I am having trouble getting jackson to respect my custom JsonDeserializer. The situation is, I have a class MyClass that contains a list of another class, OtherClass, that is outside of my control (so I can't annotate it). This OtherClass class is an interface with multiple implementations. I don't care what the original OtherClass was, I want them to always deserialize as BasicOtherClass.
Here is what I have:
#Getter
public class MyClass {
#JsonProperty("otherclasses")
#JsonSerialize(contentUsing=OtherClassSerializer.class)
#JsonDeserialize(contentUsing=OtherClassDeserializer.class)
private List<OtherClass> otherClasses;
public MyClass(
#JsonProperty("otherclasses")
#JsonDeserialize(contentUsing=OtherClassDeserializer.class)
List<OtherClass> otherClasses) {
this.otherClass = otherClass;
}
}
public static class OtherClassSerializer extends JsonSerializer<OtherClass> {
#Override
public void serialize(OtherClass otherClass, JsonGenerator gen, SerializerProvider serializers)
throws IOException, JsonProcessingException {
gen.writeStartObject();
gen.writeStringField("name", otherClass.getName());
gen.writeStringField("value", otherClass.getValue());
gen.writeEndObject();
}
/** This method is required when default typing is enabled */
#Override
public void serializeWithType(
OtherClass otherClass, JsonGenerator gen, SerializerProvider serializers, TypeSerializer typeSer)
throws JsonProcessingException, IOException {
typeSer.writeTypePrefixForScalar(value, gen, OtherClass.class);
serialize(value, gen, serializers);
typeSer.writeTypeSuffixForScalar(value, gen);
}
}
public static class OtherClassDeserializer extends JsonDeserializer<Header> {
#Override
public Header deserialize(JsonParser p, DeserializationContext ctxt) throws IOException,
JsonProcessingException {
if (p.getCurrentToken() != JsonToken.START_OBJECT) {
throw new IllegalStateException("Failed to parse OtherClass from json");
}
String name = null;
String value = null;
while (p.nextToken() != JsonToken.END_OBJECT) {
String key = p.getText();
p.nextToken();
String val = p.getText();
if (key.equals("name")) {
name = val;
} else if (key.equals("value")) {
value = val;
}
}
return new BasicOtherClass(name, value);
}
}
This is what I am trying to get to work:
ObjectMapper mapper = new ObjectMapper().enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL);
OtherClass otherClass = new BufferedOtherClass("name value");
MyClass myClass = new MyClass(Lists.newArrayList(otherClass));
String json = mapper.writeValueAsString(myClass);
// json == ["com.bschlenk.MyClass", {"otherclass": ["java.util.ArrayList", [["com.other.OtherClass", {"name": "name", "value", "value"}]]]}]
But when I try to read that json back into MyClass, it fails:
MyClass parsed = mapper.readValue(json, MyClass.class);
// com.fasterxml.jackson.databind.JsonMappingException:
// Can not construct instance of org.apache.http.Header, problem:
// abstract types either need to be mapped to concrete types,
// have custom deserializer,
// or be instantiated with additional type information
This works when I don't have type information enabled. However, it is other code that is serializing MyClass that I don't have control of, and it has type info on.
Is what I am trying to do even possible? Why doesn't mapper.readValue use my custom JsonDeserializer class? Is this by design?
I am attempting to serialise an object similar to the following:
class Header {
public String title;
public String author;
}
class Document {
public String data;
public Header header = new Header();
}
Without any annotations, jackson will serialise to:
{"data":null,"header":{"title":null,"author":null}}
Using #JsonInclude(NOT_NULL) or #JsonInclude(NOT_EMPTY) on Header and Document, it will serialise to:
{"header":{}}
I want to suppress the header property if it would be empty but this only supported for collections and strings. Ideally this would be solved via an annotation as the default BeanSerializer would be able to achieve this easily.
I can achieve what I need by writing a custom serializer but then I lose the benefit of all the advanced logic inside the default serializers.
Can anyone think of a better way to solve this? The structures above are an example only as I'm looking for a generic solution.
It's not really all that easy- basically, the bean serializer would have to hold off on writing the field name for a property until something was written by a property serializer-- this could get particularly hairy once you have several such nested within each other.
The easiest way to solve it would be for the property serializer to serialize its value to a token buffer, and then skip the property if the token buffer only contained "{" and "}". But this means that a graph of objects would end up being read in and out of buffers at each level, which defeats the purpose of having a streaming generator that doesn't build up content proportional to the size of the generated output.
If you can live with that buffer copying, this will do approximately what you ask:
#Test
public void suppress_empty_objects() throws Exception {
mapper.registerModule(new SimpleModule() {
#Override
public void setupModule(SetupContext context) {
context.addBeanSerializerModifier(new SuppressEmptyBean());
}
class SuppressEmptyBean extends BeanSerializerModifier {
#Override
public List<BeanPropertyWriter> changeProperties(SerializationConfig config,
BeanDescription beanDesc,
List<BeanPropertyWriter> beanProperties) {
// TODO: examine bean description for annotations to enable/disable suppression
ListIterator<BeanPropertyWriter> iter = beanProperties.listIterator();
while (iter.hasNext()) {
BeanPropertyWriter beanPropertyWriter = iter.next();
// TODO: only relevant to suppress properties that are themselves beans
iter.set(new SuppressEmptyPropertyWriter(beanPropertyWriter));
}
return beanProperties;
}
}
class SuppressEmptyPropertyWriter extends BeanPropertyWriter {
private final BeanPropertyWriter underlying;
SuppressEmptyPropertyWriter(BeanPropertyWriter underlying) {
super(underlying);
this.underlying = underlying;
}
#Override
public void serializeAsField(Object bean, JsonGenerator output, SerializerProvider prov)
throws Exception {
TokenBuffer tokenBuffer = new TokenBuffer(output.getCodec(), false);
underlying.serializeAsField(bean, tokenBuffer, prov);
if (!suppress(tokenBuffer, output)) {
tokenBuffer.serialize(output);
}
}
private boolean suppress(TokenBuffer tokenBuffer, JsonGenerator output) throws JsonParseException,
IOException {
if (tokenBuffer.firstToken() != JsonToken.FIELD_NAME) return false; // nope
JsonParser bufferParser = tokenBuffer.asParser();
bufferParser.nextToken(); // on field name
JsonToken valueToken1 = bufferParser.nextToken(); // on start object
if (valueToken1 != JsonToken.START_OBJECT) return false;
JsonToken valueToken2 = bufferParser.nextToken(); // on first thing inside object
return valueToken2 == JsonToken.END_OBJECT;
}
}
});
Document document = new Document();
document.data = "test";
assertThat(mapper.writeValueAsString(document), equivalentTo("{ data: 'test' }"));
document.header = new Header();
assertThat(mapper.writeValueAsString(document), equivalentTo("{ data: 'test' }"));
document.header.title = "the title";
assertThat(mapper.writeValueAsString(document),
equivalentTo("{ data: 'test', header: { title: 'the title' } }"));
}
I have the provider interface
interface IProvider<T> {
T locate();
}
and a class containing a field of type IProvider (can be another type for other fields).
class MyObject {
MyLocator<String> field;
}
I need to serialize instances of MyObject to JSON using Jackson 1.7. The output must be the same as if MyObject.field had been a String (i.e. no reference to ILocator).
I can't figure out how to build the custom serializer required to achieve this. Here is the structure I am trying to use for this task:
class MyLocatorSerializer extends SerializerBase<MyLocator<?>> {
public MyLocatorSerializer() {
super(MyLocator.class, false);
}
#Override
public void serialize(MyLocator<?> a_value, JsonGenerator a_jgen,
SerializerProvider a_provider) throws IOException, JsonGenerationException {
// Insert code here to serialize a_value.locate(), whatever its type
}
#Override
public JsonNode getSchema(SerializerProvider a_provider, Type a_typeHint)
throws JsonMappingException {
// What should I return here? I can't find documentation regarding the different schema types...
}
}
The custom serializer would be registered using
SimpleModule module = new SimpleModule("MyModule", new Version(1, 0, 0, null));
module.addSerializer(new MyLocatorSerializer());
objectMapper.registerModule(module);
Another answer using mix-in annotations following the comment from Staxman.
static class JacksonCustomModule extends SimpleModule {
public JacksonCustomModule() {
super("JacksonCustomModule", new Version(1, 0, 0, null));
}
#Override
public void setupModule(SetupContext context) {
context.setMixInAnnotations(IProvider.class, IProviderMixIn.class);
super.setupModule(context);
}
interface IProviderMixIn<T> {
#JsonValue
T locate();
}
}
Activate the module with:
objectMapper.registerModule(new JacksonCustomModule());
Apologies if I misunderstand the question, but would this be as simple as just using #JsonValue on 'Locate' method, instead of writing a custom serializer?
What #JsonValue does is take value of a property as is, and use it instead of creating a JSON Object: often this is used for serializing a POJO as a simple String or number, like so:
public class StringWrapper {
#JsonValue public String value;
}
so that for class like:
public class POJO {
public StringWrapper wrapped;
}
we would get serialization like:
{
"wrapper" : "string value of 'value'"
}
instead of what would otherwise be seen:
{
"wrapper" : {
"value" : "... string value ... "
}
}
Annotation can be used for any types of values obviously.
Following StaxMan's answer, I inspected the workings of #JsonValue and got the following serializer:
// Based on JsonValueSerializer
private static class ProviderSerializer extends SerializerBase<IProvider<?>> {
public ProviderSerializer() {
super(IProvider.class, false);
}
#Override
public void serialize(IProvider<?> value, JsonGenerator jgen, SerializerProvider provider)
throws IOException, JsonGenerationException {
Object object = value.locate();
// and if we got null, can also just write it directly
if (object == null) {
provider.defaultSerializeNull(jgen);
return;
}
Class<?> c = object.getClass();
JsonSerializer<Object> ser = provider.findTypedValueSerializer(c, true, null);
// note: now we have bundled type serializer, so should NOT call with typed version
ser.serialize(object, jgen, provider);
}
#Override
public JsonNode getSchema(SerializerProvider provider, Type typeHint)
throws JsonMappingException {
// is this right??
return JsonSchema.getDefaultSchemaNode();
}
}
After some tests, this does what I need. However, I don't fully really understand the purpose of the getSchema method, so maybe I'm doing something wrong...
I've got a bean defined as such :
public static class TestBean {
private String a;
private Long b;
public TestBean(String a, Long b) {
this.a = a;
this.b = b;
}
public String getA() {
return a;
}
public Long getB() {
return b;
}
}
It models some business and I do not get to instantiate it (using JPA). Some of my API let the user retrieve a view of this bean serialize as JSON using Jackson (through JAX-RS) and I would like to add a list of related links to this view.
The normal Jackson JSON serialization would be (for a = "aa" and b = 2L) :
{"a":"aa","b":2}
And I would like to have the links appear as
{"a":"aa","b":2,
"links":[{"rel":"rel","href":"href://"},{"rel":"rel2","href":"href://2"}]}
Possible work-around
I would rather not add a getLinks() method to my bean, it's specific to this view.
Simply using a composite object would yield a serialization like :
{"data":{"a":"aa","b":2},"links":[{"rel":"rel","href":"href://"}]}
Which I could live with but is not what I was looking for ultimately.
Current solution
I would like to avoid manipulating the JSON string or having to reload it into a Map to insert my extra values. For now the solution I've come up with seem awfully convoluted:
Current scary solution :
//a composite view object
public abstract class AddedLinksView<K> {
private final K resource;
private final Link[] links;
public AddedLinksView(K resource) {
this.resource = resource;
links = buildLinks(resource);
}
public abstract Link[] buildLinks(K resource);
public K getResource() {
return resource;
}
public Link[] getLinks() {
return links;
}
}
//a specific bean serializer
private static class RawBeanSerializer extends BeanSerializer {
public RawBeanSerializer(BeanSerializerBase ser) {
super(ser);
}
//this is like the standard serialize but without the start and end tags
public void rawSerialize(Object bean, JsonGenerator jgen, SerializerProvider provider) throws IOException,
JsonGenerationException {
if (_propertyFilterId != null) {
serializeFieldsFiltered(bean, jgen, provider);
} else {
serializeFields(bean, jgen, provider);
}
}
}
#Test
public void usingModule() throws Exception {
// basic module metadata just includes name and version (both for troubleshooting; but name needs to be unique)
SimpleModule module = new SimpleModule("EnhancedDatesModule", new Version(0, 1, 0, "alpha"));
//adding a serializer for the composite view
module.addSerializer(new JsonSerializer<AddedLinksView>() {
#Override
public Class<AddedLinksView> handledType() {
return AddedLinksView.class;
}
#Override
public void serialize(AddedLinksView value, JsonGenerator jgen, SerializerProvider provider)
throws IOException, JsonProcessingException {
jgen.writeStartObject();
//looking up the bean serializer that will be used for my resource
JsonSerializer<Object> ser = provider.findTypedValueSerializer(value.getResource().getClass(), true,
null);
if (ser instanceof BeanSerializerBase) {
//cloning it in a sub class that makes it possible to 'inline' the serialization
RawBeanSerializer openSer = new RawBeanSerializer((BeanSerializerBase) ser);
openSer.rawSerialize(value.getResource(), jgen, provider);
}
//adding my links
jgen.writeArrayFieldStart("links");
for (Link link : value.getLinks()) {
jgen.writeObject(link);
}
jgen.writeEndArray();
jgen.writeEndObject();
}
});
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);
AddedLinksView<TestBean> view = new AddedLinksView<TestBean>(new TestBean("aa", 2L)) {
#Override
public Link[] buildLinks(TestBean resource) {
return new Link[] { new Link("rel", "href://"), new Link("rel2", "href://2") };
}
};
System.out.println("useModule json output: " + mapper.writeValueAsString(view));
}
Did I miss something obvious in Jackson to achieve this? Or am I completely off the mark in my requirements already?
There is no real way to externally inject things into POJOs to serialize: but you might be interested in checking out #JsonAnyGetter, which at least allows just adding contents of a java.util.Map as extra properties for a POJO.
Would this answer your question: http://wiki.fasterxml.com/JacksonFeatureUpdateValue
I am not sure you can avoid mapping. You may use Dozer to help.
This should help you: Tools for merging java beans
I have two Java classes that I want to serialize to JSON using Jackson:
public class User {
public final int id;
public final String name;
public User(int id, String name) {
this.id = id;
this.name = name;
}
}
public class Item {
public final int id;
public final String itemNr;
public final User createdBy;
public Item(int id, String itemNr, User createdBy) {
this.id = id;
this.itemNr = itemNr;
this.createdBy = createdBy;
}
}
I want to serialize an Item to this JSON:
{"id":7, "itemNr":"TEST", "createdBy":3}
with User serialized to only include the id. I will also be able to serilize all user objects to JSON like:
{"id":3, "name": "Jonas", "email": "jonas#example.com"}
So I guess that I need to write a custom serializer for Item and tried with this:
public class ItemSerializer extends JsonSerializer<Item> {
#Override
public void serialize(Item value, JsonGenerator jgen,
SerializerProvider provider) throws IOException,
JsonProcessingException {
jgen.writeStartObject();
jgen.writeNumberField("id", value.id);
jgen.writeNumberField("itemNr", value.itemNr);
jgen.writeNumberField("createdBy", value.user.id);
jgen.writeEndObject();
}
}
I serialize the JSON with this code from Jackson How-to: Custom Serializers:
ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule",
new Version(1,0,0,null));
simpleModule.addSerializer(new ItemSerializer());
mapper.registerModule(simpleModule);
StringWriter writer = new StringWriter();
try {
mapper.writeValue(writer, myItem);
} catch (JsonGenerationException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
But I get this error:
Exception in thread "main" java.lang.IllegalArgumentException: JsonSerializer of type com.example.ItemSerializer does not define valid handledType() (use alternative registration method?)
at org.codehaus.jackson.map.module.SimpleSerializers.addSerializer(SimpleSerializers.java:62)
at org.codehaus.jackson.map.module.SimpleModule.addSerializer(SimpleModule.java:54)
at com.example.JsonTest.main(JsonTest.java:54)
How can I use a custom Serializer with Jackson?
This is how I would do it with Gson:
public class UserAdapter implements JsonSerializer<User> {
#Override
public JsonElement serialize(User src, java.lang.reflect.Type typeOfSrc,
JsonSerializationContext context) {
return new JsonPrimitive(src.id);
}
}
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(User.class, new UserAdapter());
Gson gson = builder.create();
String json = gson.toJson(myItem);
System.out.println("JSON: "+json);
But I need to do it with Jackson now, since Gson doesn't have support for interfaces.
You can put #JsonSerialize(using = CustomDateSerializer.class) over any date field of object to be serialized.
public class CustomDateSerializer extends SerializerBase<Date> {
public CustomDateSerializer() {
super(Date.class, true);
}
#Override
public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider)
throws IOException, JsonProcessingException {
SimpleDateFormat formatter = new SimpleDateFormat("EEE MMM dd yyyy HH:mm:ss 'GMT'ZZZ (z)");
String format = formatter.format(value);
jgen.writeString(format);
}
}
As mentioned, #JsonValue is a good way. But if you don't mind a custom serializer, there's no need to write one for Item but rather one for User -- if so, it'd be as simple as:
public void serialize(Item value, JsonGenerator jgen,
SerializerProvider provider) throws IOException,
JsonProcessingException {
jgen.writeNumber(id);
}
Yet another possibility is to implement JsonSerializable, in which case no registration is needed.
As to error; that is weird -- you probably want to upgrade to a later version. But it is also safer to extend org.codehaus.jackson.map.ser.SerializerBase as it will have standard implementations of non-essential methods (i.e. everything but actual serialization call).
I tried doing this too, and there is a mistake in the example code on the Jackson web page that fails to include the type (.class) in the call to addSerializer() method, which should read like this:
simpleModule.addSerializer(Item.class, new ItemSerializer());
In other words, these are the lines that instantiate the simpleModule and add the serializer (with the prior incorrect line commented out):
ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule",
new Version(1,0,0,null));
// simpleModule.addSerializer(new ItemSerializer());
simpleModule.addSerializer(Item.class, new ItemSerializer());
mapper.registerModule(simpleModule);
FYI: Here is the reference for the correct example code: http://wiki.fasterxml.com/JacksonFeatureModules
Use #JsonValue:
public class User {
int id;
String name;
#JsonValue
public int getId() {
return id;
}
}
#JsonValue only works on methods so you must add the getId method.
You should be able to skip your custom serializer altogether.
I wrote an example for a custom Timestamp.class serialization/deserialization, but you could use it for what ever you want.
When creating the object mapper do something like this:
public class JsonUtils {
public static ObjectMapper objectMapper = null;
static {
objectMapper = new ObjectMapper();
SimpleModule s = new SimpleModule();
s.addSerializer(Timestamp.class, new TimestampSerializerTypeHandler());
s.addDeserializer(Timestamp.class, new TimestampDeserializerTypeHandler());
objectMapper.registerModule(s);
};
}
for example in java ee you could initialize it with this:
import java.time.LocalDateTime;
import javax.ws.rs.ext.ContextResolver;
import javax.ws.rs.ext.Provider;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.module.SimpleModule;
#Provider
public class JacksonConfig implements ContextResolver<ObjectMapper> {
private final ObjectMapper objectMapper;
public JacksonConfig() {
objectMapper = new ObjectMapper();
SimpleModule s = new SimpleModule();
s.addSerializer(Timestamp.class, new TimestampSerializerTypeHandler());
s.addDeserializer(Timestamp.class, new TimestampDeserializerTypeHandler());
objectMapper.registerModule(s);
};
#Override
public ObjectMapper getContext(Class<?> type) {
return objectMapper;
}
}
where the serializer should be something like this:
import java.io.IOException;
import java.sql.Timestamp;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
public class TimestampSerializerTypeHandler extends JsonSerializer<Timestamp> {
#Override
public void serialize(Timestamp value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
String stringValue = value.toString();
if(stringValue != null && !stringValue.isEmpty() && !stringValue.equals("null")) {
jgen.writeString(stringValue);
} else {
jgen.writeNull();
}
}
#Override
public Class<Timestamp> handledType() {
return Timestamp.class;
}
}
and deserializer something like this:
import java.io.IOException;
import java.sql.Timestamp;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.SerializerProvider;
public class TimestampDeserializerTypeHandler extends JsonDeserializer<Timestamp> {
#Override
public Timestamp deserialize(JsonParser jp, DeserializationContext ds) throws IOException, JsonProcessingException {
SqlTimestampConverter s = new SqlTimestampConverter();
String value = jp.getValueAsString();
if(value != null && !value.isEmpty() && !value.equals("null"))
return (Timestamp) s.convert(Timestamp.class, value);
return null;
}
#Override
public Class<Timestamp> handledType() {
return Timestamp.class;
}
}
These are behavior patterns I have noticed while trying to understand Jackson serialization.
1) Assume there is an object Classroom and a class Student. I've made everything public and final for ease.
public class Classroom {
public final double double1 = 1234.5678;
public final Double Double1 = 91011.1213;
public final Student student1 = new Student();
}
public class Student {
public final double double2 = 1920.2122;
public final Double Double2 = 2324.2526;
}
2) Assume that these are the serializers we use for serializing the objects into JSON. The writeObjectField uses the object's own serializer if it is registered with the object mapper; if not, then it serializes it as a POJO. The writeNumberField exclusively only accepts primitives as arguments.
public class ClassroomSerializer extends StdSerializer<Classroom> {
public ClassroomSerializer(Class<Classroom> t) {
super(t);
}
#Override
public void serialize(Classroom value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonGenerationException {
jgen.writeStartObject();
jgen.writeObjectField("double1-Object", value.double1);
jgen.writeNumberField("double1-Number", value.double1);
jgen.writeObjectField("Double1-Object", value.Double1);
jgen.writeNumberField("Double1-Number", value.Double1);
jgen.writeObjectField("student1", value.student1);
jgen.writeEndObject();
}
}
public class StudentSerializer extends StdSerializer<Student> {
public StudentSerializer(Class<Student> t) {
super(t);
}
#Override
public void serialize(Student value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonGenerationException {
jgen.writeStartObject();
jgen.writeObjectField("double2-Object", value.double2);
jgen.writeNumberField("double2-Number", value.double2);
jgen.writeObjectField("Double2-Object", value.Double2);
jgen.writeNumberField("Double2-Number", value.Double2);
jgen.writeEndObject();
}
}
3) Register only a DoubleSerializer with DecimalFormat output pattern ###,##0.000, in SimpleModule and the output is:
{
"double1" : 1234.5678,
"Double1" : {
"value" : "91,011.121"
},
"student1" : {
"double2" : 1920.2122,
"Double2" : {
"value" : "2,324.253"
}
}
}
You can see that the POJO serialization differentiates between double and Double, using the DoubleSerialzer for Doubles and using a regular String format for doubles.
4) Register DoubleSerializer and ClassroomSerializer, without the StudentSerializer. We expect that the output is such that if we write a double as an object, it behaves like a Double, and if we write a Double as a number, it behaves like a double. The Student instance variable should be written as a POJO and follow the pattern above since it does not register.
{
"double1-Object" : {
"value" : "1,234.568"
},
"double1-Number" : 1234.5678,
"Double1-Object" : {
"value" : "91,011.121"
},
"Double1-Number" : 91011.1213,
"student1" : {
"double2" : 1920.2122,
"Double2" : {
"value" : "2,324.253"
}
}
}
5) Register all serializers. The output is:
{
"double1-Object" : {
"value" : "1,234.568"
},
"double1-Number" : 1234.5678,
"Double1-Object" : {
"value" : "91,011.121"
},
"Double1-Number" : 91011.1213,
"student1" : {
"double2-Object" : {
"value" : "1,920.212"
},
"double2-Number" : 1920.2122,
"Double2-Object" : {
"value" : "2,324.253"
},
"Double2-Number" : 2324.2526
}
}
exactly as expected.
Another important note: If you have multiple serializers for the same class registered with the same Module, then the Module will select the serializer for that class that is most recently added to the list. This should not be used - it's confusing and I am not sure how consistent this is
Moral: if you want to customize serialization of primitives in your object, you must write your own serializer for the object. You cannot rely on the POJO Jackson serialization.
Jackson's JSON Views might be a simpler way of achieving your requirements, especially if you have some flexibility in your JSON format.
If {"id":7, "itemNr":"TEST", "createdBy":{id:3}} is an acceptable representation then this will be very easy to achieve with very little code.
You would just annotate the name field of User as being part of a view, and specify a different view in your serialisation request (the un-annotated fields would be included by default)
For example:
Define the views:
public class Views {
public static class BasicView{}
public static class CompleteUserView{}
}
Annotate the User:
public class User {
public final int id;
#JsonView(Views.CompleteUserView.class)
public final String name;
public User(int id, String name) {
this.id = id;
this.name = name;
}
}
And serialise requesting a view which doesn't contain the field you want to hide (non-annotated fields are serialised by default):
objectMapper.getSerializationConfig().withView(Views.BasicView.class);
In my case (Spring 3.2.4 and Jackson 2.3.1), XML configuration for custom serializer:
<mvc:annotation-driven>
<mvc:message-converters register-defaults="false">
<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
<property name="objectMapper">
<bean class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean">
<property name="serializers">
<array>
<bean class="com.example.business.serializer.json.CustomObjectSerializer"/>
</array>
</property>
</bean>
</property>
</bean>
</mvc:message-converters>
</mvc:annotation-driven>
was in unexplained way overwritten back to default by something.
This worked for me:
CustomObject.java
#JsonSerialize(using = CustomObjectSerializer.class)
public class CustomObject {
private Long value;
public Long getValue() {
return value;
}
public void setValue(Long value) {
this.value = value;
}
}
CustomObjectSerializer.java
public class CustomObjectSerializer extends JsonSerializer<CustomObject> {
#Override
public void serialize(CustomObject value, JsonGenerator jgen,
SerializerProvider provider) throws IOException,JsonProcessingException {
jgen.writeStartObject();
jgen.writeNumberField("y", value.getValue());
jgen.writeEndObject();
}
#Override
public Class<CustomObject> handledType() {
return CustomObject.class;
}
}
No XML configuration (<mvc:message-converters>(...)</mvc:message-converters>) is needed in my solution.
The problem in your case is the ItemSerializer is missing the method handledType() which needs to be overridden from JsonSerializer
public class ItemSerializer extends JsonSerializer<Item> {
#Override
public void serialize(Item value, JsonGenerator jgen,
SerializerProvider provider) throws IOException,
JsonProcessingException {
jgen.writeStartObject();
jgen.writeNumberField("id", value.id);
jgen.writeNumberField("itemNr", value.itemNr);
jgen.writeNumberField("createdBy", value.user.id);
jgen.writeEndObject();
}
#Override
public Class<Item> handledType()
{
return Item.class;
}
}
Hence you are getting the explicit error that handledType() is not defined
Exception in thread "main" java.lang.IllegalArgumentException: JsonSerializer of type com.example.ItemSerializer does not define valid handledType()
Hope it helps someone. Thanks for reading my answer.
If your only requirement in your custom serializer is to skip serializing the name field of User, mark it as transient. Jackson will not serialize or deserialize transient fields.
[ see also: Why does Java have transient fields? ]
You have to override method handledType and everything will work
#Override
public Class<Item> handledType()
{
return Item.class;
}