如何让Traverson在资源中的资源中找到链接？

Question

我正在使用Traverson导航“发票和项目”的父子关系。我可以获取资源>并遍历它们以获得单独的发票资源，并使用for ... each循环来开发发票。我想重用Traverson对象来获取发票项目，但是Traverson不想为我合作（还）。

我已经看过问题How to access one element of a REST collection through HATEOAS links?和https://docs.spring.io/spring-hateoas/docs/0.25.0.RELEASE/reference/html/#client.traverson，这几乎可以给我一个答案，但我希望有一个更具体的例子。

String url = baseUrl + invoicesUrl;
Traverson traverson = new Traverson (new URI (url), MediaTypes.HAL_JSON);
ParameterizedTypeReference<Resources<Resource<Invoice>>> invoiceTypeReference = 
    new ParameterizedTypeReference<Resources<Resource<Invoice>>> () {};
Resources<Resource<Invoice>> invoiceResources =
    traverson.follow ("last").toObject (invoiceTypeReference);

ParameterizedTypeReference<Resources<Resource<Item>>> itemTypeReference = 
    new ParameterizedTypeReference<Resources<Resource<Item>>> () {};
for (Resource<Invoice> invoiceResource : invoiceResources) {
    Resources<Resource<Item>> itemResources =
       traverson.follow (invoiceResource.getLink ("items").getRel())
           .toObject(itemTypeReference); // fails to find the link
}

获取发票资源可以正常工作并返回

{"_embedded":
  {"invoiceList":
    [{"sourceLastModified":"2019-03-06T22:33:46.000+0000",
      "lastModified":"2019-03-06T22:33:46.000+0000",
      "validFrom":"2019-03-06T22:33:46.000+0000",
      "validTo":"9999-12-31T06:00:00.000+0000",
      "etlStageTagId":null,
      "etlCrTagId":77,
      "etlErrorInd":null,
      "uniqueRowId":35,
      "openRowInd":"Y",
      "id":35,
      "seqNum":1,
      "fileId":null,
      "fileName":null,
      "invoiceNumber":"1",
      "invoiceDate":"2019-01-18T06:00:00.000+0000",
      "invoiceTotal":0.00,
      "companyName":"TEST1",
      "approvedBy":null,
      "approvedOn":null,
      "_links":
        {"self":{"href":"https://mydomain/myService/invoices/35"},
         "invoices":{"href":"https://mydomain/myService/invoices"},
         "items": {"href":"https://mydomain/myService/items/invoice/1/company/TEST1"},
         "approve":{"href":"https://mydomain/myService/invoices/1/TEST1/approve/appName"
        }
      }
    },
    {"sourceLastModified":"2019-03-06T22:33:46.000+0000",
      "lastModified":"2019-03-06T22:33:46.000+0000",
      "validFrom":"2019-03-06T22:33:46.000+0000",
      "validTo":"9999-12-31T06:00:00.000+0000",
      "etlStageTagId":null,
      "etlCrTagId":77,
      "etlErrorInd":null,
      "uniqueRowId":36,
      "openRowInd":"Y",
      "id":36,
      "seqNum":1,
      "fileId":null,
      "fileName":null,
      "invoiceNumber":"1",
      "invoiceDate":"2019-01-18T06:00:00.000+0000",
      "invoiceTotal":-2025.10,
      "companyName":"TEST2",
      "approvedBy":null,
      "approvedOn":null,
      "_links":
        {"self":{"href":"https://mydomain/myService/invoices/36"},
         "invoices":{"href":"https://mydomain/myService/invoices"},
         "items":{"href":"https://mydomain/myService/items/invoice/1/company/TEST2"},
         "approve":{"href":"https://mydomain/myService/invoices/1/TEST2/approve/appName"
         }
        }
      }]
    },
  "_links":{"self":{"href":"https://mydomain/myService/invoices"},
            "last":{"href":"https://mydomain/myService/invoices/last"
            }
  }
}

我希望此代码在发票资源中找到链接以获取项目并执行HTTP GET调用。相反，我收到错误消息“ java.lang.IllegalStateException：期望在响应中找到带有rel'items'的链接”。

如何获取Traverson对象以跟随在for ... each循环中检索到的资源中的链接？