在Java中使用GSON从嵌套Json提取JSONObjects数组

Question

I have a nested JSON object in which one of the nested objects (keyed parameters ) is an unknown object itself. The only known part is that it's an array of further objects.

The problem is that the contents of this object are completely unknown(dynamic) and so I can't define a class with some known variable names for it.

What I need to achieve is to extract and store all these dynamic key-value pairs. For example in the below code, for parameters I need to extract both alpha and then its contents type , and store them as a Map .

js is the JSON I'm trying to parse into my class NestedJsonParse .

public class NestedJsonParse {
    String name;
    Experiment experiment;
    Parameters parameters;

    NestedJsonParse(String name, Experiment experiment, Parameters parameters) {
        this.name = name;
        this.experiment = experiment;
        this.parameters = parameters;
    }

    public String getName() { return name; }

    public String getExpName() { return experiment.getName(); }

    public String getParams() { return parameters.getParamsAsStrings(); }

    @Override
    public String toString() {
        return "NSP{" +
            "name='" + getName() + '\'' +
            ", exp-name='" + getExpName() + '\'' +
            ", params='" + getParams() + '\'' +
            "========================================"  +
            "}";
    }

    public class Experiment {
        String name;

        Experiment(String name) {
            this.name = name;
        }

        public String getName() { return name; }
    }

    public class Parameters {
        List<JSONObject>  parameters;

        Parameters(List<JSONObject> parameters) {
            this.parameters = parameters;
        }

        public String getParamsAsStrings() {
            return "abc";
        }
    }

    public static void main(String[] args) throws Exception {
        String js = "{\"name\" : \"e0\", \"experiment\" : { \"name\" : \"e1\" },  \"parameters\" : [ {\"alpha \": { \"type\" : \"float\" } } ]  }";

        JSONParser parser = new JSONParser();
        Object obj = parser.parse(js);  // Passes

        Gson GSON = new GsonBuilder().create();
        NestedJsonParse nsp = GSON.fromJson(js, NestedJsonParse.class);  // Fails
        System.out.println("NSP : " + nsp.toString());
    }
}

This passes if the JSON does not contain a parameters block but fails otherwise with this error:

Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 206 path $.parameters

Answer 1

One simple option is just to deserialize parameters as an Object so no class Parameters at all. Your NestedJsonParse could (greatly simplified) be like:

@Getter @Setter
public class NestedJsonParse {
    private String name;
    private Experiment experiment;
    private Object parameters;
}

Then when you do

NestedJsonParse nsp = GSON.fromJson(js, NestedJsonParse.class);

your parameters in nsp could be:

• null
• com.google.gson.internal.LinkedTreeMap
• ArrayList containing above mentioned or other ArrayList etc...

depending on what is in the JSON.

Based on what you find from parameters you can take appropriate action after deserialization.