Spring Boot解析嵌套JSON为独立对象时遇MismatchedInputException错误

我有一段包含嵌套对象的JSON数据：

{
  "process": "create",
  "processDescription": "some description",
  "book": {
    "id": "some unique id",
    "bookName": "some name",
    "bookISBN": "some number",
    "bookDesc": "some description",    
    "author": {
      "id": "a different ID",
      "firstName": "some name",
      "lastName": "some description"
    },
    "timestampUpdated": null,
    "timestampCreated": 1672753599223,
    "createdByUser": "Jane Doe",
    "updatedByUser": null
  },
  "someField": "some data",
  "anotherField": "more data"
}

我仅关注其中的book对象和author对象，且已为它们定义了对应的Java类。尝试通过以下代码读取JSON字符串时出现错误：

JsonNode jsonObject = new ObjectMapper().readTree(jsonString);
JsonNode bookObj = jsonObject.get("book");
JsonNode authorObj = bookObj.get("author");

ObjectMapper objectMapper = new ObjectMapper();
AuthorClass author = objectMapper.readValue(authorObj.asText(), AuthorClass.class);

错误信息：

com.fasterxml.jackson.databind.exc.MismatchedInputException: No content to map due to end-of-input at [Source: (String)""; line: 1, column: 0]

问题原因 #

调用authorObj.asText()会尝试将author这个JSON对象转换为字符串，但Jackson对非字符串类型的JsonNode调用asText()会返回空字符串，后续readValue解析空字符串时自然会报“无内容可映射”的错误。

解决方法 #

以下几种方式可以正确解析嵌套的author对象：

方法1：直接从JsonNode转换为Java对象 #

使用treeToValue方法直接将JsonNode转换为目标类实例，无需转成字符串：

ObjectMapper objectMapper = new ObjectMapper();
JsonNode jsonObject = objectMapper.readTree(jsonString);
// 直接定位到author节点
JsonNode authorNode = jsonObject.get("book").get("author");
// 转换为AuthorClass对象
AuthorClass author = objectMapper.treeToValue(authorNode, AuthorClass.class);

方法2：先解析Book对象再取Author #

如果已经定义了包含AuthorClass字段的BookClass，可以先解析整个book节点，再从中获取author：

ObjectMapper objectMapper = new ObjectMapper();
JsonNode jsonObject = objectMapper.readTree(jsonString);
// 解析book节点为BookClass实例
BookClass book = objectMapper.treeToValue(jsonObject.get("book"), BookClass.class);
// 从book对象中获取author
AuthorClass author = book.getAuthor();

方法3：使用JsonParser解析 #

通过JsonNode.traverse()获取JsonParser，再传入readValue方法：

ObjectMapper objectMapper = new ObjectMapper();
JsonNode jsonObject = objectMapper.readTree(jsonString);
JsonNode authorNode = jsonObject.get("book").get("author");
// 用traverse()获取解析器，再转换对象
AuthorClass author = objectMapper.readValue(authorNode.traverse(), AuthorClass.class);

内容的提问来源于stack exchange，提问作者jason