Correct way to map JSON to Java Object

Question

Im trying to map some json elements to a java object but cant seem to figure out the correct way. Theres obviously loads of examples online on how to map json to pojos but I have a big JSON string and I only want to map a few keys in the JSON String to build up an object...

{
"glossary": {
    "title": "example glossary",
    "GlossDiv": {
        "title": "S",
        "GlossList": {
            "GlossEntry": {
                "ID": "SGML",
                "SortAs": "SGML",
                "GlossTerm": "Standard Generalized Markup Language",
                "Acronym": "SGML",
                "Abbrev": "ISO 8879:1986",
                "GlossDef": {
                    "para": "A meta-markup language, used to create markup languages such as DocBook.",
                    "GlossSeeAlso": ["GML", "XML"]
                },
                "GlossSee": "markup"
            }
        }
    }
}

}

lets say I only want the title and GlossEntry: id, SortAs and Abbrev,

What's the cleanest why to map it to a pojo with only those fields without having to map the whole json to an object and get those elements

Answer 1

If you are open to use Jackson , a popular JSON parser for Java, this answer may give you some insights. Here's a tailored solution for your question.

First define a class as following to hold the values:

@JsonIgnoreProperties(ignoreUnknown = true)
public class GlossaryEntry {

    @JsonProperty("ID")
    private String id;

    @JsonProperty("SortAs")
    private String sortAs;

    @JsonProperty("Abbrev")
    private String abbrev;

    // Constructor, getters and setters omitted
}

Then parse the whole JSON into a JsonNode with ObjectMapper :

String json = "{\"glossary\":{\"title\":\"example glossary\",\"GlossDiv\":{\"title\":\"S\",\"GlossList\":{\"GlossEntry\":{\"ID\":\"SGML\",\"SortAs\":\"SGML\",\"GlossTerm\":\"Standard Generalized Markup Language\",\"Acronym\":\"SGML\",\"Abbrev\":\"ISO 8879:1986\",\"GlossDef\":{\"para\":\"A meta-markup language, used to create markup languages such as DocBook.\",\"GlossSeeAlso\":[\"GML\",\"XML\"]},\"GlossSee\":\"markup\"}}}}}";

ObjectMapper mapper = new ObjectMapper();
JsonNode tree = mapper.readTree(json);

Use JSON Pointer to query for the GlossEntry node and finally use ObjectMapper to parse it into a GlossaryEntry instance:

JsonNode node = tree.at("/glossary/GlossDiv/GlossList/GlossEntry");
GlossaryEntry glossaryEntry = mapper.treeToValue(node, GlossaryEntry.class);

JSON Pointer is a path language to traverse JSON. For more details, check the RFC 6901 . It is available in Jackson since the version 2.3.

Answer 2

With googles gson library you can map the json to a generic JsonElement. From there you can access fields and traverse subobjects as you like.

JsonElement elem = new JsonParser.parse(input);

This gets more complicated the more fields you require. I'd suggest building a deserialization DTO providing the right object hierarchy. You can then use standard mapping approaches to map your json. In a second step you can map this DTO to your own domain model. Projects like ModelMapper make this task fairly easy.