I'm working with a 3rd party JSON API, it returns data like this:
{details: {...}, ...}
I use Java Jackson to deserialize this JSON string into a POJO object, the field declaration is :
#JsonProperty("details")
public Details getDetails(){...}
and Details is another class.
Everything is fine until I found that API may return data like this:
{details: false, ...}
If details is empty, it returns false!!! And jackson gave me this exception:
com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value of type [simple type, class Details] from Boolean value; no single-boolean/Boolean-arg constructor/factory method (through reference chain: ...["details"])
So, how to handle this kind of JSON string? I only need this field to set to null if empty.
The error message from Jackson hints that the library has bulit in support for static factory methods. This is (perhaps) a simpler solution than a custom deserializer:
I created this example POJO, with a static factory method, annotated so that Jackson uses it:
public class Details {
public String name; // example property
#JsonCreator
public static Details factory(Map<String,Object> props) {
if (props.get("details") instanceof Boolean) return null;
Details details = new Details();
Map<String,Object> detailsProps = (Map<String,Object>)props.get("details");
details.name = (String)detailsProps.get("name");
return details;
}
}
test method:
public static void main(String[] args)
{
String fullDetailsJson = "{\"details\": {\"name\":\"My Details\"}} ";
String emptyDetailsJson = "{\"details\": false} ";
ObjectMapper mapper = new ObjectMapper();
try {
Details details = mapper.readValue(fullDetailsJson, Details.class);
System.out.println(details.name);
details = mapper.readValue(emptyDetailsJson, Details.class);
System.out.println(details);
} catch (Exception e) {
e.printStackTrace();
}
}
result is as expected:
My Details
null
Make a custom JsonDeserializer to handle deserializing your Details object in which you either return null if you get false or pass the object to the default deserializer if it's an actual object. Pseudocode:
public class CustomDeserializer extends JsonDeserializer<Details>{
#Override
public Details deserialize(JsonParser jsonParser, DeserializationContext ctx){
//if object use default deserializer else return null
}
}
You'll also have to write an ObjectMapperProvider to register your deserializer like so:
#Provider
public class ObjectMapperProvider implements ContextResolver<ObjectMapper>{
private ObjectMapper mapper;
public ObjectMapperProvider(){
mapper = new ObjectMapper();
SimpleModule sm = new SimpleModule();
sm.addDeserializer(Details.class, new CustomDeserializer());
mapper.registerModule(sm);
}
public ObjectMapper getContext(Class<?> arg0){
return mapper;
}
}
Related
I accept from a server a json like this:
{
"": "hello"
}
And in Jackson I did
#JsonProperty("")
private String string
When deserialising the object it ignores the property completely.
How can I make an empty string count as a key?
Thank you
I found a way to achieve what you want with custom deserializer by following steps.
Step 1: Create the POJO class you want to deserialize to
public class MyPojo {
private String emptyFieldName;
//constructor, getter, setter and toString
}
Step 2: Create your custom deserializer
public class MyDeserializer extends StdDeserializer<MyPojo> {
public MyDeserializer () {
this(null);
}
protected MyDeserializer (Class<?> vc) {
super(vc);
}
#Override
public MyObject deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
JsonNode jsonNode = jsonParser.getCodec().readTree(jsonParser);
String emptyFieldName = jsonNode.get("").asText();
return new MyPojo(emptyFieldName);
}
}
Step 3: Register this custom deserializer
ObjectMapper objectMapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(MyPojo.class, new MyDeserializer());
objectMapper.registerModule(module);
MyPojo myPojo = objectMapper.readValue(jsonStr, MyPojo.class);
System.out.println(myPojo.getEmptyFieldName());
Console output:
hello
BTW, you could also directly register this custom deserializer on the class:
#JsonDeserialize(using = MyDeserializer.class)
public class MyPojo {
...
}
For more information, please refer to Getting Started with Custom Deserialization in Jackson.
I managed to configure Jackson to serialize a class without any getters or annotations on the private fields inside the class by just using
objectMapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
But I don't manage to make a JSON string of this non-static class to get deserialized into an object without any parameterized constructor or setters. Is this possible at all - I think it should through reflection but I don't get exactly how...
Here are 2 tests which need to pass to achieve what I need:
public class ObjectMapperTest {
private ObjectMapper mapper;
#Before
public void init() {
mapper = new ObjectMapper();
mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
}
#Test
public void serialize() throws Exception {
Payload payloadToSerialize = new Payload();
payloadToSerialize.data = "testData";
String serializedPayload = mapper.writeValueAsString(payloadToSerialize);
assertThat(serializedPayload, is("{\"data\":\"testData\"}"));
// --> OK
}
#Test
public void deserialize() throws Exception {
Payload deserializedPayload = mapper.readValue("{\"data\":\"testData\"}", Payload.class);
assertThat(deserializedPayload.data, is("testData"));
// com.fasterxml.jackson.databind.JsonMappingException:
// No suitable constructor found for type [simple type, class ...ObjectMapperTest$Payload]:
// can not instantiate from JSON object (missing default constructor or creator, or perhaps need to add/enable type information?)
// at [Source: {"data":"testData"}; line: 1, column: 2]
}
public class Payload {
private String data;
public Payload() {
// empty constructor for Jackson
}
}
}
Making the Payload class static would fix the test but static classes are not an option for me as I am not working with inner payload classes in the project. Any ideas how to fix it through object mapper configuration change?
EDIT
As I am using the Jackson object mapper in a Spring MVC application to serialize / deserialize under the hood I need a solution which changes or extends the object mapper configuration only.
You can write your own deserializer to parse the JSON and create the instance of Payload, and then set the data value using reflection.
Exemple :
#Before
public void init() {
mapper = new ObjectMapper();
mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
mapper.registerModule(
new SimpleModule()
.addDeserializer(Payload.class, new JsonDeserializer<Payload>() {
#Override
public Payload deserialize(JsonParser parser, DeserializationContext ctx)
throws IOException, JsonProcessingException {
JsonNode obj = parser.readValueAsTree(); // Read the JSON as a node
Payload payload = new Payload();
if (obj.isObject() && obj.has("data")) { // The node is an object and has a "data" field
try {
// Use reflection to set the value
Field dataField = payload.getClass().getDeclaredField("data");
dataField.setAccessible(true);
dataField.set(payload, obj.get("data").asText());
} catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException ex) {
throw new IOException("Reflection error", ex);
}
}
return payload;
}
}));
}
Edit: If you want something more "generic" you can try to create the instance yourself and change the accessibility of all the fields. Then you tell Jackson to update the values using the JSON.
public <T> T deserialize(final String json, final T instance) throws Exception {
for (Field field : instance.getClass().getDeclaredFields()) {
field.setAccessible(true);
}
mapper.readerForUpdating(instance).readValue(json);
return instance;
}
#Test
public void deserializeUpdate() throws Exception {
Payload deserializedPayload = deserialize("{\"data\":\"testData\"}", new Payload());
assertThat(deserializedPayload.data, is("testData"));
}
I tested this on your Payload class, maybe it doesn't work on more complex objects.
I have written a custom deserializer to map only the required fields using jackson. Here goes.
public class GeneralDeserializer extends JsonDeserializer<GeneralDomain> {
#Override
public GeneralDomain deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
final JsonNode jsonNode = jp.getCodec().readTree(jp);
final Map<String, String> map = new ObjectMapper().convertValue(jsonNode, Map.class);
final String event = "Proxy";
return new GeneralDomain(map.get("id"), event, map.get("name"), map.get("lastLogin"));
}
#Override
public Class<GeneralDomain> handledType() {
return GeneralDomain.class;
}
}
I have a mixin class too for this to add extra annotations.
#JsonDeserialize(using = GeneralDeserializer.class)
public class GeneralDomainMixIn{}
I fetch the object in this way,
ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(GeneralDomain.class, SimpleRevealPublicEventMixIn.class);
String json = "{\"id\": 111, \"name\": David, \"lastLogin\": \"02-10-2016 10:32:00 AM\"}";
GeneralDomain readValue = mapper.readValue(json, GeneralDomain.class);
This works great. But as you can see in the custom deserializer, I am hard coding the event field value. This will be passed on by some other instance variable in the main class. I have to pass this field to the custom deserializer. So is there a way to access this variable inside the deserializer? Or is there any other alternative way to achieve this? Please help me out. Thanks.
Got the answer finally. Thanks to Philip in this link.
All I had to do was this.
Create an instance of InjectableValues
private InjectableValues injectEventType() {
return new InjectableValues.Std()
.addValue("event", "proxy")
}
use this method to set the injectEventType method in mapper class
GeneralDomain readValue = mapper.setInjectableValues(injectEventType()).readValue(json, GeneralDomain.class);
In my deserialize method I had to retrieve the values provided by the InjectableValues:
String event = String.valueOf(ctxt.findInjectableValue("event", null, null));
I need to convert the following JSON to Java object. The property providerResponse in the JSON contains map of properties but they are escaped and wrapped in doubleQuotes. As a result, it does not deserialize the property providerResponse into a Java object (it comes as String). I use objectMapper.readValue(msgStr, classType) to deserialize the JSON. The message is generated by AWS for SNS delivery status notifications and I don't have control to change the JSON message. Is it possible to configure ObjectMapper to unescape the property and deserialize into a Java object instead of String?
{
"delivery":{
"providerResponse":"{\"sqsRequestId\":\"308ee0c6-7d51-57b0-a472-af8e6c41be0b\",\"sqsMessageId\":\"88dd59eb-c34d-4e4d-bb27-7e0d226daa2a\"}"
}
}
#JsonProperty("providerResponse")
private String providerResponse;
There doesn't seem to be a way to configure ObjectMapper to handle this behavior by default. The solution is to create a custom JsonDeserializer:
public class Wrapper {
public Delivery delivery;
}
public class Delivery {
#JsonDeserialize(using = ProviderResponseDeserializer.class)
public ProviderResponse providerResponse;
}
public class ProviderResponse {
public String sqsRequestId;
public String sqsMessageId;
}
public class ProviderResponseDeserializer extends JsonDeserializer<ProviderResponse> {
private static final ObjectMapper mapper = new ObjectMapper();
#Override
public ProviderResponse deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
return mapper.readValue(jsonParser.getText(), ProviderResponse.class);
}
}
Then you can deserialize the JSON by using your ObjectMapper:
ObjectMapper mapper = new ObjectMapper();
Wrapper wrapper = mapper.readValue(JSON, Wrapper.class);
I faced this similar issue. This gets resolved if we define a constructor in ProviderResponse which takes a single string argument (which is actually json) and then map the json in the constructor to the instance of ProviderResponse and use this temp instance to initialise the properties.
public class Wrapper {
public Delivery delivery;
}
public class Delivery {
public ProviderResponse providerResponse;
}
public class ProviderResponse {
public String sqsRequestId;
public String sqsMessageId;
private static ObjectMapper objMapper = new ObjectMapper();
public ProviderResponse(String json) {
ProviderResponse temp = objMapper.readValue(json, ProviderResponse.class);
this.sqsMessageId = temp.sqsMessageId;
this.sqsRequestId = temp.sqsRequestId;
}
}
The key is to keep the ObjectMapper instance and the its usage somewhere in your utility class and use it from there.
Imagine the following scenario:
class <T> Foo<T> {
....
}
class Bar {
Foo<Something> foo;
}
I want to write a custom Jackson deserializer for Foo. In order to do that (for example, in order to deserialize Bar class that has Foo<Something> property), I need to know the concrete type of Foo<T>, used in Bar, at deserialization time (e.g. I need to know that T is Something in that particluar case).
How does one write such a deserializer? It should be possible to do it, since Jackson does it with typed collections and maps.
Clarifications:
It seems there are 2 parts to solution of the problem:
1) Obtain declared type of property foo inside Bar and use that to deserialize Foo<Somehting>
2) Find out at deserialization time that we are deserializing property foo inside class Bar in order to successfully complete step 1)
How does one complete 1 and 2 ?
You can implement a custom JsonDeserializer for your generic type which also implements ContextualDeserializer.
For example, suppose we have the following simple wrapper type that contains a generic value:
public static class Wrapper<T> {
public T value;
}
We now want to deserialize JSON that looks like this:
{
"name": "Alice",
"age": 37
}
into an instance of a class that looks like this:
public static class Person {
public Wrapper<String> name;
public Wrapper<Integer> age;
}
Implementing ContextualDeserializer allows us to create a specific deserializer for each field in the Person class, based on the generic type parameters of the field. This allows us to deserialize the name as a string, and the age as an integer.
The complete deserializer looks like this:
public static class WrapperDeserializer extends JsonDeserializer<Wrapper<?>> implements ContextualDeserializer {
private JavaType valueType;
#Override
public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) throws JsonMappingException {
JavaType wrapperType = property.getType();
JavaType valueType = wrapperType.containedType(0);
WrapperDeserializer deserializer = new WrapperDeserializer();
deserializer.valueType = valueType;
return deserializer;
}
#Override
public Wrapper<?> deserialize(JsonParser parser, DeserializationContext ctxt) throws IOException {
Wrapper<?> wrapper = new Wrapper<>();
wrapper.value = ctxt.readValue(parser, valueType);
return wrapper;
}
}
It is best to look at createContextual here first, as this will be called first by Jackson. We read the type of the field out of the BeanProperty (e.g. Wrapper<String>) and then extract the first generic type parameter (e.g. String). We then create a new deserializer and store the inner type as the valueType.
Once deserialize is called on this newly created deserializer, we can simply ask Jackson to deserialize the value as the inner type rather than as the whole wrapper type, and return a new Wrapper containing the deserialized value.
In order to register this custom deserializer, we then need to create a module that contains it, and register that module:
SimpleModule module = new SimpleModule()
.addDeserializer(Wrapper.class, new WrapperDeserializer());
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(module);
If we then try to deserialize the example JSON from above, we can see that it works as expected:
Person person = objectMapper.readValue(json, Person.class);
System.out.println(person.name.value); // prints Alice
System.out.println(person.age.value); // prints 37
There are some more details about how contextual deserializers work in the Jackson documentation.
If the target itself is a generic type then property will be null, for that you'll need to get the valueTtype from the DeserializationContext:
#Override
public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) throws JsonMappingException {
if (property == null) { // context is generic
JMapToListParser parser = new JMapToListParser();
parser.valueType = ctxt.getContextualType().containedType(0);
return parser;
} else { // property is generic
JavaType wrapperType = property.getType();
JavaType valueType = wrapperType.containedType(0);
JMapToListParser parser = new JMapToListParser();
parser.valueType = valueType;
return parser;
}
}
This is how you can access/resolve {targetClass} for a Custom Jackson Deserializer. Of course you need to implement ContextualDeserializer interface for this.
public class WPCustomEntityDeserializer extends JsonDeserializer<Object>
implements ContextualDeserializer {
private Class<?> targetClass;
#Override
public Object deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
ObjectCodec oc = jp.getCodec();
JsonNode node = oc.readTree(jp);
//Your code here to customize deserialization
// You can access {target class} as targetClass (defined class field here)
//This should build some {deserializedClasObject}
return deserializedClasObject;
}
#Override
public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property){
//Find here the targetClass to be deserialized
String targetClassName=ctxt.getContextualType().toCanonical();
try {
targetClass = Class.forName(targetClassName);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
return this;
}
}
For my use case, none of the above solutions worked, so I had to write a custom module. You can find my implementation on GitHub.
I wanted to write a deserializer that automatically removes blank Strings from Lists.