convert json to java POJO using stream api - gson

Question

I have a json file and using below code to convert json into java POJO
reader = new JsonReader(new InputStreamReader(responseStream, "UTF-8"));
Gson gson = new GsonBuilder().create();
reader.beginObject();
while (reader.hasNext()) {
Example st = gson.fromJson(reader, Example.class);
}
my json structure is as:
{
"$id": "students.json",
"type": "object",
"properties": {
"project": {
"$id": "project",
"projectList": [
"ABC"
]
},
"students": {
"$id": "/properties/students",
"type": "array",
"subelements": {
"properties": {
"id": {
"$id": "/properties/students/subelements/properties/id",
"examples": [
"Y"
]
},
"dep": {
"$id": "/properties/students/subelements/properties/dep",
"examples": [
"X"
]
}
},
"required": [
"id",
"dep"
]
}
}
},
"required": [
"project"
]
}
And I only need students.subelements.id.examples[0] and students.subelements.dep.examples[0] from list of students
currently my java object classes are:
#JsonInclude(JsonInclude.Include.NON_NULL)
#JsonPropertyOrder({
"project",
"elements"
})
public class Example {
/**
* The project
* (Required)
*
*/
#JsonProperty("project")
#JsonPropertyDescription("The project code")
private String project;
#JsonProperty("elements")
private List<Student> elements = null;
#JsonIgnore
private Map<String, Object> additionalProperties = new HashMap<String, Object>();
}
//student class
public class Student{
private String id;
private String dep;
}
and I am facing below exception:
com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was NAME at line 2 column 4 path $.
so please help me what will be my exact java object class according to provided json and I will get only required fields from that class ?

Answer 1

Reason for error
To begin, the reason for the error is that, after you first call reader.beginObject();, the JSON reader will go to second line "$id", which is a NAME type for JSONToken.
And gson.fromJson(reader, Student.class); is expecting the next JSON value is of BEGIN_OBJECT type, hence the error occur.
Solution
Since only a small part from the JSON is required, and the path is not trivial, we can not create a POJO to retrieve data by direct mapping. As #user10375692 suggests, we can implement JsonDeserializer interface for more flexible mapping. In the deserialize method, we can use JSONObject API to retrieve data from specific path. Following is an example.
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonArray;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
public class JsonToPojo {
public static void main(String[] args) {
String json = getJson();
GsonBuilder gsonBuilder = new GsonBuilder();
JsonDeserializer<Example> deserializer = new JsonDeserializer<Example>() {
#Override
public Example deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
throws JsonParseException {
Example example = new Example();
JsonObject jsonObject = json.getAsJsonObject();
example.setProject(jsonObject.getAsJsonObject("properties").getAsJsonObject("project")
.getAsJsonArray("projectList").get(0).getAsString());
JsonObject subElementProperties = jsonObject.getAsJsonObject("properties").getAsJsonObject("students")
.getAsJsonObject("subelements").getAsJsonObject("properties");
JsonArray idExamples = subElementProperties.getAsJsonObject("id").getAsJsonArray("examples");
JsonArray depExamples = subElementProperties.getAsJsonObject("dep").getAsJsonArray("examples");
List<Student> students = new ArrayList<Student>();
for (int i = 0; i < idExamples.size(); i++) {
Student student = new Student();
student.setId(idExamples.get(i).getAsString());
student.setDep(depExamples.get(i).getAsString());
students.add(student);
}
example.setStudents(students);
return example;
}
};
gsonBuilder.registerTypeAdapter(Example.class, deserializer);
Gson customGson = gsonBuilder.create();
Example customObject = customGson.fromJson(json, Example.class);
System.out.println(customObject.getStudents() + ", " + customObject.getProject());
}
private static String getJson() {
return "{ "
+ " \"$id\": \"students.json\", "
+ " \"type\": \"object\", "
+ " \"properties\": { "
+ " \"project\": { "
+ " \"$id\": \"project\", "
+ " \"projectList\": [ "
+ " \"ABC\" "
+ " ] "
+ " }, "
+ " \"students\": { "
+ " \"$id\": \"subproject\", "
+ " \"type\": \"array\", "
+ " \"subelements\": { "
+ " \"properties\": { "
+ " \"id\": { "
+ " \"$id\": \"id\", "
+ " \"examples\": [ "
+ " \"Y\" "
+ " ] "
+ " }, "
+ " \"dep\": { "
+ " \"$id\": \"dep\", "
+ " \"examples\": [ "
+ " \"X\" "
+ " ] "
+ " } "
+ " }, "
+ " \"required\": [ "
+ " \"id\", "
+ " \"dep\" "
+ " ] "
+ " } "
+ " } "
+ " }, "
+ " \"required\": [ "
+ " \"project\" "
+ " ] "
+ "} ";
}
}