I want to define my own serialize and deserialize methods for Student class, so I extended TypeAdapter and override its methods, but now deserialization does not work. Why this happens?
public class GSONFormat {
#Test
public void run()
{
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(Student.class, new StudentAdapter());
Gson gson = builder.create();
Student s = new Student();
s.setAge(11);
s.setName("hiway");
System.out.println(gson.toJson(s));
String str = "{\"age\":11,\"name\":\"hiway\"}";
s = gson.fromJson(str, Student.class);
System.out.println(s);
}
}
class Student{
private int age;
private String name;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
class StudentAdapter extends TypeAdapter<Student>
{
#Override
public void write(JsonWriter out, Student s) throws IOException {
out.beginObject();
out.name("age");
out.value(s.getAge());
out.name("name");
out.value(s.getName());
out.endObject();
}
#Override
public Student read(JsonReader in) throws IOException {
in.beginObject();
Student s = new Student();
s.setAge(in.nextInt());
s.setName(in.nextString());
in.endObject();
return s;
}
}
As Matt Ball commented is not a good idea at all writing your type adapter if you do not have a very good reason at all. If you remove this line:
builder.registerTypeAdapter(Student.class, new StudentAdapter());
your parsing will work without any effort. However you might be interested in understand a bit further what you can do with type adapter. So I give you a possible implementation. Keep in mind that if your class Student get more complex, writing your own type adapter can be hard. It's better to let Gson do its own work. Anycase, here is something that patches your code (compiles under Java7, if you use lower version, change switch with an if chain).
class StudentAdapter extends TypeAdapter<Student>
{
#Override
public void write(JsonWriter out, Student s) throws IOException {
out.beginObject();
out.name("age");
out.value(s.getAge());
out.name("name");
out.value(s.getName());
out.endObject();
}
#Override
public Student read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
return null;
}
in.beginObject();
Student s = new Student();
while (in.peek() == JsonToken.NAME){
String str = in.nextName();
fillField(in, s, str);
}
in.endObject();
return s;
}
private void fillField(JsonReader in, Student s, String str)
throws IOException {
switch(str){
case "age": s.setAge(in.nextInt());
break;
case "name": s.setName(in.nextString());
break;
}
}
}
Related
I've managed to deserialize Json data to my private variables using Jackson ObjectMapper, however, it makes use of public setters. I have also tried Gson, however I need to create a TypeAdapter which uses a switch statement (with package-private setters, which is atleast better than public setters) in its "read" method (breaking the OCP), as well as not following liskov substitution principle because it implements a TypeAdapter and its "write" function is left empty. Is there any way to deserialize in a way which does not use public setters or break lots of principles?
Sources for which may give a better understanding the implementation of my code:
https://www.tutorialspoint.com/gson/gson_custom_adapters.htm#
public class Food {
private String name;
private double calories;
#JsonAlias("serving_size_g")
private double servingSize;
public void setName(String name) {
this.name = name;
}
public double getCalories() {
return calories;
}
public void setCalories(double calories) {
this.calories = calories;
}
public double getServingSize() {
return servingSize;
}
}
And another class uses this function to create the food object:
private Food getFood(String foodName) throws JsonProcessingException {
String nutritionJsonString = apiConnector.getNutritionAsStringFromAPI(foodName);
Food food = objectMapper.readValue(nutritionJsonString, Food.class);
return food;
}
The other solution that I tried using Gson:
public class NutritionTypeAdapter extends TypeAdapter<Food> {
#Override
public void write(JsonWriter jsonWriter, Food food) throws IOException {
}
#Override
public Food read(JsonReader jsonReader) throws IOException {
final Food food = new Food();
jsonReader.beginObject();
while(jsonReader.hasNext()){
switch (jsonReader.nextName()){
case "name":
food.setName(jsonReader.nextString());
break;
case "calories":
food.setCalories(jsonReader.nextDouble());
break;
case "serving_size_g":
food.setServingSize(jsonReader.nextDouble());
break;
default:
jsonReader.skipValue();
}
}
jsonReader.endObject();
return food;
}
}
You can define constructor:
public class Main {
public static void main(String[] args) {
ObjectMapper objectMapper = new ObjectMapper();
try {
Food food = objectMapper.readValue("""
{
"name" : "x"
}
""", Food.class);
} catch (JsonProcessingException e) {
throw new RuntimeException(e);
}
}
static class Food {
#JsonCreator
public Food(
#JsonProperty("name")
String name,
#JsonProperty("calories")
double calories,
#JsonProperty("serving_size_g")
double servingSize
) {
// assign to fields
}
}
}```
So i have this JSON file
{
"results":[
"result":{},
"result":{}
]
}
I wish to deserialize it to a java object which contains an array of result objects.
public class Foo(){
#JsonProperty("results")
private Result[] results;
public void setResults(Result[] results){
this.results = results;
}
public Result[] getResults(){
return this.results;
}
}
public class JsonToObject(){
ObjectMapper mp = new ObjectMapper();
public void createObject(String jsonFile){
Foo bar = mp.readValue(jsonFile, Foo.Class)
}
}
My issue is I keep getting deserialization issues as I have not definied "result".
One way I can get around this is to have result as a class variable inside Result but that seems stupid to do and also may cause issues with re-serializing.
How can I convert the JSON so that my class contains an array of result?
Your question program is wrong. There are many problems with your code. Please use below sample:
public class Foo {
#JsonProperty("results")
private Result[] results;
public Foo() {
}
public Result[] getResults() {
return results;
}
public void setResults(Result[] results) {
this.results = results;
}
}
public class Result {
private String name;
public Result() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
public static void main(String[] args) throws IOException {
ObjectMapper mp = new ObjectMapper();
String content = "{\"results\":[{\"name\":\"apple\"},{\"name\":\"lemon\"}]}";
Foo bar = mp.readValue(content, Foo.class);
}
I want to use jackson json library for a generic method as follows:
public MyRequest<T> tester() {
TypeReference<MyWrapper<T>> typeRef = new TypeReference<MyWrapper<T>>();
MyWrapper<T> requestWrapper = (MyWrapper<T>) JsonConverter.fromJson(jsonRequest, typeRef);
return requestWrapper.getRequest();
}
public class MyWrapper<T> {
private MyRequest<T> request;
public MyRequest<T> getRequest() {
return request;
}
public void setRequest(MyRequest<T> request) {
this.request = request;
}
}
public class MyRequest<T> {
private List<T> myobjects;
public void setMyObjects(List<T> ets) {
this.myobjects = ets;
}
#NotNull
#JsonIgnore
public T getMyObject() {
return myobjects.get(0);
}
}
Now the problem is that when I call getMyObject() which is inside the request object Jackson returns the nested custom object as a LinkedHashMap. Is there any way in which I specify that T object needs to be returned? For example: if I sent object of type Customer then Customer should be returned from that List?
This is a well-known problem with Java type erasure: T is just a type variable, and you must indicate actual class, usually as Class argument. Without such information, best that can be done is to use bounds; and plain T is roughly same as 'T extends Object'. And Jackson will then bind JSON Objects as Maps.
In this case, tester method needs to have access to Class, and you can construct
JavaType type = mapper.getTypeFactory().
constructCollectionType(List.class, Foo.class)
and then
List<Foo> list = mapper.readValue(new File("input.json"), type);
'JavaType' works !!
I was trying to unmarshall (deserialize) a List in json String to ArrayList java Objects and was struggling to find a solution since days.
Below is the code that finally gave me solution.
Code:
JsonMarshallerUnmarshaller<T> {
T targetClass;
public ArrayList<T> unmarshal(String jsonString) {
ObjectMapper mapper = new ObjectMapper();
AnnotationIntrospector introspector = new JacksonAnnotationIntrospector();
mapper.getDeserializationConfig()
.withAnnotationIntrospector(introspector);
mapper.getSerializationConfig()
.withAnnotationIntrospector(introspector);
JavaType type = mapper.getTypeFactory().
constructCollectionType(
ArrayList.class,
targetclass.getClass());
try {
Class c1 = this.targetclass.getClass();
Class c2 = this.targetclass1.getClass();
ArrayList<T> temp = (ArrayList<T>)
mapper.readValue(jsonString, type);
return temp ;
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null ;
}
}
I modified rushidesai1's answer to include a working example.
JsonMarshaller.java
import java.io.*;
import java.util.*;
public class JsonMarshaller<T> {
private static ClassLoader loader = JsonMarshaller.class.getClassLoader();
public static void main(String[] args) {
try {
JsonMarshallerUnmarshaller<Station> marshaller = new JsonMarshallerUnmarshaller<>(Station.class);
String jsonString = read(loader.getResourceAsStream("data.json"));
List<Station> stations = marshaller.unmarshal(jsonString);
stations.forEach(System.out::println);
System.out.println(marshaller.marshal(stations));
} catch (IOException e) {
e.printStackTrace();
}
}
#SuppressWarnings("resource")
public static String read(InputStream ios) {
return new Scanner(ios).useDelimiter("\\A").next(); // Read the entire file
}
}
Output
Station [id=123, title=my title, name=my name]
Station [id=456, title=my title 2, name=my name 2]
[{"id":123,"title":"my title","name":"my name"},{"id":456,"title":"my title 2","name":"my name 2"}]
JsonMarshallerUnmarshaller.java
import java.io.*;
import java.util.List;
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector;
public class JsonMarshallerUnmarshaller<T> {
private ObjectMapper mapper;
private Class<T> targetClass;
public JsonMarshallerUnmarshaller(Class<T> targetClass) {
AnnotationIntrospector introspector = new JacksonAnnotationIntrospector();
mapper = new ObjectMapper();
mapper.getDeserializationConfig().with(introspector);
mapper.getSerializationConfig().with(introspector);
this.targetClass = targetClass;
}
public List<T> unmarshal(String jsonString) throws JsonParseException, JsonMappingException, IOException {
return parseList(jsonString, mapper, targetClass);
}
public String marshal(List<T> list) throws JsonProcessingException {
return mapper.writeValueAsString(list);
}
public static <E> List<E> parseList(String str, ObjectMapper mapper, Class<E> clazz)
throws JsonParseException, JsonMappingException, IOException {
return mapper.readValue(str, listType(mapper, clazz));
}
public static <E> List<E> parseList(InputStream is, ObjectMapper mapper, Class<E> clazz)
throws JsonParseException, JsonMappingException, IOException {
return mapper.readValue(is, listType(mapper, clazz));
}
public static <E> JavaType listType(ObjectMapper mapper, Class<E> clazz) {
return mapper.getTypeFactory().constructCollectionType(List.class, clazz);
}
}
Station.java
public class Station {
private long id;
private String title;
private String name;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#Override
public String toString() {
return String.format("Station [id=%s, title=%s, name=%s]", id, title, name);
}
}
data.json
[{
"id": 123,
"title": "my title",
"name": "my name"
}, {
"id": 456,
"title": "my title 2",
"name": "my name 2"
}]
I've a generic field in User.java. I want to use the value of T in json.
public class User<T> {
public enum Gender {MALE, FEMALE};
private T field;
private Gender _gender;
private boolean _isVerified;
private byte[] _userImage;
public T getField() { return field; }
public boolean isVerified() { return _isVerified; }
public Gender getGender() { return _gender; }
public byte[] getUserImage() { return _userImage; }
public void setField(T f) { field = f; }
public void setVerified(boolean b) { _isVerified = b; }
public void setGender(Gender g) { _gender = g; }
public void setUserImage(byte[] b) { _userImage = b; }
}
and mapper class is:
public class App
{
public static void main( String[] args ) throws JsonParseException, JsonMappingException, IOException
{
ObjectMapper mapper = new ObjectMapper();
Name n = new Name();
n.setFirst("Harry");
n.setLast("Potter");
User<Name> user = new User<Name>();
user.setField(n);
user.setGender(Gender.MALE);
user.setVerified(false);
mapper.writeValue(new File("user1.json"), user);
}
}
and the json output is :
{"field":{"first":"Harry","last":"Potter"},"gender":"MALE","verified":false,"userImage":null}
In the output, i want Name to be appeared in place of field. How do i do that. Any help?
I think what u ask is not JSON's default behavior. Field name is the "key" of the json map, not the variable name. U should rename the field or make some String process to do it.
private T field;
change the above to this:
private T name;
You need a custom serializer to do that. That's a runtime data transformation and Jackson has no support for data transformation other than with a custom serializer (well, there's wrapping/unwrapping of value, but let's not go there). Also, you will need to know in advance every type of transformation you want to apply inside your serializer. The following works:
public class UserSerializer extends JsonSerializer<User<?>> {
private static final String USER_IMAGE_FIELD = "userImage";
private static final String VERIFIED_FIELD = "verified";
private static final String FIELD_FIELD = "field";
private static final String NAME_FIELD = "name";
#Override
public void serialize(User<?> value, JsonGenerator jgen, SerializerProvider provider) throws IOException,
JsonProcessingException {
jgen.writeStartObject();
if (value.field instanceof Name) {
jgen.writeFieldName(NAME_FIELD);
} else {
jgen.writeFieldName(FIELD_FIELD);
}
jgen.writeObject(value.field);
jgen.writeStringField("gender", value._gender.name());
jgen.writeBooleanField(VERIFIED_FIELD, value._isVerified);
if (value._userImage == null) {
jgen.writeNullField(USER_IMAGE_FIELD);
} else {
jgen.writeBinaryField(USER_IMAGE_FIELD, value._userImage);
}
jgen.writeEndObject();
}
}
I want to use jackson json library for a generic method as follows:
public MyRequest<T> tester() {
TypeReference<MyWrapper<T>> typeRef = new TypeReference<MyWrapper<T>>();
MyWrapper<T> requestWrapper = (MyWrapper<T>) JsonConverter.fromJson(jsonRequest, typeRef);
return requestWrapper.getRequest();
}
public class MyWrapper<T> {
private MyRequest<T> request;
public MyRequest<T> getRequest() {
return request;
}
public void setRequest(MyRequest<T> request) {
this.request = request;
}
}
public class MyRequest<T> {
private List<T> myobjects;
public void setMyObjects(List<T> ets) {
this.myobjects = ets;
}
#NotNull
#JsonIgnore
public T getMyObject() {
return myobjects.get(0);
}
}
Now the problem is that when I call getMyObject() which is inside the request object Jackson returns the nested custom object as a LinkedHashMap. Is there any way in which I specify that T object needs to be returned? For example: if I sent object of type Customer then Customer should be returned from that List?
This is a well-known problem with Java type erasure: T is just a type variable, and you must indicate actual class, usually as Class argument. Without such information, best that can be done is to use bounds; and plain T is roughly same as 'T extends Object'. And Jackson will then bind JSON Objects as Maps.
In this case, tester method needs to have access to Class, and you can construct
JavaType type = mapper.getTypeFactory().
constructCollectionType(List.class, Foo.class)
and then
List<Foo> list = mapper.readValue(new File("input.json"), type);
'JavaType' works !!
I was trying to unmarshall (deserialize) a List in json String to ArrayList java Objects and was struggling to find a solution since days.
Below is the code that finally gave me solution.
Code:
JsonMarshallerUnmarshaller<T> {
T targetClass;
public ArrayList<T> unmarshal(String jsonString) {
ObjectMapper mapper = new ObjectMapper();
AnnotationIntrospector introspector = new JacksonAnnotationIntrospector();
mapper.getDeserializationConfig()
.withAnnotationIntrospector(introspector);
mapper.getSerializationConfig()
.withAnnotationIntrospector(introspector);
JavaType type = mapper.getTypeFactory().
constructCollectionType(
ArrayList.class,
targetclass.getClass());
try {
Class c1 = this.targetclass.getClass();
Class c2 = this.targetclass1.getClass();
ArrayList<T> temp = (ArrayList<T>)
mapper.readValue(jsonString, type);
return temp ;
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null ;
}
}
I modified rushidesai1's answer to include a working example.
JsonMarshaller.java
import java.io.*;
import java.util.*;
public class JsonMarshaller<T> {
private static ClassLoader loader = JsonMarshaller.class.getClassLoader();
public static void main(String[] args) {
try {
JsonMarshallerUnmarshaller<Station> marshaller = new JsonMarshallerUnmarshaller<>(Station.class);
String jsonString = read(loader.getResourceAsStream("data.json"));
List<Station> stations = marshaller.unmarshal(jsonString);
stations.forEach(System.out::println);
System.out.println(marshaller.marshal(stations));
} catch (IOException e) {
e.printStackTrace();
}
}
#SuppressWarnings("resource")
public static String read(InputStream ios) {
return new Scanner(ios).useDelimiter("\\A").next(); // Read the entire file
}
}
Output
Station [id=123, title=my title, name=my name]
Station [id=456, title=my title 2, name=my name 2]
[{"id":123,"title":"my title","name":"my name"},{"id":456,"title":"my title 2","name":"my name 2"}]
JsonMarshallerUnmarshaller.java
import java.io.*;
import java.util.List;
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector;
public class JsonMarshallerUnmarshaller<T> {
private ObjectMapper mapper;
private Class<T> targetClass;
public JsonMarshallerUnmarshaller(Class<T> targetClass) {
AnnotationIntrospector introspector = new JacksonAnnotationIntrospector();
mapper = new ObjectMapper();
mapper.getDeserializationConfig().with(introspector);
mapper.getSerializationConfig().with(introspector);
this.targetClass = targetClass;
}
public List<T> unmarshal(String jsonString) throws JsonParseException, JsonMappingException, IOException {
return parseList(jsonString, mapper, targetClass);
}
public String marshal(List<T> list) throws JsonProcessingException {
return mapper.writeValueAsString(list);
}
public static <E> List<E> parseList(String str, ObjectMapper mapper, Class<E> clazz)
throws JsonParseException, JsonMappingException, IOException {
return mapper.readValue(str, listType(mapper, clazz));
}
public static <E> List<E> parseList(InputStream is, ObjectMapper mapper, Class<E> clazz)
throws JsonParseException, JsonMappingException, IOException {
return mapper.readValue(is, listType(mapper, clazz));
}
public static <E> JavaType listType(ObjectMapper mapper, Class<E> clazz) {
return mapper.getTypeFactory().constructCollectionType(List.class, clazz);
}
}
Station.java
public class Station {
private long id;
private String title;
private String name;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#Override
public String toString() {
return String.format("Station [id=%s, title=%s, name=%s]", id, title, name);
}
}
data.json
[{
"id": 123,
"title": "my title",
"name": "my name"
}, {
"id": 456,
"title": "my title 2",
"name": "my name 2"
}]