使用 Jackson 解析 Java 中的 JSON 子集

Question

Given a Json, is it possible to use Jackson to only parse out a section of the message? Say that the data I'm interested in is buried in a deep hierarchy of fields and I simply do not care about creating DTO classes for each and every class.

Given a very simplified scenario I'd like to model the Telephone class without knowing anything about the structure before it:

...{
  "firstName": "John",
  "lastName" : "doe",
  "age"      : 26,
  "address"  : {
    "streetAddress": "naist street",
    "city"         : "Nara",
    "postalCode"   : "630-0192"
  },
  "phoneNumbers": [
    {
      "type"  : "iPhone",
      "number": "0123-4567-8888"
    },
    {
      "type"  : "home",
      "number": "0123-4567-8910"
    }
  ]
}....

I'm thinking something in the terms of using json-path together with deserializing just the parts I'm interested of. Some pseudo:

List<Telephone> phoneNrs = parse(".my.deep.structure.persons.phoneNumbers", List<Telephone.class>);

Answer 1

ObjectMapper mapper = new ObjectMapper();
JsonNode json = mapper.readTree("... your JSON ...");

Using the JsonNode object you can then call get("my").get("deep").get("structure") to get the node you want.

Once you got your hands on that node, a simple call to mapper.treeToValue(myDeepJsonNode, Telephone[].class) will get you your array of Telephone . You can get a list using a TypeReference as well.

To get to your deep JsonNode you can also use the findValue and findPath methods.

The Javadoc: https://fasterxml.github.io/jackson-databind/javadoc/2.2.0/com/fasterxml/jackson/databind/JsonNode.html

Answer 2

You can use JsonPath library. With this library you can map your JsonPath output directly into POJO's.

Pseudo:

List<Telephone> phoneNrs = JsonPath.parse(json).read("$.my.deep.structure.persons.phoneNumbers", List.class);

Answer 3

Yes, it is possible the way you have mentioned in the Pseudo code. "phoneNumbers" is a key and value returned can be passed on to Jackson deserialiying.

If the response is an array of maps then you can iterate through each one of them and use the yourResponseAsJSONObject.get('phoneNumbers') method to get the value and pass it on to Jackson

or use JsonPath as mentioned by @dimas

Answer 4

To do this efficiently with Jackson, use the Streaming API via the JsonParser class ( http://fasterxml.github.io/jackson-core/javadoc/2.5/com/fasterxml/jackson/core/JsonParser.html ).

This approach will allocate no additional memory and will not incur the cost of deserializing values for all of the skipped data. Since the code will be much longer and more difficult to read than using Jackson's ObjectMapper, only do this if profiling shows unacceptable GC activity or CPU usage during parsing.

You can skip all of the nodes that you are uninterested in until you hit the "phoneNumbers" key. Then you can call the readValueAs function to deserialize the array of phone number dictionaries like so readValueAs(new TypeReference<MyPhoneNumberType[]>()) .

See also:

• a tutorial on reading and writing with JsonParser: http://www.cowtowncoder.com/blog/archives/2009/01/entry_132.html
• The main documentation: https://github.com/FasterXML/jackson-core