I have a structure where there is a group which may contain leaf items or other groups too, so I implemented the Composite design pattern using a "SavedItem" interface with 2 subclasses "Rectangle" which represents the leaf node and "Group" where Group contains an arraylist of SavedItem objects to implement this composition hierarchy.
I used Gson to export the structure I have as a json file. The exported object is basically an arraylist of SavedItem objects and everything worked fine. However, the application requires loading a json file as well and converting it back to java objects, so I also used Gson but as I am trying to retrieve an arraylist of the interface "SavedItem" I am getting this exception:
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: Unable to invoke no-args constructor for interface SavedItem. Register an InstanceCreator with Gson for this type may fix this problem.
This is the "SavedItem" interface:
public interface SavedItem {
public String getId();
public String getDescription();
public Boolean getIsGroup();
public void addSavedItem(SavedItem savedItem);
}
This is the leaf node class "Rectangle" (of course with all setters and getters):
public class Rectangle implements SavedItem{
private String id;
private String type;
private String description;
private Point startingPoint ;
private double width;
private double height;
private boolean isGroup;
public Rectangle(){}
public Rectangle(Point startingPoint, double width, double height, String type , String description) {
this.id = "FIELD-" + UUID.randomUUID().toString();
this.startingPoint = startingPoint;
this.width = width;
this.height = height;
this.type= type;
this.description = description;
this.isGroup = false;
}
And this is the Group class (in addition to setters and getters):
public class Group implements SavedItem{
private String id;
private String description;
ArrayList<SavedItem> groupFields = new ArrayList<SavedItem>();
private boolean isGroup;
public Group(){}
public Group(String description) {
this.id = "GROUP-" + UUID.randomUUID().toString();
this.description = description;
this.isGroup = true;
}
The main object (called MedicalForm) contains this arraylist:
private ArrayList<SavedItem> StructuredFields = new ArrayList<>();
Creating the json file is done like this:
try {
Writer writer = new FileWriter(path+"/"+ form.getFormName()+".json");
Gson gson = new GsonBuilder().setPrettyPrinting().create();
JsonObject object = new JsonObject();
object.addProperty("id", form.getId());
object.addProperty("formName", form.getFormName());
JsonElement element = gson.toJsonTree(form.getStructuredFields());
object.add("StructuredFields", element);
object.addProperty("imageString", form.getImageString());
gson.toJson(object,writer);
writer.close();
} catch (IOException e) {
e.printStackTrace();
}
And loading a json and converting it back to java objects is done like this:
JsonReader reader = new JsonReader(new FileReader(path));
Gson gson = new Gson();
MedicalForm response = gson.fromJson(reader, MedicalForm.class);
form.setId(response.getId());
form.setFormName(response.getFormName());
form.setStructuredFields(response.getStructuredFields());
form.setImageString(response.getImageString());
I searched for the problem and I found solutions saying I should add a default constructor and so I did in both Group and Rectangle classes in addition to the class where I am trying to convert json to java object. However, I can't add a constructor in the interface, and replacing the interface with a superclass will prevent the flexibility of casting.
How can I solve this issue and convert the json properly to java objects?
Related
I am working with some json objects that I call verbose:
{
"user": {
"name": "username",
"email": "blah#blah.com",
"time_zone": "America/New_York"
}
}
But I'd prefer to just deal with them in terms of java POJOs like:
class UserDetails {
String name;
String email;
String timeZone;
...
}
Note that I have no control over the POJO as it is generated code.
My two requirements for (de)serialization is that
the timeZone field maps to time_zone in JSON
the outer user is ignored
So I have some customer (de)serializers:
class UserDeserializer implements JsonDeserializer<UserDetails> {
#Override
public UserDetails deserialize(JsonElement je, Type type, JsonDeserializationContext jdc)
throws JsonParseException {
JsonElement content = je.getAsJsonObject().get("user");
UserDetails userDetails = new GsonBuilder()
.setFieldNamingStrategy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
.create()
.fromJson(content, UserDetails.class);
return userDetails;
}
}
class UserSerializer implements JsonSerializer<UserDetails> {
#Override
public JsonElement serialize(UserDetails userDetails, Type typeOfSrc,
JsonSerializationContext context) {
JsonObject obj = new JsonObject();
JsonElement je = new GsonBuilder()
.setFieldNamingStrategy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
.create().toJsonTree(userDetails);
obj.add("user", je);
return obj;
}
}
I feel like creating new Gson objects in the (de)serializer logic is not ideal/efficient just to add and remove the outermost user key.
EDIT: Actually .setFieldNamingStrategy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES) does work fine on deserialization.
I don't really think it's a good idea in general, and you should probably better have a single Wrapper<T> for all "top-most" purposes (if you don't want your inner objects to be considered "verbose").
But you're right when you say
I feel like creating new Gson objects in the (de)serializer logic is not ideal/efficient just to add and remove the outermost user key.
So:
Creating a Gson is a relatively expensive operation.
This just creates unnecessary objects and hits the heap.
Gson may be configured in a special way and you might want to share the same Gson configuration everywhere.
JsonSerializer and JsonDeserializer operate on JSON trees (JsonElement and its subclasses), therefore it creates an intermediate in-memory tree representations before/after serialization/deserialization.
You might consider a faster solution, that's free of those items.
final class VerboseTypeAdapterFactory
implements TypeAdapterFactory {
private final Map<Class<?>, String> mappings;
private VerboseTypeAdapterFactory(final Map<Class<?>, String> mappings) {
this.mappings = mappings;
}
static TypeAdapterFactory get(final Map<Class<?>, String> mappings) {
// Create a defensive copy to make sure the map is not modified from outside
final Map<Class<?>, String> mappingsCopy = mappings
.entrySet()
.stream()
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
return new VerboseTypeAdapterFactory(mappingsCopy);
}
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
final Class<? super T> rawType = typeToken.getRawType();
// Not something we can handle?
if ( !mappings.containsKey(rawType) ) {
// Then let Gson do its job elsewhere
return null;
}
// Getting a property name we want to use for a particular class
final String propertyName = mappings.get(rawType);
// And getting the original type adapter for this class (effectively ReflectiveTypeAdapterFactory.Adapter)
final TypeAdapter<T> delegateTypeAdapter = gson.getDelegateAdapter(this, typeToken);
return VerboseTypeAdapter.get(propertyName, delegateTypeAdapter);
}
private static final class VerboseTypeAdapter<T>
extends TypeAdapter<T> {
private final String propertyName;
private final TypeAdapter<T> delegateTypeAdapter;
private VerboseTypeAdapter(final String propertyName, final TypeAdapter<T> delegateTypeAdapter) {
this.propertyName = propertyName;
this.delegateTypeAdapter = delegateTypeAdapter;
}
private static <T> TypeAdapter<T> get(final String propertyName, final TypeAdapter<T> delegateTypeAdapter) {
return new VerboseTypeAdapter<>(propertyName, delegateTypeAdapter)
// A convenient method to simplify null-handling
.nullSafe();
}
#Override
#SuppressWarnings("resource")
public void write(final JsonWriter out, final T object)
throws IOException {
// Open the object with `{`
out.beginObject();
// Prepend the object with its reserved name
out.name(propertyName);
// Write the object
delegateTypeAdapter.write(out, object);
// And close the object with `}`
out.endObject();
}
#Override
public T read(final JsonReader in)
throws IOException {
// Assume the very first token is `{`
in.beginObject();
// Peeking what's the actual property name
final String actualPropertyName = in.nextName();
// And if it's not we expect, throw a JSON parse exception
if ( !actualPropertyName.equals(propertyName) ) {
throw new JsonParseException("Expected " + propertyName + " but was " + actualPropertyName);
}
// Otherwise read the value led by the property name
final T object = delegateTypeAdapter.read(in);
// And make sure there are no more properties
if ( in.hasNext() ) {
throw new JsonParseException(propertyName + " is expected to be the only top-most property");
}
// Assume the very last token is `}` (this works for the check above, but we made it more semantical)
in.endObject();
return object;
}
}
}
So, for example, the following code
private static final Gson gson = new GsonBuilder()
.setFieldNamingStrategy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
.registerTypeAdapterFactory(VerboseTypeAdapterFactory.get(ImmutableMap.of(UserDetails.class, "user")))
.create();
...
final UserDetails userDetails = gson.fromJson(jsonReader, UserDetails.class);
System.out.println(userDetails.name);
System.out.println(userDetails.email);
System.out.println(userDetails.timeZone);
final String json = gson.toJson(userDetails);
System.out.println(json);
produces
username
blah#blah.com
America/New_York
{"user":{"name":"username","email":"blah#blah.com","time_zone":"America/New_York"}}
As the conclusion:
No more excessive Gson instantiation.
Original Gson instance configuration inherited (i.e. FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES set once).
No intermediate JsonElement instances.
Please note: There are many questions on this site about how to use custom Jackson deserializers...this question is not one more of those! This questions has to do with using a deserializer under very unique circumstances (none of which have previous questions/answers on this site!).
Spring Boot using Jackson for JSON serialization here. I have two POJOs that are used in the #RequestBody (HTTP request entity) for a POST endpoint:
#JsonDeserialize(using = FizzDeserializer.class)
public class Fizz {
private String name;
private String label;
private Integer code;
// Getters, setters & ctors
}
#JsonDeserialize(using = BuzzDeserializer.class)
public class Buzz {
private String id;
private String locale;
private Set<Fizz> fizzes;
// Getters, setters & ctors
}
#RestController
#RequestMapping("v1/data/buzzes")
public class BuzzController {
#PostMapping
public void updateBuzz(#RequestBody Buzz buzz) {
// do whatever
}
}
I want HTTP clients to be able to POST the following JSON to this endpoint:
{
"id" : "12345-67890",
"locale" : "en_US",
"fizzes" : [
"foo",
"bar"
]
}
...where "foo" and "bar" are the Fizz#names of two different Fizz instances. In other words, I don't want the client to have to specify the entire Fizz object, just specify its name as a JSON string (my app + DB guarantee Fizzes have unique names).
So I'm using a custom JsonDeserializer to accomplish all this mapping:
public BuzzDeserializer extends JsonDeserializer<Buzz> {
Buzz deserialize(JsonParser jsonParser, DeserializationContext ctxt) throws IOException, JsonProcessingException {
JsonNode buzzNode = jsonParser.readValueAsTree();
String id = buzzNode.get("id");
String locale = buzzNode.get("locale");
// TODO: How to read "foo" and "bar" (etc.) into a Set<Fizz> instances?
Set<Fizz> fizzes = ???
new Buzz(id, locale, fizzes);
}
}
public FizzDeserializer extends JsonDeserializer<Fizz> {
private FizzDAO fizzDAO;
// Getters, setters & ctors...
Fizz deserialize(JsonParser jsonParser, DeserializationContext ctxt) throws IOException, JsonProcessingException {
JsonNode fizzNode = jsonParser.readValueAsTree();
// If I can get access to "foo"/"bar"/etc. string somehow, I can look up the Fizz using the DAO:
String fooBarEtcStr = ???
return fizzDAO.findFizzByName(fooBarEtcStr);
}
However I'm not sure how I can read the JSON fizzes array into a Set<Fizz> inside this deserializer. Any ideas?
Taken from the comments to the question, the only problem seems to be to get the JSON array. Provided that the service or DAO to lookup is already injected or provided in the serializer, try something along the lines:
final JsonNode arr = buzzNode.get("fizzes");
if (arr.isArray()) {
final Set<Fizz> fizzes = Sets.newHashSetWithExpectedSize(arr.size());
for (JsonNode obj : arr) {
final String name = obj.asText();
Fizz fizz = // load from DAO
fizzes.add(fizz);
}
}
This can of course be optimised by collecting the String values and use only one DAO call. Also some java8 streaming could make the code less verbose.
I am working on an embedded jersey instance which will run a JAXB RESTful service. I have configured Jackson with two steps:
Adding this to my POM
<dependency>
<groupId>org.glassfish.jersey.media</groupId>
<artifactId>jersey-media-json-jackson</artifactId>
<version>2.23.2</version>
</dependency>
Registering it in my application
public HandheldApplication() {
scripts.add(HandheldServer.class);
scripts.add(BasicScript.class);
// Add JacksonFeature.
scripts.add(JacksonFeature.class);
scripts.add(LoggingFilter.class);
}
I have a complex object being passed back and forth as shown below:
package com.ziath.handheldserver.valueobjects;
import java.util.ArrayList;
import java.util.List;
import javax.xml.bind.annotation.*;
#SuppressWarnings("restriction")
#XmlRootElement
public class Widget {
private String key;
private String name;
private List<String> options = new ArrayList<String>();
private String value;
private String type;
public Widget(){
super();
}
public Widget(String key, String name, List<String> options, String value,
String type) {
super();
this.key = key;
this.name = name;
this.options = options;
this.value = value;
this.type = type;
}
public String getKey() {
return key;
}
public void setKey(String key) {
this.key = key;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<String> getOptions() {
return options;
}
public void setOptions(List<String> options) {
this.options = options;
}
public String getValue() {
return value;
}
public void setValue(String value) {
this.value = value;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
}
When I execute this in a GET method as shown below:
#Override
#GET
#Path("getKeys")
#Produces(MediaType.APPLICATION_JSON)
public List<Widget> getKeys(#QueryParam(value = "page") int page)
This works fine and I get JSON back; however when I execute it is a PUT as shown below:
#Override
#PUT
#Path("validateKeys")
#Produces({MediaType.APPLICATION_JSON})
#Consumes(MediaType.APPLICATION_JSON)
public boolean validateKeys(#QueryParam(value = "page")int page, #QueryParam(value = "widgets")List<Widget> widgets)
When I execute a PUT to access this method I get a stack trace as follows:
Caused by: org.glassfish.jersey.internal.inject.ExtractorException: Error un-marshalling JAXB object of type: class com.ziath.handheldserver.valueobjects.Widget.
at org.glassfish.jersey.jaxb.internal.JaxbStringReaderProvider$RootElementProvider$1.fromString(JaxbStringReaderProvider.java:195)
at org.glassfish.jersey.server.internal.inject.AbstractParamValueExtractor.convert(AbstractParamValueExtractor.java:139)
at org.glassfish.jersey.server.internal.inject.AbstractParamValueExtractor.fromString(AbstractParamValueExtractor.java:130)
at org.glassfish.jersey.server.internal.inject.CollectionExtractor.extract(CollectionExtractor.java:88)
at org.glassfish.jersey.server.internal.inject.CollectionExtractor$ListValueOf.extract(CollectionExtractor.java:107)
at org.glassfish.jersey.server.internal.inject.QueryParamValueFactoryProvider$QueryParamValueFactory.provide(QueryParamValueFactoryProvider.java:89)
... 38 more
Caused by: javax.xml.bind.UnmarshalException
- with linked exception:
[org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Content is not allowed in prolog.]
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.createUnmarshalException(AbstractUnmarshallerImpl.java:335)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.createUnmarshalException(UnmarshallerImpl.java:563)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:249)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:214)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:140)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:123)
at org.glassfish.jersey.jaxb.internal.JaxbStringReaderProvider$RootElementProvider$1.fromString(JaxbStringReaderProvider.java:190)
... 43 more
So it seems to me that Jackson is correctly marshalling my POJO into JSON but trying to unmarshall it as XML. Note that I switched to Jackson away from MOXy because I needed to be able to handle collections coming back and forth and apparently MOXy cannot do that.
Is there a setting I've missed to tell Jackson/Jersey to go both ways for JSON?
Try removing #QueryParam(value = "widgets") because you should pass it as entity body - not query param.
#PUT
#Path("validateKeys")
#Produces({MediaType.APPLICATION_JSON})
#Consumes(MediaType.APPLICATION_JSON)
public boolean validateKeys(#QueryParam(value = "page")int page, List<Widget> widgets)
Also you can make wrapper class:
#XmlRootElement
public class Widgets {
private List<Widget> widgets;
// other fields, setters and getters
}
And then:
#PUT
#Path("validateKeys")
#Produces({MediaType.APPLICATION_JSON})
#Consumes(MediaType.APPLICATION_JSON)
public boolean validateKeys(#QueryParam(value = "page")int page, Widgets widgets)
I would suggest to read some discussions about REST design because you're using verbs in your paths:
Is this a bad REST URL?
Understanding REST: Verbs, error codes, and authentication
I was switching between QueryParam and FormParam to try and get one of them to work. If I use FormParam I also need to change the consumes to APPLICATION_FORM_URLENCODED.
The actual issue was that the default unmarshalling with Jackson was using XML because it was tagged as an XML resource - take that out! I finally managed to work out how to unmarshall from JSON by using a static fromString method. Then to handle the list; I cannot use a wrapper class because this needs to be highly cross language and exposing a wrapper with a list would have complicated the implementation from Python, C#, etc. The way to get it to accept a list with a wrapper is to post the name of the param (in this case widgets) multiple time. Then each JSON passed in will be called against the fromString method.
I need to create custom jackson generic typed bean deserializer.
T is bean implementing IEntity.
public interface IEntity {
public int getId();
}
#JsonDeserialize(using = RestLoaderRequestDeserializer.class)
#Produces(MediaType.APPLICATION_JSON)
public class RestLoaderRequest<T extends IEntity> Serializable {
private T entity; // entity to load field to
private String className; // actual class of entity
private String fieldName; // fieldName to lazy REST load
// constructors(non parameter & all parameter), getters, setters, hashCode, equals, toString
}
Now what I have is this:
public class RestLoaderRequestDeserializer extends JsonDeserializer<RestLoaderRequest<IEntity>> {
#Override
public RestLoaderRequest<IEntity> deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
try {
ObjectCodec objectCodec = jp.getCodec();
JsonNode node = objectCodec.readTree(jp);
String className = node.get("className").textValue();
String fieldName = node.get("fieldName").textValue();
Class<?> clazz = Class.forName(className);
JsonNode rawEntityNode = node.get("entity");
// How to deserialize rawEntityNode to T based on className ?
RestLoaderRequest<IEntity> request = new RestLoaderRequest<IEntity>();
request.setClassName(className);
request.setFieldName(fieldName);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
return null;
}
}
1) How to do auto-deserialization of entity based on className?
Keep in mind that handling of type information and handling of payload data are separate.
So regular JSON (de)serializers are not supposed to handle types at all: rather, separate TypeSerializer and TypeDeserializer are used. This is done to prevent explosion in number of combinations; there are couple of TypeSerializer/-Deserializer implementations (for different inclusion mechanism), but hundreds of value (de)serializers.
Custom (de)serializers can obviously break this separation, when you have full control. But core Jackson has this separation, and you can see examples in standard handlers.
gson is such a great serialize/deserialization tool. It's really simple to get a JSON representation of an arbitrary object by using the toJson-function.
Now I want to send the data of my object to the browser to be used within javascript/jQuery. Thus, I need one additional JSON element defining the dom class of the object which is coded within my object as a dynamic/memberless function
public String buildDomClass()
How to add this string to my String created by the toJson function?
Any ideas?
Thanks a lot
An easy way is to combine a TypeAdapterFactory and an interface.
First an interface for your method :
public interface MyInterface {
public String buildDomClass();
}
then the factory :
final class MyAdapter implements TypeAdapterFactory {
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> tokenType) {
final TypeAdapter<T> adapter = gson.getDelegateAdapter(this, tokenType);
return new TypeAdapter<T>() {
#Override
public T read(JsonReader reader) throws IOException {
return adapter.read(reader);
}
#Override
public void write(JsonWriter writer, T value) throws IOException {
JsonElement tree = adapter.toJsonTree(value);
if (value instanceof MyInterface) {
String dom = ((MyInterface) value).buildDomClass();
JsonObject jo = (JsonObject) tree;
jo.addProperty("dom", dom );
}
gson.getAdapter(JsonElement.class).write(writer, tree);
}
};
}
}
Easy to understand, if the object you want to serialize implement the interface, you delegate the serializing, and then you add an extra property holding you DOM.
In case you don't know, you register a factory like this
Gson gson = new GsonBuilder().registerTypeAdapterFactory(new MyAdapter()).create();