Related
I am coming to a problem where I am trying to deserialize empty string as null value with my code below. is there a way to deserialize it properly. I created a class EmptyStringDeserializer but I got stuck creating the custom deserializer. Below you can find my EmptyStringDeserializer to see what I did wrong.
You could enable ACCEPT_EMPTY_STRING_AS_NULL_OBJECT in your ObjectMapper:
ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
Alternatively, you could define a custom deserializer:
public class CustomStringDeserializer extends JsonDeserializer<String> {
#Override
public String deserialize(JsonParser p, DeserializationContext ctxt)
throws IOException {
String value = StringDeserializer.instance.deserialize(p, ctxt);
if (value == null || value.trim().isEmpty()) {
return null;
}
return value;
}
}
And register it to a module in your ObjectMapper:
SimpleModule module = new SimpleModule();
module.addDeserializer(String.class, new CustomStringDeserializer());
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);
This deserializer will be used to deserialize all strings.
Your #JsonDeserialize should be on field level not on class level. Add on every String field.
I have a problem in my custom deserializer in Jackson. I want to access the default serializer to populate the object I am deserializing into. After the population I will do some custom things but first I want to deserialize the object with the default Jackson behavior.
This is the code that I have at the moment.
public class UserEventDeserializer extends StdDeserializer<User> {
private static final long serialVersionUID = 7923585097068641765L;
public UserEventDeserializer() {
super(User.class);
}
#Override
#Transactional
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
ObjectCodec oc = jp.getCodec();
JsonNode node = oc.readTree(jp);
User deserializedUser = null;
deserializedUser = super.deserialize(jp, ctxt, new User());
// The previous line generates an exception java.lang.UnsupportedOperationException
// Because there is no implementation of the deserializer.
// I want a way to access the default spring deserializer for my User class.
// How can I do that?
//Special logic
return deserializedUser;
}
}
What I need is a way to initialize the default deserializer so that I can pre-populate my POJO before I start my special logic.
When calling deserialize from within the custom deserializer It seems the method is called from the current context no matter how I construct the serializer class. Because of the annotation in my POJO. This causes a Stack Overflow exception for obvious reasons.
I have tried initializing a BeanDeserializer but the process is extremely complex and I haven't managed to find the right way to do it. I have also tried overloading the AnnotationIntrospector to no avail, thinking that it might help me ignore the annotation in the DeserializerContext. Finally it seams I might have had some success using JsonDeserializerBuilders although this required me to do some magic stuff to get hold of the application context from Spring. I would appreciate any thing that could lead me to a cleaner solution for example how Can I construct a deserialization context without reading the JsonDeserializer annotation.
As StaxMan already suggested you can do this by writing a BeanDeserializerModifier and registering it via SimpleModule. The following example should work:
public class UserEventDeserializer extends StdDeserializer<User> implements ResolvableDeserializer
{
private static final long serialVersionUID = 7923585097068641765L;
private final JsonDeserializer<?> defaultDeserializer;
public UserEventDeserializer(JsonDeserializer<?> defaultDeserializer)
{
super(User.class);
this.defaultDeserializer = defaultDeserializer;
}
#Override public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException
{
User deserializedUser = (User) defaultDeserializer.deserialize(jp, ctxt);
// Special logic
return deserializedUser;
}
// for some reason you have to implement ResolvableDeserializer when modifying BeanDeserializer
// otherwise deserializing throws JsonMappingException??
#Override public void resolve(DeserializationContext ctxt) throws JsonMappingException
{
((ResolvableDeserializer) defaultDeserializer).resolve(ctxt);
}
public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException
{
SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new BeanDeserializerModifier()
{
#Override public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer)
{
if (beanDesc.getBeanClass() == User.class)
return new UserEventDeserializer(deserializer);
return deserializer;
}
});
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);
User user = mapper.readValue(new File("test.json"), User.class);
}
}
The DeserializationContext has a readValue() method you may use. This should work for both the default deserializer and any custom deserializers you have.
Just be sure to call traverse() on the JsonNode level you want to read to retrieve the JsonParser to pass to readValue().
public class FooDeserializer extends StdDeserializer<FooBean> {
private static final long serialVersionUID = 1L;
public FooDeserializer() {
this(null);
}
public FooDeserializer(Class<FooBean> t) {
super(t);
}
#Override
public FooBean deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
JsonNode node = jp.getCodec().readTree(jp);
FooBean foo = new FooBean();
foo.setBar(ctxt.readValue(node.get("bar").traverse(), BarBean.class));
return foo;
}
}
I found an answer at https://stackoverflow.com/a/51927577/14731 which is much more readable than the accepted answer.
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
User user = jp.readValueAs(User.class);
// some code
return user;
}
It really doesn't get easier than this.
If it is possible for you to declare extra User class then you can implement it just using annotations
// your class
#JsonDeserialize(using = UserEventDeserializer.class)
public class User {
...
}
// extra user class
// reset deserializer attribute to default
#JsonDeserialize
public class UserPOJO extends User {
}
public class UserEventDeserializer extends StdDeserializer<User> {
...
#Override
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
// specify UserPOJO.class to invoke default deserializer
User deserializedUser = jp.ReadValueAs(UserPOJO.class);
return deserializedUser;
// or if you need to walk the JSON tree
ObjectMapper mapper = (ObjectMapper) jp.getCodec();
JsonNode node = oc.readTree(jp);
// specify UserPOJO.class to invoke default deserializer
User deserializedUser = mapper.treeToValue(node, UserPOJO.class);
return deserializedUser;
}
}
There are couple of ways to do this, but to do it right involves bit more work. Basically you can not use sub-classing, since information default deserializers need is built from class definitions.
So what you can most likely use is to construct a BeanDeserializerModifier, register that via Module interface (use SimpleModule). You need to define/override modifyDeserializer, and for the specific case where you want to add your own logic (where type matches), construct your own deserializer, pass the default deserializer you are given.
And then in deserialize() method you can just delegate call, take the result Object.
Alternatively, if you must actually create and populate the object, you can do so and call overloaded version of deserialize() that takes third argument; object to deserialize into.
Another way that might work (but not 100% sure) would be to specify Converter object (#JsonDeserialize(converter=MyConverter.class)). This is a new Jackson 2.2 feature.
In your case, Converter would not actually convert type, but simplify modify the object: but I don't know if that would let you do exactly what you want, since the default deserializer would be called first, and only then your Converter.
Along the lines of what Tomáš Záluský has suggested, in cases where using BeanDeserializerModifier is undesirable you can construct a default deserializer yourself using BeanDeserializerFactory, although there is some extra setup necessary. In context, this solution would look like so:
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
ObjectCodec oc = jp.getCodec();
JsonNode node = oc.readTree(jp);
User deserializedUser = null;
DeserializationConfig config = ctxt.getConfig();
JavaType type = TypeFactory.defaultInstance().constructType(User.class);
JsonDeserializer<Object> defaultDeserializer = BeanDeserializerFactory.instance.buildBeanDeserializer(ctxt, type, config.introspect(type));
if (defaultDeserializer instanceof ResolvableDeserializer) {
((ResolvableDeserializer) defaultDeserializer).resolve(ctxt);
}
JsonParser treeParser = oc.treeAsTokens(node);
config.initialize(treeParser);
if (treeParser.getCurrentToken() == null) {
treeParser.nextToken();
}
deserializedUser = (User) defaultDeserializer.deserialize(treeParser, context);
return deserializedUser;
}
You are bound to fail if you try to create your custom deserializer from scratch.
Instead, you need to get hold of the (fully configured) default deserializer instance through a custom BeanDeserializerModifier, and then pass this instance to your custom deserializer class:
public ObjectMapper getMapperWithCustomDeserializer() {
ObjectMapper objectMapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new BeanDeserializerModifier() {
#Override
public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config,
BeanDescription beanDesc, JsonDeserializer<?> defaultDeserializer)
if (beanDesc.getBeanClass() == User.class) {
return new UserEventDeserializer(defaultDeserializer);
} else {
return defaultDeserializer;
}
}
});
objectMapper.registerModule(module);
return objectMapper;
}
Note: This module registration replaces the #JsonDeserialize annotation, i.e. the User class or User fields should no longer be annotated with this annotation.
The custom deserializer should then be based on a DelegatingDeserializer so that all methods delegate, unless you provide an explicit implementation:
public class UserEventDeserializer extends DelegatingDeserializer {
public UserEventDeserializer(JsonDeserializer<?> delegate) {
super(delegate);
}
#Override
protected JsonDeserializer<?> newDelegatingInstance(JsonDeserializer<?> newDelegate) {
return new UserEventDeserializer(newDelegate);
}
#Override
public User deserialize(JsonParser p, DeserializationContext ctxt)
throws IOException {
User result = (User) super.deserialize(p, ctxt);
// add special logic here
return result;
}
}
I was not ok with using BeanSerializerModifier since it forces to declare some behavioral changes in central ObjectMapper rather than in custom deserializer itself and in fact it is parallel solution to annotating entity class with JsonSerialize. If you feel it the similar way, you might appreciate my answer here: https://stackoverflow.com/a/43213463/653539
Using BeanDeserializerModifier works well, but if you need to use JsonDeserialize there is a way to do it with AnnotationIntrospector
like this:
ObjectMapper originalMapper = new ObjectMapper();
ObjectMapper copy = originalMapper.copy();//to keep original configuration
copy.setAnnotationIntrospector(new JacksonAnnotationIntrospector() {
#Override
public Object findDeserializer(Annotated a) {
Object deserializer = super.findDeserializer(a);
if (deserializer == null) {
return null;
}
if (deserializer.equals(MyDeserializer.class)) {
return null;
}
return deserializer;
}
});
Now copied mapper will now ignore your custom deserializer (MyDeserializer.class) and use default implementation. You can use it inside deserialize method of your custom deserializer to avoid recursion by making copied mapper static or wire it if using Spring.
A simpler solution for me was to just add another bean of ObjectMapper and use that to deserialize the object (thanks to https://stackoverflow.com/users/1032167/varren comment) - in my case I was interested to either deserialize to its id (an int) or the whole object https://stackoverflow.com/a/46618193/986160
import com.fasterxml.jackson.annotation.JsonAutoDetect;
import com.fasterxml.jackson.annotation.PropertyAccessor;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import org.springframework.context.annotation.Bean;
import java.io.IOException;
public class IdWrapperDeserializer<T> extends StdDeserializer<T> {
private Class<T> clazz;
public IdWrapperDeserializer(Class<T> clazz) {
super(clazz);
this.clazz = clazz;
}
#Bean
public ObjectMapper objectMapper() {
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
mapper.configure(MapperFeature.DEFAULT_VIEW_INCLUSION, true);
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.NONE);
mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
return mapper;
}
#Override
public T deserialize(JsonParser jp, DeserializationContext dc) throws IOException, JsonProcessingException {
String json = jp.readValueAsTree().toString();
// do your custom deserialization here using json
// and decide when to use default deserialization using local objectMapper:
T obj = objectMapper().readValue(json, clazz);
return obj;
}
}
for each entity that needs to be going through custom deserializer we need to configure it in the global ObjectMapper bean of the Spring Boot App in my case (e.g for Category):
#Bean
public ObjectMapper objectMapper() {
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
mapper.configure(MapperFeature.DEFAULT_VIEW_INCLUSION, true);
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.NONE);
mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
SimpleModule testModule = new SimpleModule("MyModule")
.addDeserializer(Category.class, new IdWrapperDeserializer(Category.class))
mapper.registerModule(testModule);
return mapper;
}
Here is a short solution using default ObjectMapper
private static final ObjectMapper MAPPER = new ObjectMapper(); // use default mapper / mapper without customization
public MyObject deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
MyObject object = MAPPER.readValue(p, MyObject.class);
// do whatever you want
return object;
}
And please: There is really no need to use any String value or something else. All needed information are given by JsonParser, so use it.
I created custom JsonDeserializer for that can be applied to any field with type String.
public class EmptyToNullStringDeserializer extends JsonDeserializer<String> {
#Override
public String deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
String text = jp.getText();
return "" == text ? null : text;
}
}
It can be used in models.
class SomeClass {
#JsonDeserialize(using = EmptyToNullStringDeserializer.class)
private String someField;
}
It converts JSON
{"someField": ""}
into Java object where someField equals to null (not "")
Question: How to create generic JsonDeserializer that sets null to all Java object fields that equals to "" in JSON?
It should be used as:
#JsonDeserialize(using = EmptyToNullStringDeserializer.class)
class SomeClass {
private String someField;
}
This more of a Jackson question than a Spring question. You would just need to register your custom deserializer with the ObjectMapper...
SimpleModule module = new SimpleModule("MyModule", new Version(1, 0, 0, null));
module.addDeserializer(String.class, new EmptyToNullStringDeserializer());
mapper.registerModule(module);
How you get access to that ObjectMapper depends on if you are using Spring Boot or just plain Spring.
You need to register your custom deserializer with ObjectMapper
I am trying a simple JSON to de-serialize in to java object. I am however, getting empty String values for java.lang.String property values. In rest of the properties, blank values are converting to null values(which is what I want).
My JSON and related Java class are listed below.
JSON string:
{
"eventId" : 1,
"title" : "sample event",
"location" : ""
}
EventBean class POJO:
public class EventBean {
public Long eventId;
public String title;
public String location;
}
My main class code:
ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
try {
File file = new File(JsonTest.class.getClassLoader().getResource("event.txt").getFile());
JsonNode root = mapper.readTree(file);
// find out the applicationId
EventBean e = mapper.treeToValue(root, EventBean.class);
System.out.println("It is " + e.location);
}
I was expecting print "It is null". Instead, I am getting "It is ". Obviously, Jackson is not treating blank String values as NULL while converting to my String object type.
I read somewhere that it is expected. However, this is something I want to avoid for java.lang.String too. Is there a simple way?
Jackson will give you null for other objects, but for String it will give empty String.
But you can use a Custom JsonDeserializer to do this:
class CustomDeserializer extends JsonDeserializer<String> {
#Override
public String deserialize(JsonParser jsonParser, DeserializationContext context) throws IOException, JsonProcessingException {
JsonNode node = jsonParser.readValueAsTree();
if (node.asText().isEmpty()) {
return null;
}
return node.toString();
}
}
In class you have to use it for location field:
class EventBean {
public Long eventId;
public String title;
#JsonDeserialize(using = CustomDeserializer.class)
public String location;
}
It is possible to define a custom deserializer for the String type, overriding the standard String deserializer:
this.mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(String.class, new StdDeserializer<String>(String.class) {
#Override
public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
String result = StringDeserializer.instance.deserialize(p, ctxt);
if (StringUtils.isEmpty(result)) {
return null;
}
return result;
}
});
mapper.registerModule(module);
This way all String fields will behave the same way.
You might first like to see if there has been any progress on the Github issue requesting this exact feature.
For those using Spring Boot: The answer from jgesser was the most helpful to me, but I spent a while trying to work out the best way to configure it in Spring Boot.
Actually, the documentation says:
Any beans of type com.fasterxml.jackson.databind.Module are
automatically registered with the auto-configured
Jackson2ObjectMapperBuilder and are applied to any ObjectMapper
instances that it creates.
So here's jgesser's answer expanded into something you can copy-paste into a new class in a Spring Boot application
#Configuration
public class EmptyStringAsNullJacksonConfiguration {
#Bean
SimpleModule emptyStringAsNullModule() {
SimpleModule module = new SimpleModule();
module.addDeserializer(
String.class,
new StdDeserializer<String>(String.class) {
#Override
public String deserialize(JsonParser parser, DeserializationContext context)
throws IOException {
String result = StringDeserializer.instance.deserialize(parser, context);
if (StringUtils.isEmpty(result)) {
return null;
}
return result;
}
});
return module;
}
}
I could get this by following configuration.
final ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, true);
it is possible to use JsonCreator annotation. It worked for me
public class Foo {
private String field;
#JsonCreator
public Foo(
#JsonProrerty("field") String field) {
this.field = StringUtils.EMPTY.equals(field) ? null : field ;
}
}
I have a problem in my custom deserializer in Jackson. I want to access the default serializer to populate the object I am deserializing into. After the population I will do some custom things but first I want to deserialize the object with the default Jackson behavior.
This is the code that I have at the moment.
public class UserEventDeserializer extends StdDeserializer<User> {
private static final long serialVersionUID = 7923585097068641765L;
public UserEventDeserializer() {
super(User.class);
}
#Override
#Transactional
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
ObjectCodec oc = jp.getCodec();
JsonNode node = oc.readTree(jp);
User deserializedUser = null;
deserializedUser = super.deserialize(jp, ctxt, new User());
// The previous line generates an exception java.lang.UnsupportedOperationException
// Because there is no implementation of the deserializer.
// I want a way to access the default spring deserializer for my User class.
// How can I do that?
//Special logic
return deserializedUser;
}
}
What I need is a way to initialize the default deserializer so that I can pre-populate my POJO before I start my special logic.
When calling deserialize from within the custom deserializer It seems the method is called from the current context no matter how I construct the serializer class. Because of the annotation in my POJO. This causes a Stack Overflow exception for obvious reasons.
I have tried initializing a BeanDeserializer but the process is extremely complex and I haven't managed to find the right way to do it. I have also tried overloading the AnnotationIntrospector to no avail, thinking that it might help me ignore the annotation in the DeserializerContext. Finally it seams I might have had some success using JsonDeserializerBuilders although this required me to do some magic stuff to get hold of the application context from Spring. I would appreciate any thing that could lead me to a cleaner solution for example how Can I construct a deserialization context without reading the JsonDeserializer annotation.
As StaxMan already suggested you can do this by writing a BeanDeserializerModifier and registering it via SimpleModule. The following example should work:
public class UserEventDeserializer extends StdDeserializer<User> implements ResolvableDeserializer
{
private static final long serialVersionUID = 7923585097068641765L;
private final JsonDeserializer<?> defaultDeserializer;
public UserEventDeserializer(JsonDeserializer<?> defaultDeserializer)
{
super(User.class);
this.defaultDeserializer = defaultDeserializer;
}
#Override public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException
{
User deserializedUser = (User) defaultDeserializer.deserialize(jp, ctxt);
// Special logic
return deserializedUser;
}
// for some reason you have to implement ResolvableDeserializer when modifying BeanDeserializer
// otherwise deserializing throws JsonMappingException??
#Override public void resolve(DeserializationContext ctxt) throws JsonMappingException
{
((ResolvableDeserializer) defaultDeserializer).resolve(ctxt);
}
public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException
{
SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new BeanDeserializerModifier()
{
#Override public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer)
{
if (beanDesc.getBeanClass() == User.class)
return new UserEventDeserializer(deserializer);
return deserializer;
}
});
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);
User user = mapper.readValue(new File("test.json"), User.class);
}
}
The DeserializationContext has a readValue() method you may use. This should work for both the default deserializer and any custom deserializers you have.
Just be sure to call traverse() on the JsonNode level you want to read to retrieve the JsonParser to pass to readValue().
public class FooDeserializer extends StdDeserializer<FooBean> {
private static final long serialVersionUID = 1L;
public FooDeserializer() {
this(null);
}
public FooDeserializer(Class<FooBean> t) {
super(t);
}
#Override
public FooBean deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
JsonNode node = jp.getCodec().readTree(jp);
FooBean foo = new FooBean();
foo.setBar(ctxt.readValue(node.get("bar").traverse(), BarBean.class));
return foo;
}
}
I found an answer at https://stackoverflow.com/a/51927577/14731 which is much more readable than the accepted answer.
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
User user = jp.readValueAs(User.class);
// some code
return user;
}
It really doesn't get easier than this.
If it is possible for you to declare extra User class then you can implement it just using annotations
// your class
#JsonDeserialize(using = UserEventDeserializer.class)
public class User {
...
}
// extra user class
// reset deserializer attribute to default
#JsonDeserialize
public class UserPOJO extends User {
}
public class UserEventDeserializer extends StdDeserializer<User> {
...
#Override
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
// specify UserPOJO.class to invoke default deserializer
User deserializedUser = jp.ReadValueAs(UserPOJO.class);
return deserializedUser;
// or if you need to walk the JSON tree
ObjectMapper mapper = (ObjectMapper) jp.getCodec();
JsonNode node = oc.readTree(jp);
// specify UserPOJO.class to invoke default deserializer
User deserializedUser = mapper.treeToValue(node, UserPOJO.class);
return deserializedUser;
}
}
There are couple of ways to do this, but to do it right involves bit more work. Basically you can not use sub-classing, since information default deserializers need is built from class definitions.
So what you can most likely use is to construct a BeanDeserializerModifier, register that via Module interface (use SimpleModule). You need to define/override modifyDeserializer, and for the specific case where you want to add your own logic (where type matches), construct your own deserializer, pass the default deserializer you are given.
And then in deserialize() method you can just delegate call, take the result Object.
Alternatively, if you must actually create and populate the object, you can do so and call overloaded version of deserialize() that takes third argument; object to deserialize into.
Another way that might work (but not 100% sure) would be to specify Converter object (#JsonDeserialize(converter=MyConverter.class)). This is a new Jackson 2.2 feature.
In your case, Converter would not actually convert type, but simplify modify the object: but I don't know if that would let you do exactly what you want, since the default deserializer would be called first, and only then your Converter.
Along the lines of what Tomáš Záluský has suggested, in cases where using BeanDeserializerModifier is undesirable you can construct a default deserializer yourself using BeanDeserializerFactory, although there is some extra setup necessary. In context, this solution would look like so:
public User deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
ObjectCodec oc = jp.getCodec();
JsonNode node = oc.readTree(jp);
User deserializedUser = null;
DeserializationConfig config = ctxt.getConfig();
JavaType type = TypeFactory.defaultInstance().constructType(User.class);
JsonDeserializer<Object> defaultDeserializer = BeanDeserializerFactory.instance.buildBeanDeserializer(ctxt, type, config.introspect(type));
if (defaultDeserializer instanceof ResolvableDeserializer) {
((ResolvableDeserializer) defaultDeserializer).resolve(ctxt);
}
JsonParser treeParser = oc.treeAsTokens(node);
config.initialize(treeParser);
if (treeParser.getCurrentToken() == null) {
treeParser.nextToken();
}
deserializedUser = (User) defaultDeserializer.deserialize(treeParser, context);
return deserializedUser;
}
You are bound to fail if you try to create your custom deserializer from scratch.
Instead, you need to get hold of the (fully configured) default deserializer instance through a custom BeanDeserializerModifier, and then pass this instance to your custom deserializer class:
public ObjectMapper getMapperWithCustomDeserializer() {
ObjectMapper objectMapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new BeanDeserializerModifier() {
#Override
public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config,
BeanDescription beanDesc, JsonDeserializer<?> defaultDeserializer)
if (beanDesc.getBeanClass() == User.class) {
return new UserEventDeserializer(defaultDeserializer);
} else {
return defaultDeserializer;
}
}
});
objectMapper.registerModule(module);
return objectMapper;
}
Note: This module registration replaces the #JsonDeserialize annotation, i.e. the User class or User fields should no longer be annotated with this annotation.
The custom deserializer should then be based on a DelegatingDeserializer so that all methods delegate, unless you provide an explicit implementation:
public class UserEventDeserializer extends DelegatingDeserializer {
public UserEventDeserializer(JsonDeserializer<?> delegate) {
super(delegate);
}
#Override
protected JsonDeserializer<?> newDelegatingInstance(JsonDeserializer<?> newDelegate) {
return new UserEventDeserializer(newDelegate);
}
#Override
public User deserialize(JsonParser p, DeserializationContext ctxt)
throws IOException {
User result = (User) super.deserialize(p, ctxt);
// add special logic here
return result;
}
}
I was not ok with using BeanSerializerModifier since it forces to declare some behavioral changes in central ObjectMapper rather than in custom deserializer itself and in fact it is parallel solution to annotating entity class with JsonSerialize. If you feel it the similar way, you might appreciate my answer here: https://stackoverflow.com/a/43213463/653539
Using BeanDeserializerModifier works well, but if you need to use JsonDeserialize there is a way to do it with AnnotationIntrospector
like this:
ObjectMapper originalMapper = new ObjectMapper();
ObjectMapper copy = originalMapper.copy();//to keep original configuration
copy.setAnnotationIntrospector(new JacksonAnnotationIntrospector() {
#Override
public Object findDeserializer(Annotated a) {
Object deserializer = super.findDeserializer(a);
if (deserializer == null) {
return null;
}
if (deserializer.equals(MyDeserializer.class)) {
return null;
}
return deserializer;
}
});
Now copied mapper will now ignore your custom deserializer (MyDeserializer.class) and use default implementation. You can use it inside deserialize method of your custom deserializer to avoid recursion by making copied mapper static or wire it if using Spring.
A simpler solution for me was to just add another bean of ObjectMapper and use that to deserialize the object (thanks to https://stackoverflow.com/users/1032167/varren comment) - in my case I was interested to either deserialize to its id (an int) or the whole object https://stackoverflow.com/a/46618193/986160
import com.fasterxml.jackson.annotation.JsonAutoDetect;
import com.fasterxml.jackson.annotation.PropertyAccessor;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import org.springframework.context.annotation.Bean;
import java.io.IOException;
public class IdWrapperDeserializer<T> extends StdDeserializer<T> {
private Class<T> clazz;
public IdWrapperDeserializer(Class<T> clazz) {
super(clazz);
this.clazz = clazz;
}
#Bean
public ObjectMapper objectMapper() {
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
mapper.configure(MapperFeature.DEFAULT_VIEW_INCLUSION, true);
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.NONE);
mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
return mapper;
}
#Override
public T deserialize(JsonParser jp, DeserializationContext dc) throws IOException, JsonProcessingException {
String json = jp.readValueAsTree().toString();
// do your custom deserialization here using json
// and decide when to use default deserialization using local objectMapper:
T obj = objectMapper().readValue(json, clazz);
return obj;
}
}
for each entity that needs to be going through custom deserializer we need to configure it in the global ObjectMapper bean of the Spring Boot App in my case (e.g for Category):
#Bean
public ObjectMapper objectMapper() {
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
mapper.configure(MapperFeature.DEFAULT_VIEW_INCLUSION, true);
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.NONE);
mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
SimpleModule testModule = new SimpleModule("MyModule")
.addDeserializer(Category.class, new IdWrapperDeserializer(Category.class))
mapper.registerModule(testModule);
return mapper;
}
Here is a short solution using default ObjectMapper
private static final ObjectMapper MAPPER = new ObjectMapper(); // use default mapper / mapper without customization
public MyObject deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
MyObject object = MAPPER.readValue(p, MyObject.class);
// do whatever you want
return object;
}
And please: There is really no need to use any String value or something else. All needed information are given by JsonParser, so use it.