Develop Reference

Jackson custom value for nested class's field

Good day everyone,
Im struggling with serializing object with nested object having custom name with # JsonProperty.
So, here it is:
public class Wrapper {
public final Payload payload;
public Wrapper(String name){
this.payload = new Payload(name);
}
public static final class Payload{
#JsonProperty("customName")
public final String name;
#JsonCreator
public Payload(#JsonProperty ("customName") String name){
this.name = name;
}
}
}
So, in every test I see non-custom name - "name". I added getter with #JsonProperty without any success.
My test class:
#JsonTest
public class SerializeWrapperTest {
#Test
void whenSerialiseThanCorrect() throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
Wrapper wrapper =
new Wrapper("name");
String json = objectMapper.writeValueAsString(wrapper);
Assertions.assertEquals("{"payload":{"customName":"name"}}",json);
}
}
Test results:
Expected :{"payload":{"customName":"name"}}
Actual :{"payload":{"name":"name"}}
org.opentest4j.AssertionFailedError.
Edited.

If I understood it correct , you need to have a custom property in json for the field , "name".
For that you need to do following -
public class Wrapper {
public final Payload payload;
public Wrapper(String name){
this.payload = new Payload(name);
}
public static final class Payload{
#JsonProperty("customName")
public final String name;
public Payload(String name){
this.name = name;
}
}
JUnit -
public class WrapperTest {
#Before
public void setUp() throws Exception {
}
#After
public void tearDown() throws Exception {
}
#Test
public void whenSerialiseThanCorrect() throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
Wrapper wrapper =
new Wrapper("Ajeet");
String json = objectMapper.writeValueAsString(wrapper);
System.out.println(json);
}
}
}

Finding all private fields and their corresponding getters / setters for nested classes

Situation: few apps communicate using Java DTOs.
I have classes which holds as its fields another classes and they hold another another classes (up to three levels down from top DTO).
Fields could be single DTO or as (exclusively) ArrayList of other classes (DTOs).
All classes are DTO. Just private fields and public setters and getters.
Now, when I get top DTO is there any way to inspect it and get all getters, including nested ones, read fields through getters and then do what I have to do (change some data, specifically remove/change some characters (I have method which does that, all final fields are eventually Strings or Integers), and then write data back using appropriate setter. I guess the best would be to find getter/setter pair per final field and do operation then move to next. Upon finding final (lowest level field) I should check if it is String (do the operation) and if Integer skip operation.
I know there is similar question but it doesn't deal with nested DTOs.
Java reflection get all private fields
If possible I would avoid any 3rd party library.
Any advice on this?
UPDATE: Almost there. Here is kind of demo code, I wish it is so simple, but conceptually it is more less like that:
class SymposiaDTO
import java.util.ArrayList;
public class SymposiaDTO {
private ProgramDTO programDTO;
private ArrayList<PaperDTO> papersDTO;
public ProgramDTO getProgramDTO() {
return programDTO;
}
public void setProgramDTO(ProgramDTO programDTO) {
this.programDTO = programDTO;
}
public ArrayList<PaperDTO> getPapersDTO() {
return papersDTO;
}
public void setPapersDTO(ArrayList<PaperDTO> papersDTO) {
this.papersDTO = papersDTO;
}
}
class ProgramDTO
public class ProgramDTO {
String programTitle;
Integer programID;
public String getProgramTitle() {
return programTitle;
}
public void setProgramTitle(String programTitle) {
this.programTitle = programTitle;
}
public Integer getProgramID() {
return programID;
}
public void setProgramID(Integer programID) {
this.programID = programID;
}
}
class PaperDTO
import java.util.ArrayList;
public class PaperDTO {
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public ArrayList<AuthorDTO> getAuthrosDTO() {
return authrosDTO;
}
public void setAuthrosDTO(ArrayList<AuthorDTO> authrosDTO) {
this.authrosDTO = authrosDTO;
}
private String title;
private ArrayList<AuthorDTO> authrosDTO;
}
class AuthorDTO
public class AuthorDTO {
private String address;
private String name;
private String title;
private String age;
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getAge() {
return age;
}
public void setAge(String age) {
this.age = age;
}
}
class Controller <--- by Carlos if I got his instructions right, this version gives no output at all, never even get's single iteration in for loop.
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
public class Controller {
#SuppressWarnings({ "unused", "rawtypes" })
public static void main(String[] args) {
SymposiaDTO symposiaDTO = new SymposiaDTO();
ProgramDTO programDTO = new ProgramDTO();
PaperDTO paperDTO = new PaperDTO();
AuthorDTO authorDTO = new AuthorDTO();
Class<?> topClass = symposiaDTO.getClass();
for (Class<?> innerClass : topClass.getDeclaredClasses()) {
for (Field field : innerClass.getDeclaredFields()) {
if (Modifier.isPrivate(field.getModifiers())) {
String name = Character.toUpperCase(field.getName().charAt(0)) + field.getName().substring(1);
Method getter;
try {
getter = innerClass.getDeclaredMethod("get" + name);
} catch (Exception ex) {
getter = null;
}
Method setter;
try {
setter = innerClass.getDeclaredMethod("set" + name, field.getType());
} catch (Exception ex) {
setter = null;
}
// TODO real work...
System.out.printf("%s: getter=%s, setter=%s%n", innerClass.getSimpleName(), getter, setter);
}
}
}
}
}
class Controller2 <--- slightly modified previous version, this gets into the loop, but runs twice, and it never gets deeper into nested DTOs.
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Controller2 {
#SuppressWarnings({ "unused", "rawtypes" })
public static void main(String[] args) {
SymposiaDTO symposiaDTO = new SymposiaDTO();
ProgramDTO programDTO = new ProgramDTO();
PaperDTO paperDTO = new PaperDTO();
AuthorDTO authorDTO = new AuthorDTO();
Class<?> topClass = symposiaDTO.getClass();
List<Class> classesToWalk = new ArrayList<Class>();
for (Field field : topClass.getDeclaredFields()) {
Class symposiaDTO2 = field.getDeclaringClass();
classesToWalk.add(symposiaDTO2);
}
for (Class<?> innerClass : classesToWalk) {
Field[] fields = Arrays.stream(innerClass.getDeclaredFields())
.filter(field -> Modifier.isPrivate(field.getModifiers())).toArray(Field[]::new);
for (Field field : fields) {
String name = Character.toUpperCase(field.getName().charAt(0)) + field.getName().substring(1);
Method getter;
try {
getter = innerClass.getDeclaredMethod("get" + name);
} catch (Exception ex) {
getter = null;
}
Method setter;
try {
setter = innerClass.getDeclaredMethod("set" + name, field.getType());
} catch (Exception ex) {
setter = null;
}
// TODO real work...
System.out.printf("%s: getter=%s, setter=%s%n", innerClass.getSimpleName(), getter, setter);
}
}
}
}
This is output from Controller2:
SymposiaDTO: getter=public ProgramDTO SymposiaDTO.getProgramDTO(),
setter=public void SymposiaDTO.setProgramDTO(ProgramDTO)
SymposiaDTO: getter=public java.util.ArrayList
SymposiaDTO.getPapersDTO(), setter=public void
SymposiaDTO.setPapersDTO(java.util.ArrayList)
SymposiaDTO: getter=public ProgramDTO SymposiaDTO.getProgramDTO(),
setter=public void SymposiaDTO.setProgramDTO(ProgramDTO)
SymposiaDTO: getter=public java.util.ArrayList
SymposiaDTO.getPapersDTO(), setter=public void
SymposiaDTO.setPapersDTO(java.util.ArrayList)

You could use getDeclaredClasses to find nested classes, then find the private fields and finally the getters and setters:
Class<?> topClass = ...
for (Class<?> innerClass : topClass.getDeclaredClasses()) {
for (Field field : innerClass.getDeclaredFields()) {
if (Modifier.isPrivate(field.getModifiers())) {
String name = Character.toUpperCase(field.getName().charAt(0))
+ field.getName().substring(1);
Method getter;
try {
getter = innerClass.getDeclaredMethod("get" + name);
} catch (Exception ex) {
getter = null;
}
Method setter;
try {
setter = innerClass.getDeclaredMethod("set" + name, field.getType());
} catch (Exception ex) {
setter = null;
}
// TODO real work...
System.out.printf("%s: getter=%s, setter=%s%n",
innerClass.getSimpleName(), getter, setter);
}
}
}
Edit: above code is valid for "nested classes" as mentioned in the questions title. After the sample code was added to the question it seems like the question is about getters and setters of the fields of the class:
Use getDeclaredFields to get all fields of the class and find the corresponding getter and setter as above; use getType to get the type (class) of each field and (recursively) start over with that class.

Serialization - not working

I create class with methods like a How to serialize an object into a string
and it every say error "java.lang.ClassCastException: java.lang.String cannot be cast to Myclass"
My codes:
1)
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import javax.xml.bind.DatatypeConverter;
public class Serialization {
public static Object fromString(String s) throws IOException,
ClassNotFoundException {
byte[] data = DatatypeConverter.parseBase64Binary(s);
ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(
data));
Object o = ois.readObject();
ois.close();
return o;
}
public static String toString(Serializable o) throws IOException {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(o);
oos.close();
return DatatypeConverter.printBase64Binary(baos.toByteArray());
}
}
2) - calling
MyClass hl = (MyClass) Serialization.fromString(items
.getString("data"));
hl.load(); // this is my method from class
3) MyClass - Hologram
public class Hologram implements Serializable {
/**
*
*/
private static final long serialVersionUID = 1L;
private Location loc;
private String name;
private String displayname;
public ArmorStand stand;
public Hologram(String name, String displayname, Location loc) {
this.loc = loc;
this.name = name;
this.displayname = displayname;
ArmorStand as = (ArmorStand) loc.getWorld().spawnEntity(loc,
EntityType.ARMOR_STAND);
as.setGravity(false);
as.setCanPickupItems(false);
as.setCustomName(displayname);
as.setCustomNameVisible(true);
as.setVisible(false);
this.stand = as;
HologramManager.holograms.put(name, this);
}
public void move(Location loc) {
this.loc = loc;
stand.teleport(loc);
}
public Location getLocation() {
return this.loc;
}
public void remove() {
stand.remove();
HologramManager.holograms.remove(name);
}
public void removeHologram() {
HologramManager.remove(name);
}
public void changeName(String name) {
HologramManager.holograms.remove(this.name);
this.name = name;
HologramManager.holograms.put(name, this);
}
public void changeDisplayName(String displayName) {
this.displayname = displayName;
stand.setCustomName(displayname);
stand.setCustomNameVisible(true);
}
public void load() {
//todo
}
}

Based on the linked answer, the problem most likely lies in the code you aren't showing us. When you serialize your MyClass object, you are probably doing something like this:
MyClass hl;
String base64String = Serialization.toString(hl.toString());
However you should be calling it like this:
MyClass hl;
String base64String = Serialization.toString(hl);
If you pass a String to the serialization function, you'll get a String back when you call Serialization.fromString(). You want to get an object back that you can cast to a MyClass instance, so pass one of those into Serialization.toString().

The fromString() method in Serilization returns an Object, which you wouldnt be able to cast to the class MyClass. The below line is causing the classCastException
MyClass hl = (MyClass) Serialization.fromString(items
.getString("data"));

Gson dynamic class type

I am trying to deserialize JSON objects in my Java application using gson
.
However, I cant figure how to dynamically apply the class type.
I have the class-name in a String
User user = gson.fromJson(userJSON, User.class);
How can I put String myClass as class type?

I believe what is suggested in the previous answer is perfectly OK. Just to avoid all confusions and make it much obvious, here is the full code:
package com.test;
import com.google.gson.Gson;
import com.google.gson.JsonSyntaxException;
public class TestGSON {
public static void main(String[] args) {
Gson gson = new Gson();
try{
Object fromJson = gson.fromJson("{\"name\":\"Dharam\"}", Class.forName("com.test.MyClass"));
if(fromJson instanceof MyClass){
System.out.println(fromJson);
}
} catch (JsonSyntaxException | ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
class MyClass {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#Override
public String toString() {
return "MyClass [name=" + name + "]";
}
}
Output from the above code:
MyClass [name=Dharam]

Class<?> cls = Class.forName(className);
But your className should be fully-qualified - i.e. com.mycompany.MyClass

Self circular reference in Gson

I'm having some issues to deserialize a Json array that follows this format:
[
{
"ChildList":[
{
"ChildList":[
],
"Id":110,
"Name":"Books",
"ApplicationCount":0
}
],
"Id":110,
"Name":"Books",
"ApplicationCount":0
}
]
It's basically an array of Categories where each category can also have a List of sub-categories, and so on and so on.
My class model looks a little like this:
public class ArrayOfCategory{
protected List<Category> category;
}
public class Category{
protected ArrayOfCategory childList;
protected int id;
protected String name;
protected int applicationCount;
}
Now, Gson obviously complains about the circular reference. Is there any way to parse this Json input given that I can't assume how many levels of categories there are?
Thanks in advance.
Edit:
Just in case someone has a similar problem, based on Spaeth answer I adapted the solution to a more general case using reflection. The only requirement is that the List of objects represented by the JSON array is wrapped in another class (like Category and ArrayOfCategory in my example). With the following code applied to my original sample, you can just call "deserializeJson(jsonString,ArrayOfCategory.class)" and it will work as expected.
private <T> T deserializeJson(String stream, Class<T> clazz) throws PluginException {
try {
JsonElement je = new JsonParser().parse(stream);
if (je instanceof JsonArray) {
return deserializeJsonArray(clazz, je);
} else {
return new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE).create().fromJson(stream, clazz);
}
} catch (Exception e) {
throw new PluginException("Failed to parse json string: " + ((stream.length() > 20) ? stream.substring(0, 20) : stream) + "... to class " + clazz.getName());
}
}
private <T> T deserializeJsonArray(Class<T> clazz, JsonElement je) throws InstantiationException, IllegalAccessException {
ParameterizedType listField = (ParameterizedType) clazz.getDeclaredFields()[0].getGenericType();
final Type listType = listField.getActualTypeArguments()[0];
T ret = clazz.newInstance();
final Field retField = ret.getClass().getDeclaredFields()[0];
retField.setAccessible(true);
retField.set(ret, getListFromJsonArray((JsonArray) je,(Class<?>) listType));
return ret;
}
private <E> List<E> getListFromJsonArray(JsonArray je, Class<E> listType) {
Type collectionType = new TypeToken<List<E>>(){}.getType();
final GsonBuilder builder = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE);
Gson jsonParser = builder.create();
return jsonParser.fromJson(je, collectionType);
}

Maybe you could try this:
com.google.gson.Gson gson = new GsonBuilder().create();
InputStreamReader reader = new InputStreamReader(new FileInputStream(new File("/tmp/gson.txt")));
Collection<Category> fromJson = gson.fromJson(reader, new TypeToken<Collection<Category>>() {}.getType());
System.out.println(fromJson);
you will get a good result.
The "magic" occurs here: new TypeToken<Collection<Category>>() {}.getType()
The entire code is:
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStreamReader;
import java.util.Collection;
import java.util.List;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonIOException;
import com.google.gson.JsonSyntaxException;
import com.google.gson.reflect.TypeToken;
public class GsonCircularReference {
public class Category {
protected List<Category> childList;
protected int id;
protected String name;
protected int applicationCount;
public List<Category> getChildList() {
return childList;
}
public void setChildList(final List<Category> childList) {
this.childList = childList;
}
public int getId() {
return id;
}
public void setId(final int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
public int getApplicationCount() {
return applicationCount;
}
public void setApplicationCount(final int applicationCount) {
this.applicationCount = applicationCount;
}
#Override
public String toString() {
return "Category [category=" + childList + ", id=" + id + ", name=" + name + ", applicationCount="
+ applicationCount + "]";
}
}
public static void main(final String[] args) throws JsonSyntaxException, JsonIOException, FileNotFoundException {
com.google.gson.Gson gson = new GsonBuilder().create();
InputStreamReader reader = new InputStreamReader(new FileInputStream(new File("/tmp/gson.txt")));
Collection<Category> fromJson = gson.fromJson(reader, new TypeToken<Collection<Category>>() {}.getType());
System.out.println(fromJson);
}
}
JSON file is:
[
{
"childList":[
{
"childList":[
],
"id":110,
"Name":"Books",
"applicationCount":0
}
],
"id":110,
"name":"Books",
"applicationCount":0
}
]

Take a look at GraphAdapterBuilder. You'll need to include it in your app, but it can serialize arbitrary graphs of objects.