I have a concrete class called "Clerk" that implements an interface called "Worker". I want to deserialize my Clerk object and store it into a Worker object, just like we can normally do if I had this:
Worker clerkWorker = new Clerk("abc", "alice white");
Only instead I have the clerk json that I want to deserialize and store into clerkWorker:
{
"uid": "1a-2wq"
"name": "Maryanne Chen"
}
The interface:
public interface Worker {
int getUID();
String getName();
void work();
}
The class implementing it:
private class Clerk implements Worker {
int uid;
String name;
public Clerk(id, fullname) {
uid = id;
name = fullname;
}
public void work() {
System.out.println("Clerk is processing files");
}
}
Following the second solution described here: Using Gson with Interface Types I copied RuntimeTypeAdapterFactory locally to use it:
RuntimeTypeAdapterFactory typeFactory =
RuntimeTypeAdapterFactory.of(Worker.class, "type");
typeFactory.registerSubtype(Clerk.class);
Gson gson = new GsonBuilder().registerTypeAdapterFactory(typeFactory).create();
final Reader r = getReader(jsonFile);
return gson.fromJson(r, typeToken.getType());
I've verified my getReader function is working. I get an exception:
"cannot deserialize interface Worker because it does not define a field named type".
If Clerk is a Worker, then there shouldn't be any reason you can't just do exactly what you already showed with the constructor example.
Worker w = (new Gson()).fromJson( jsonData, Clerk.class ) ;
I'm not sure why you'd want so much to cast upward like this right away, since anything that wants a Worker should be able to take the Clerk directly...
When you tell Gson to deserialize something with an interface type, it has to somehow figure out which implementing class it should use. It does this by checking a special field in the json data. When you call RuntimeTypeAdapterFactory.of(Worker.class, "type");, you are telling Gson that the field it should check to figure out which class it should use is named "type". When you then give it a chunk of json data that doesn't have that field, it complains.
To fix this, you will need to either add the "type" field to your json data, or tell Gson in the code to deserialize as Clerk, not Worker.
Related
This seems to be a missing part in the documentation of Vaadin...
I call an API to get data in my UI like this:
#Override
public URI getUri(String url, PageRequest page) {
return UriComponentsBuilder.fromUriString(url)
.queryParam("page", page.getPageNumber())
.queryParam("size", page.getPageSize())
.queryParam("sort", (page.getSort().isSorted() ? page.getSort() : ""))
.build()
.toUri();
}
#Override
public Mono<Page<SomeDto>> getDataByPage(PageRequest pageRequest) {
return webClient.get()
.uri(getUri(URL_API + "/page", pageRequest))
.retrieve()
.bodyToMono(new ParameterizedTypeReference<>() {
});
}
In the Vaadin documentation (https://vaadin.com/docs/v10/flow/binding-data/tutorial-flow-data-provider), I found an example with DataProvider.fromCallbacks but this expects streams and that doesn't feel like the correct approach as I need to block on the requests to get the streams...
DataProvider<SomeDto, Void> lazyProvider = DataProvider.fromCallbacks(
q -> service.getData(PageRequest.of(q.getOffset(), q.getLimit())).block().stream(),
q -> service.getDataCount().block().intValue()
);
When trying this implementation, I get the following error:
org.springframework.core.codec.CodecException: Type definition error: [simple type, class org.springframework.data.domain.Page]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `org.springframework.data.domain.Page` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
at [Source: (io.netty.buffer.ByteBufInputStream); line: 1, column: 1]
grid.setItems(lazyProvider);
I don't have experience with vaadin, so i'll talk about the deserialization problem.
Jackson needs a Creator when deserializing. That's either:
the default no-arg constructor
another constructor annotated with #JsonCreator
static factory method annotated with #JsonCreator
If we take a look at spring's implementations of Page - PageImpl and GeoPage, they have neither of those. So you have two options:
Write your custom deserializer and register it with the ObjectMapper instance
The deserializer:
public class PageDeserializer<T> extends StdDeserializer<Page<T>> {
public PageDeserializer() {
super(Page.class);
}
#Override
public Page<T> deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JacksonException {
//TODO implement for your case
return null;
}
}
And registration:
SimpleModule module = new SimpleModule();
module.addDeserializer(Page.class, new PageDeserializer<>());
objectMapper.registerModule(module);
Make your own classes extending PageImpl, PageRequest, etc. and annotate their constructors with #JsonCreator and arguments with #JsonProperty.
Your page:
public class MyPage<T> extends PageImpl<T> {
#JsonCreator
public MyPage(#JsonProperty("content_prop_from_json") List<T> content, #JsonProperty("pageable_obj_from_json") MyPageable pageable, #JsonProperty("total_from_json") long total) {
super(content, pageable, total);
}
}
Your pageable:
public class MyPageable extends PageRequest {
#JsonCreator
public MyPageable(#JsonProperty("page_from_json") int page, #JsonProperty("size_from_json") int size, #JsonProperty("sort_object_from_json") Sort sort) {
super(page, size, sort);
}
}
Depending on your needs for Sort object, you might need to create MySort as well, or you can remove it from constructor and supply unsorted sort, for example, to the super constructor. If you are deserializing from input manually you need to provide type parameters like this:
JavaType javaType = TypeFactory.defaultInstance().constructParametricType(MyPage.class, MyModel.class);
Page<MyModel> deserialized = objectMapper.readValue(pageString, javaType);
If the input is from request body, for example, just declaring the generic type in the variable is enough for object mapper to pick it up.
#PostMapping("/deserialize")
public ResponseEntity<String> deserialize(#RequestBody MyPage<MyModel> page) {
return ResponseEntity.ok("OK");
}
Personally i would go for the second option, even though you have to create more classes, it spares the tediousness of extracting properties and creating instances manually when writing deserializers.
There are two parts to this question.
The first one is about asynchronously loading data for a DataProvider in Vaadin. This isn't supported since Vaadin has prioritized the typical case with fetching data straight through JDBC. This means that you end up blocking a thread while the data is loading. Vaadin 23 will add support for doing that blocking on a separate thread instead of keeping the UI thread blocked, but it will still be blocking.
The other half of your problem doesn't seem to be directly related to Vaadin. The exception message says that the Jackson instance used by the REST client isn't configured to support creating instances of org.springframework.data.domain.Page. I don't have direct experience with this part of the problem, so I cannot give any advice on exactly how to fix it.
It is a Springboot project. The code snip is as below. At line 59, the desired retrun type for restTemplate.getForEntity is List<Template>. While debugging, I find that the actual return type is an ArrayList contains many LinkedHashMap.
While LinkedHashMap is not sub class of Template. I don't know why the expect result type and the actual result type match.
Could anyone tell why it doesn't return ArrayList<Template>, instead of ArrayList<LinkedHashMap>? Thanks.
Template is an model defined in our project.
public class Template {
private String id;
private String name;
private String content;
xxx getters and setters
}
And it is a controller where the resttemplate is invoked.
#PostMapping(value = "/getTemplatesByGroup", produces = "application/json;charset=UTF-8")
#ResponseBody
public EUDataGrid<Template> getTemplatesByGroup(#RequestParam(defaultValue = "-1") Integer groupId) {
EUDataGrid<Template> grid = new EUDataGrid<>();
xxxx
List<Template> list = restTemplate.getForEntity(urlFullTemplates, ArrayList.class).getBody();
xxxx
return grid;
}
Json result format as below
[
{
"id": 1788,
"name": "xxxx",
"content": "xxxxx."
},
{
"id": 1787,
"name": "xxxxx",
"content": "xxxx"
}
]
Edit:
I googled a lot for this issue again. It is a common problem. There are similar scenarios some guys also encountered. I add the link in the foot of this post.
It seems that this is a bug of RestTemplate to handle generic properly. And there are ways to resolve this.
Here I want to know, why it doesn't throw exception when restTemplate returns ArrayList<LinkedHashMap> and assign it to List<Template>? They are differnt types. This is some kind of like assgin an int to a string.
I guess there is some magic with generic type. Could someone tell more about this? Thanks.
Unable to get a generic ResponseEntity<T> where T is a generic class "SomeClass<SomeGenericType>"
Using Spring RestTemplate in generic method with generic parameter
RestTemplate: how to get generic List response
Here is the problem in JSON response you are getting List<Template> or Array of Template, but in the responseType you just specified ArrayList where jackson doesn't know which type of ArrayList it is
limitation See the limitation
getForEntity(URI url, Class<T> responseType)
This sends a request to the specified URI using the GET verb and converts the response body into the requested Java type. This works great for most classes, but it has a limitation: we cannot get lists of objects.
One way is simple just specify Array type
Template[] list = restTemplate.getForEntity(urlFullTemplates, Template[].class).getBody();
Or use the exchange method
I'm new to GraphQL and I'm currently implementing a GraphQL API into an established Java code, using GraphQL-SPQR and I'm running into a couple issues when it comes extracting data from hierarchical classes.
The issues that I am running into are as follows.
Firstly I don't if there is an easy way to get all the data associated with a returned node. If there is, this would be most useful for my more complex classes.
Secondly when a method returns an abstract class, I only seem able to request the variables on the abstract class. I'm sure this should be possible I am just hitting my head against a wall.
As a simple example
public abstract class Animal {
private String name;
private int age;
// Constructor
#GraphQLQuery(name = "name")
public String getName() {
return name;
}
// Age getter
}
public class Dog extends Animal {
private String favouriteFood;
// Constructor
#GraphQLQuery(name = "favouriteFood")
public String getFavouriteFood() {
return favouriteFood;
}
}
public class Database {
#GraphQLQuery(name = "getanimal")
public Animal getAnimal(#GraphQLArgument(name = "animalname") String animalname) {
return database.get(name);
}
}
So in my first question what I am currently querying is.
"{animalname(name: \"Daisy\") {name age}}"
This works fine as expected. If you imagine the class however had 10 variables I would like to merely be able to write the equivalent of the following without having to look them up.
"{node(name: \"Daisy\") {ALL}}"
Is this possible?
In terms of my second question.
The follow query, throws an error ('Field 'favouriteFood' in type 'Animal' is undefined')
"{animalname(name: \"Bones\") {name age favouriteFood}}"
likewise (reading Inline Fragments of https://graphql.org/learn/queries/)
"{animalname(name: \"Bones\") {name age ... on Dog{favouriteFood}}}"
throws an error Unknown type Dog
This is annoying as I have a number of sub classes which could be returned and may require handling in different fashions. I think I can understand why this is occuring as GraphQL has no knowledge as to what the true class is, only the super class I have returned. However I'm wondering if there is a way to fix this.
Ultimately while I can get past both these issues by simply serialising all the data to JSON and sending it back, it kind of gets rid of the point of GraphQL and I would rather find an alternate solution.
Thank you for any response.
Apologies if these are basic questions.
Answering my own question to help anyone else who has this issue.
The abstract class needs to have #GraphQLInterface included, as shown below
#GraphQLInterface(name = "Animal ", implementationAutoDiscovery = true)
public abstract class Animal {
private String name;
private int age;
// Constructor
#GraphQLQuery(name = "name")
public String getName() {
return name;
}
// Age getter
}
The following code was found after much solution and was created by the creator of SPQR. Effectively, when setting up your schema you need to declare an interface mapping strategy. The code below can be copied wholesale with only the "nodeQuery" variable being replaced with the service you are using to containing your "#GraphQLQuery" and "#GraphQLMutation" methods.
final GraphQLSchema schema = new GraphQLSchemaGenerator()
.withInterfaceMappingStrategy(new InterfaceMappingStrategy() {
#Override
public boolean supports(final AnnotatedType interfase) {
return interfase.isAnnotationPresent(GraphQLInterface.class);
}
#Override
public Collection<AnnotatedType> getInterfaces(final AnnotatedType type) {
Class clazz = ClassUtils.getRawType(type.getType());
final Set<AnnotatedType> interfaces = new HashSet<>();
do {
final AnnotatedType currentType = GenericTypeReflector.getExactSuperType(type, clazz);
if (supports(currentType)) {
interfaces.add(currentType);
}
Arrays.stream(clazz.getInterfaces())
.map(inter -> GenericTypeReflector.getExactSuperType(type, inter))
.filter(this::supports).forEach(interfaces::add);
} while ((clazz = clazz.getSuperclass()) != Object.class && clazz != null);
return interfaces;
}
}).withOperationsFromSingleton(nodeQuery)// register the service
.generate(); // done ;)
graphQL = new GraphQL.Builder(schema).build();
As this solution took some hunting, I'm going to start a blog soon with the other solutions I've stumbled on.
With regards to having a query that just returns all results. This is not possible in GraphQL. One workaround I might write is to have a endpoint that returns JSON of the entire object and the name of the object, then I can just use ObjectMapper to convert it back.
I hope this helps other people. I'm still looking into an answer for my first question and will update this post when I find one.
I have an API the returns a standard reply for all requests that gets parsed by gson/retrofit.
public class ServerReply<T> {
#Expose
private String status;
#Expose
private T data;
#Expose
private String message;
}
I have an interface for Retrofit that will return a list of users inside of serverReply.
public interface Test {
#POST("/Test")
void runTest(#Body Body body, Callback<ServerReply<List<User>>> response);
}
I would like to get a different list of objects depending on the content of the body. Is it possible to use templating/generics to accomplish this?(see below)
public interface Test<T> {
#POST("/Test")
void runTest(#Body Body body, Callback<ServerReply<List<T>>> response);
}
No, but it's a Java limitation not a missing Retrofit feature. Due to type erasure there is no way for Retrofit to resolve what the type variable T actually is to pass to the deserializer without a concrete class.
I am trying to submit a form from Ext JS 4 to a Spring 3 Controller using JSON. I am using Jackson 1.9.8 for the serialization/deserialization using Spring's built-in Jackson JSON support.
I have a status field that is initially null in the Domain object for a new record. When the form is submitted it generates the following json (scaled down to a few fields)
{"id":0,"name":"someName","status":""}
After submitted the following is seen in the server log
"nested exception is org.codehaus.jackson.map.JsonMappingException: Can not construct instance of com.blah.domain.StatusEnum from String value '': value not one of the declared Enum instance names"
So it appears that Jackson is expecting a valid Enum value or no value at all including an empty string. How do I fix this whether it is in Ext JS, Jackson or Spring?
I tried to create my own ObjectMapper such as
public class MyObjectMapper extends Object Mapper {
public MyObjectMapper() {
configure(DeserializationConfig.Feature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, true);
}
}
and send this as a property to MappingJacksonMappingView but this didn't work. I also tried sending it in to MappingJacksonHttpMessageConverter but that didn't work. Side question: Which one should I be sending in my own ObjectMapper?
Suggestions?
The other thing you could do is create a specialized deserializer (extends org.codehaus.jackson.map.JsonDeserializer) for your particular enum, that has default values for things that don't match. What I've done is to create an abstract deserializer for enums that takes the class it deserializes, and it speeds this process along when I run into the issue.
public abstract class EnumDeserializer<T extends Enum<T>> extends JsonDeserializer<T> {
private Class<T> enumClass;
public EnumDeserializer(final Class<T> iEnumClass) {
super();
enumClass = iEnumClass;
}
#Override
public T deserialize(final JsonParser jp,
final DeserializationContext ctxt) throws IOException, JsonProcessingException {
final String value = jp.getText();
for (final T enumValue : enumClass.getEnumConstants()) {
if (enumValue.name().equals(value)) {
return enumValue;
}
}
return null;
}
}
That's the generic class, basically just takes an enum class, iterates over the values of the enum and checks the next token to match any name. If they do it returns it otherwise return null;
Then If you have an enum MyEnum you'd make a subclass of EnumDeserializer like this:
public class MyEnumDeserializer extends EnumDeserializer<MyEnum> {
public MyEnumDeserializer() {
super(MyEnum.class);
}
}
Then wherever you declare MyEnum:
#JsonDeserialize(using = MyEnumDeserializer.class)
public enum MyEnum {
...
}
I'm not familiar with Spring, but just in case, it may be easier to handle that on the client side:
Ext.define('My.form.Field', {
extend: 'Ext.form.field.Text',
getSubmitValue: function() {
var me = this,
value;
value = me.getRawValue();
if ( value === '' ) {
return ...;
}
}
});
You can also disallow submitting empty fields by setting their allowBlank property to false.
Ended up adding defaults in the EXT JS Model so there is always a value. Was hoping that I didn't have to this but it's not that big of a deal.
I have the same issue. I am reading a JSON stream with some empty strings. I am not in control of the JSON stream, because it is from a foreign service. And I am always getting the same error message. I tried this here:
mapper.getDeserializationConfig().with(DeserializationConfig.Feature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
But without any effect. Looks like a Bug.