与POJO

奇怪的例子 gson 解析器可以将JSON字符串解析为 String ,但无法将该JSON字符串解析为它所表示的原始对象。

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.util.ArrayList;

public class Reader {
    public class Beta {
        String beta1;
        @Override
        public String toString() {
            return "{ beta1: " + this.beta1 + "}";
        }
    }
    public class Alpha {
        Beta beta;
        @Override
        public String toString() {
            return "{" + "beta: " + beta.toString() + "}";
        }
    }
    public static void main(String[] args) {
        Gson gson = new Gson();
        String example1 = "[[{beta:{beta1:beta2}}]]";
        String json = gson.toJson(example1);
        // this throws an error
        ArrayList<ArrayList<Alpha>> dog = gson.fromJson(json, new TypeToken<ArrayList<ArrayList<Alpha>>>(){}.getType());
        // this does not
        ArrayList<ArrayList<Alpha>> dog1 = gson.fromJson("[[{beta:{beta1:beta2}}]]", new TypeToken<ArrayList<ArrayList<Alpha>>>(){}.getType());
    }
}