Suppose I am writing custom serialization for some class, but would like to process one of its field with default methods.
How to do that?
While serializing we have JsonGenerator#writeObjectField().
But what is corresponding method for deserialization?
Regard the code below:
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;
import java.io.IOException;
import java.util.Objects;
public class TryDelegate {
public static class MyOuterClassSerializer extends JsonSerializer<MyOuterClass> {
#Override
public void serialize(MyOuterClass value, JsonGenerator gen, SerializerProvider serializers) throws IOException, JsonProcessingException {
gen.writeStartObject();
gen.writeObjectField("inner", value.getInner());
gen.writeEndObject();
}
}
public static class MyOuterClassDeserializer extends JsonDeserializer<MyOuterClass> {
#Override
public MyOuterClass deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
MyOuterClass ans = new MyOuterClass();
JsonToken token;
token = p.getCurrentToken();
if( token != JsonToken.START_OBJECT ) {
throw new JsonParseException("Start object expected", p.getCurrentLocation());
}
if( !"inner".equals(p.nextFieldName() ) ) {
throw new JsonParseException("'inner; field expected", p.getCurrentLocation());
}
MyInnerClass inner = null;// how to desrialize inner from here with default processing???
ans.setInner(inner);
token = p.nextToken();
if( token != JsonToken.END_OBJECT ) {
throw new JsonParseException("End object expected", p.getCurrentLocation());
}
return ans;
}
}
public static class MyInnerClass {
private int value;
public int getValue() {
return value;
}
public void setValue(int value) {
this.value = value;
}
#Override
public String toString() {
return "{\"value\":" + value + "}";
}
}
#JsonDeserialize(using = MyOuterClassDeserializer.class)
#JsonSerialize(using = MyOuterClassSerializer.class)
public static class MyOuterClass {
private MyInnerClass inner;
public MyInnerClass getInner() {
return inner;
}
public void setInner(MyInnerClass inner) {
this.inner = inner;
}
#Override
public String toString() {
return "{\"inner\":" + Objects.toString(inner) + "}";
}
}
public static void main(String[] args) throws IOException {
ObjectMapper mapper = new ObjectMapper();
String string;
MyInnerClass inner = new MyInnerClass();
inner.setValue(12);
MyOuterClass outer = new MyOuterClass();
outer.setInner(inner);
string = mapper.writeValueAsString(outer);
System.out.println(string);
MyOuterClass outer2 = mapper.readValue(string, MyOuterClass.class);
System.out.println(outer2); // inner was not deserialized
}
}
How to implement MyOuterDeserializer?
The DeserializationContext offers these tools.
After checking the field name for "inner", move to the next token, the beginning of the JSON object and use the DeserializationContext to deserialize the JSON object into a MyInnerClass object.
if (!"inner".equals(p.nextFieldName())) {
throw new JsonParseException("'inner; field expected", p.getCurrentLocation());
}
p.nextToken(); // consumes the field name token
MyInnerClass inner = ctxt.readValue(p, MyInnerClass.class);
The javadoc states
Convenience method that may be used by composite or container
deserializers, for reading one-off values contained (for sequences, it
is more efficient to actually fetch deserializer once for the whole
collection).
Careful while using the DeserializationContext. Don't try to recursively deserialize types for which you have have registered custom deserializers.
Related
I'm using jackson-databind version 2.12.3 to serialize the return of an object that should return like this:
{
"field1":"value1",
"field2":"value2",
"links":{
"field":{
"href":"/link"
},
"test":{
"href":"/test"
}
}
}
My classes are these:
public class HrefType {
private String href = null;
...
}
public class Link extends HashMap<String, HrefType> {
private HrefType field = null;
...
}
public class MyObject {
private String field1 = null;
private String field2 = null;
private Link links = null;
...
}
The return is myObject:
MyObject myObject = new MyObject();
myObject.setField1("value1");
myObject.setField2("value2");
Link link = new Link();
link.setField(new HrefType().href("/link"));
link.put("test",new HrefType().href("/test"));
myObject.setLinks(link);
However with the default ObjectMapper the "link.setField" is ignored and the returned json is:
{
"field1":"value1",
"field2":"value2",
"links":{
"test":{
"href":"/test"
}
}
}
I tried doing some tests with JsonSerializer but couldn't do something generic for all classes that extend HashMap (these classes are generated from BerlinGroup's PSD2 YAML, so I wouldn't want to change the generated class).
Is there a generic way to do it, or should I make a serialize class for each class that extends the HashMap?
Composition
First of all, I suggest you use composition instead of inheritance in this particular case. Your code will look like the next:
private class Link {
private final HrefType field;
private final HashMap<String, HrefType> test;
public Link(HrefType field) {
this.field = field;
}
public HrefType getField() {
return field;
}
public HashMap<String, HrefType> getTest() {
return test;
}
}
And serialization will work fine, as expected.
Serializer
But in case, if you can't change the original code, you might to write your own StdSerializer. For example:
private class LinkSerializer extends StdSerializer<Link> {
public LinkSerializer() {
super(Link.class);
}
#Override
public void serialize(Link link, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
jsonGenerator.writeStartObject();
final HrefType field = link.getField();
jsonGenerator.writeObjectField("field", field);
jsonGenerator.writeObjectField("test", new HashMap<>(link));
jsonGenerator.writeEndObject();
}
}
And declare it over your Link class:
#JsonSerialize(using = LinkSerializer.class)
private static class Link extends HashMap<String, HrefType> {
private final HrefType field;
public Link(HrefType field) {
this.field = field;
}
public HrefType getField() {
return field;
}
}
based on this answer I developed this generic method of making for all objects that extend a Map:
import java.io.IOException;
import java.lang.reflect.Field;
import java.util.Map;
import org.springframework.util.ReflectionUtils;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
public class MyClassSerializer extends JsonSerializer<Object> {
private final JsonSerializer<Object> defaultSerializer;
public MyClassSerializer(JsonSerializer<Object> defaultSerializer) {
this.defaultSerializer = (defaultSerializer);
}
#SuppressWarnings({ "unchecked", "rawtypes" })
#Override
public void serialize(Object src, JsonGenerator gen, SerializerProvider provider) throws IOException {
Field[] fields = src.getClass().getDeclaredFields();
for (Field field : fields) {
try {
boolean fieldAccessible = field.isAccessible();
field.setAccessible(true);
Object object = ReflectionUtils.getField(field, src);
if (object != null && object instanceof Map) {
Field[] fieldsMap = object.getClass().getDeclaredFields();
Map map = (Map) object;
for (Field fieldMap : fieldsMap) {
boolean fieldMapAccessible = fieldMap.isAccessible();
fieldMap.setAccessible(true);
Object fieldObject = ReflectionUtils.getField(fieldMap, object);
if (fieldObject != null) {
map.put(fieldMap.getName(), fieldObject);
}
fieldMap.setAccessible(fieldMapAccessible);
}
}
field.setAccessible(fieldAccessible);
} catch (Exception e) {
e.printStackTrace();
}
}
defaultSerializer.serialize(src, gen, provider);
}
#Override
public Class<Object> handledType() {
return Object.class;
}
}
which goes through all fields, when I find one that extends from a Map I go through all the fields of this one and add it to the Map ignoring the object's fields, so the Serializer works perfectly.
EDIT: to Deserializer properly I do this:
import java.io.IOException;
import java.lang.reflect.Field;
import java.util.Map;
import org.springframework.util.ReflectionUtils;
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.JsonMappingException;
import com.fasterxml.jackson.databind.deser.ResolvableDeserializer;
#SuppressWarnings("rawtypes")
public class MyClassDeserializer extends JsonDeserializer implements ResolvableDeserializer {
private JsonDeserializer defaultDeserializer;
protected MyClassDeserializer(JsonDeserializer deserializer) {
this.defaultDeserializer = deserializer;
}
#SuppressWarnings("unchecked")
#Override
public Object deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
Object obj = defaultDeserializer.deserialize(p, ctxt);
Field[] fields = obj.getClass().getDeclaredFields();
for (Field field : fields) {
try {
boolean fieldAccessible = field.isAccessible();
field.setAccessible(true);
Object object = ReflectionUtils.getField(field, obj);
if (object != null && object instanceof Map) {
Field[] fieldsMap = object.getClass().getDeclaredFields();
Map map = (Map) object;
for (Object key : map.keySet()) {
for (Field fieldMap : fieldsMap) {
if (fieldMap.getName().equals((String) key)) {
if (fieldMap.getName().equalsIgnoreCase("serialVersionUID")) {
continue;
}
boolean fieldMapAccessible = fieldMap.isAccessible();
fieldMap.setAccessible(true);
Object fieldObject = ReflectionUtils.getField(fieldMap, object);
if (fieldObject == null) {
fieldMap.set(object, map.get(key));
map.replace(key, null);
}
fieldMap.setAccessible(fieldMapAccessible);
}
}
}
Object[] keys = map.keySet().toArray();
for (int i = 0; i < keys.length; i++) {
if(map.get(keys[i])==null) {
map.remove(keys[i]);
}
}
}
field.setAccessible(fieldAccessible);
} catch (Exception e) {
e.printStackTrace();
}
}
return obj;
}
#Override
public void resolve(DeserializationContext ctxt) throws JsonMappingException {
((ResolvableDeserializer) defaultDeserializer).resolve(ctxt);
}
}
I'm looking for a way to (de-)serialize a List of items without using Annotations in Jackson. Is this possible? What I'm doing up to now is trying to replace the <item>-tag with a tag telling about the item's class, but no avail. And even if this worked, I'm not sure whether Jackson would offer a way to process this tag information.
To give a better of what I'm aiming at, here's a sample:
public class JacksonTest {
private static class ListElement {
private boolean value;
// getters, setters, constructors omitted
}
#Test
public void testDeSerialization() throws Exception {
final List<ListElement> existing = Arrays.asList(new ListElement(true));
final ObjectMapper mapper = new XmlMapper();
final JavaType listJavaType = mapper.getTypeFactory().constructCollectionType(List.class, ListElement.class);
final String listString = mapper.writerFor(listJavaType).writeValueAsString(existing);
System.out.println(listString);
// "<List><item><value>true</value></item></List>"
}
}
So, the result is <List><item><value>true</value></item></List>, while I want the <item>-tag to be replaced with the (qualified) class name or offering a type-attribute.
Of course, even this would not help if there's no way in Jackson to process this class name.
Do I have reached a dead end here or is there a way to go?
You can define your own JsonSerializer (also used for XML) and add it to a JacksonXmlModule.
ToXmlGenerator has a setNextName function that allows you to override the default item name
private class MyListSerializer extends JsonSerializer<List> {
#Override
public void serialize(List list, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
throws IOException {
for (Object obj : list) {
if (jsonGenerator instanceof ToXmlGenerator) {
ToXmlGenerator xmlGenerator = (ToXmlGenerator) jsonGenerator;
String className = obj.getClass().getSimpleName();
xmlGenerator.setNextName(new QName(className));
}
jsonGenerator.writeObject(obj);
// this is overridden at the next iteration
// and ignored at the last
jsonGenerator.writeFieldName("dummy");
}
}
#Override
public Class<List> handledType() {
return List.class;
}
}
#Test
public void testDeSerialization() throws Exception {
final List<ListElement> existing = Arrays.asList(new ListElement(true));
JacksonXmlModule module = new JacksonXmlModule();
module.addSerializer(new MyListSerializer());
final ObjectMapper mapper = new XmlMapper(module);
final JavaType listJavaType = mapper.getTypeFactory().constructCollectionType(List.class, ListElement.class);
final ObjectWriter writer = mapper.writerFor(listJavaType);
final String listString = writer.writeValueAsString(existing);
System.out.println(listString);
// "<List><ListElement><value>true</value></ListElement></List>"
}
Okay, after some tinkering and debugging with Evertude's proposal I've figured out a solution. I'm not really happy with the serialization part and honestly I don't know why I was supposed to do it this way. When debugging I've noticed that XmlGenerator::setNextName is required to be called once but does not have any effect on the next call, so I had to implement a switch there and set the field name for the next item in the loop directly.
I'ld be glad if somebody has an idea what I'm doing wrong, but at least my attempt is working for now:
#Test
public void testDeSerialization() throws Exception {
final List<ListElement> existing = Arrays.asList(new ListElement(true), new ListElement(false));
JacksonXmlModule module = new JacksonXmlModule();
module.addSerializer(new MyListSerializer());
final ObjectMapper mapper = new XmlMapper(module);
final JavaType listJavaType = mapper.getTypeFactory().constructCollectionType(List.class, ListElement.class);
final ObjectWriter writer = mapper.writerFor(listJavaType);
final String listString = writer.writeValueAsString(existing);
module.addDeserializer(List.class, new MyListDeserializer());
List<ListElement> deserialized = mapper.readValue(listString, List.class);
assertEquals(existing, deserialized); // provided there're proper hash() and equals() methods
}
private class MyListSerializer extends JsonSerializer<List> {
#Override
public void serialize(List list, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
throws IOException {
boolean done = false;
for (Object obj : list) {
if (jsonGenerator instanceof ToXmlGenerator) {
ToXmlGenerator xmlGenerator = (ToXmlGenerator) jsonGenerator;
String className = obj.getClass().getSimpleName();
// weird switch
if (!done) xmlGenerator.setNextName(new QName(className));
else jsonGenerator.writeFieldName(className);
done = true;
}
jsonGenerator.writeObject(obj);
}
}
#Override
public Class<List> handledType() {
return List.class;
}
}
private class MyListDeserializer extends JsonDeserializer<List> {
#Override
public List deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
List<Object> items = new ArrayList<>();
JsonToken nextToken;
while ((nextToken = p.nextToken()) != JsonToken.END_OBJECT) {
String currentName = p.currentName();
try {
String className = "my.test.project.JacksonCustomSerializer$" + currentName;
Class<?> loadClass = getClass().getClassLoader().loadClass(className);
p.nextToken();
items.add(p.readValueAs(loadClass));
} catch (ClassNotFoundException e) {
// some handling
}
}
return items;
}
#Override
public Class<List> handledType() {
return List.class;
}
}
We have a service which currently consumes JSON. We want to slightly restructure this JSON (move one property one level up) but also implement graceful migration so that our service could process old structure as well as new structure. We're using Jackson for JSON deserialization.
How do we restructure JSON prior to deserialization with Jackson?
Here's a MCVE.
Assume our old JSON looks as follows:
{"reference": {"number" : "one", "startDate" : [2016, 11, 16], "serviceId" : "0815"}}
We want to move serviceId one level up:
{"reference": {"number" : "one", "startDate" : [2016, 11, 16]}, "serviceId" : "0815"}
This are the classes we want to deserialize from both old an new JSONs:
public final static class Container {
public final Reference reference;
public final String serviceId;
#JsonCreator
public Container(#JsonProperty("reference") Reference reference, #JsonProperty("serviceId") String serviceId) {
this.reference = reference;
this.serviceId = serviceId;
}
}
public final static class Reference {
public final String number;
public final LocalDate startDate;
#JsonCreator
public Reference(#JsonProperty("number") String number, #JsonProperty("startDate") LocalDate startDate) {
this.number = number;
this.startDate = startDate;
}
}
We only want serviceId in Container, not in both classes.
What I've got working is the following deserializer:
public static class ServiceIdMigratingContainerDeserializer extends JsonDeserializer<Container> {
private final ObjectMapper objectMapper;
{
objectMapper = new ObjectMapper();
objectMapper.registerModule(new JavaTimeModule());
objectMapper.configure(DeserializationFeature.READ_UNKNOWN_ENUM_VALUES_AS_NULL, true);
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
}
#Override
public Container deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
ObjectNode node = p.readValueAsTree();
migrate(node);
return objectMapper.treeToValue(node, Container.class);
}
private void migrate(ObjectNode containerNode) {
TreeNode referenceNode = containerNode.get("reference");
if (referenceNode != null && referenceNode.isObject()) {
TreeNode serviceIdNode = containerNode.get("serviceId");
if (serviceIdNode == null) {
TreeNode referenceServiceIdNode = referenceNode.get("serviceId");
if (referenceServiceIdNode != null && referenceServiceIdNode.isValueNode()) {
containerNode.set("serviceId", (ValueNode) referenceServiceIdNode);
}
}
}
}
}
This deserializer first retrieves the tree, manipulates it and then deserializers it using an own instance of ObjectMapper. It works but we really dislike the fact that we have another instance of ObjectMapper here. If we don't create it and somehow use the system-wide instance of ObjectMapper we get an infinite cycle because when we try to call objectMapper.treeToValue, our deserializer gets called recursively. So this works (with an own instance of ObjectMapper) but it is not an optimal solution.
Another method I've tried was using a BeanDeserializerModifier and a own JsonDeserializer which "wraps" the default serializer:
public static class ServiceIdMigrationBeanDeserializerModifier extends BeanDeserializerModifier {
#Override
public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc,
JsonDeserializer<?> defaultDeserializer) {
if (beanDesc.getBeanClass() == Container.class) {
return new ModifiedServiceIdMigratingContainerDeserializer((JsonDeserializer<Container>) defaultDeserializer);
} else {
return defaultDeserializer;
}
}
}
public static class ModifiedServiceIdMigratingContainerDeserializer extends JsonDeserializer<Container> {
private final JsonDeserializer<Container> defaultDeserializer;
public ModifiedServiceIdMigratingContainerDeserializer(JsonDeserializer<Container> defaultDeserializer) {
this.defaultDeserializer = defaultDeserializer;
}
#Override
public Container deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
ObjectNode node = p.readValueAsTree();
migrate(node);
return defaultDeserializer.deserialize(new TreeTraversingParser(node, p.getCodec()), ctxt);
}
private void migrate(ObjectNode containerNode) {
TreeNode referenceNode = containerNode.get("reference");
if (referenceNode != null && referenceNode.isObject()) {
TreeNode serviceIdNode = containerNode.get("serviceId");
if (serviceIdNode == null) {
TreeNode referenceServiceIdNode = referenceNode.get("serviceId");
if (referenceServiceIdNode != null && referenceServiceIdNode.isValueNode()) {
containerNode.set("serviceId", (ValueNode) referenceServiceIdNode);
}
}
}
}
}
"Wrapping" a default deserializer seems to be a better approach, but this fails with an NPE:
java.lang.NullPointerException
at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:157)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:150)
at de.db.vz.rikernpushadapter.migration.ServiceIdMigrationTest$ModifiedServiceIdMigratingContainerDeserializer.deserialize(ServiceIdMigrationTest.java:235)
at de.db.vz.rikernpushadapter.migration.ServiceIdMigrationTest$ModifiedServiceIdMigratingContainerDeserializer.deserialize(ServiceIdMigrationTest.java:1)
at com.fasterxml.jackson.databind.ObjectReader._bindAndClose(ObjectReader.java:1623)
at com.fasterxml.jackson.databind.ObjectReader.readValue(ObjectReader.java:1217)
at ...
The whole MCVE code is in the following PasteBin. It is a single-class all-containing test case which demonstrates both approaches. The migratesViaDeserializerModifierAndUnmarshalsServiceId fails.
So this leaves me with a question:
How do we restructure JSON prior to deserialization with Jackson?
In the best traditions, right after posting the question, I've managed to solve this.
Two things:
I had to do newJsonParser.nextToken(); to avoid NPE.
Extend DelegatingDeserializer
Here's a working DelegatingDeserializer:
public static class ModifiedServiceIdMigratingContainerDeserializer
extends DelegatingDeserializer {
public ModifiedServiceIdMigratingContainerDeserializer(JsonDeserializer<?> defaultDeserializer) {
super(defaultDeserializer);
}
#Override
protected JsonDeserializer<?> newDelegatingInstance(JsonDeserializer<?> newDelegatee) {
return new ModifiedServiceIdMigratingContainerDeserializer(newDelegatee);
}
#Override
public Object deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
return super.deserialize(restructure(p), ctxt);
}
#Override
public Object deserialize(JsonParser p, DeserializationContext ctxt, Object intoValue) throws IOException,
JsonProcessingException {
return super.deserialize(restructure(p), ctxt, intoValue);
}
public Object deserializeWithType(JsonParser jp, DeserializationContext ctxt, TypeDeserializer typeDeserializer)
throws IOException, JsonProcessingException {
return super.deserializeWithType(restructure(jp), ctxt, typeDeserializer);
}
public JsonParser restructure(JsonParser p) throws IOException, JsonParseException {
final ObjectNode node = p.readValueAsTree();
migrate(node);
final TreeTraversingParser newJsonParser = new TreeTraversingParser(node, p.getCodec());
newJsonParser.nextToken();
return newJsonParser;
}
private void migrate(ObjectNode containerNode) {
TreeNode referenceNode = containerNode.get("reference");
if (referenceNode != null && referenceNode.isObject()) {
TreeNode serviceIdNode = containerNode.get("serviceId");
if (serviceIdNode == null) {
TreeNode referenceServiceIdNode = referenceNode.get("serviceId");
if (referenceServiceIdNode != null && referenceServiceIdNode.isValueNode()) {
containerNode.set("serviceId", (ValueNode) referenceServiceIdNode);
}
}
}
}
}
I'm deserializing a large json value. Deeply nested within that value is a json object like the following:
{
"fieldOne": "valueOne",
"fieldTwo": {
"innerField": "innerValue"
}
}
I'm using the Jackson ObjectMapper to deserialize the large json value into a 3rd party class. Deeply nested within that 3rd party class is another 3rd party class:
public class DeepThirdPartyClass {
public String fieldOne;
}
which unfortunately is missing the fieldTwo property. I can create my own class which adds the missing field:
public class MyClass extends DeepThirdPartyClass {
public MySubObject fieldTwo;
}
How do I configure jackson so that whenever it attempts to deserialize a value to DeepThirdPartyClass, it deserializes to MyClass instead?
I had similar requirement when I have to filter any not allowed characters in all String values.
To create Object Mapper:
public class CustomObjectMapper extends ObjectMapper {
public CustomObjectMapper() {
super();
SimpleModule module = new SimpleModule("HTML XSS Serializer", new Version(1, 0, 0, "FINAL", "com.crowdoptic", "web"));
module.addSerializer(String.class, new JsonHtmlXssSerializer());
module.addDeserializer(String.class, new JsonHtmlXssDeserializer());
this.registerModule(module);
}
}
public class JsonHtmlXssDeserializer extends StdScalarDeserializer<String> {
private static final Logger LOG = LogManager.getLogger(JsonHtmlXssDeserializer.class);
public JsonHtmlXssDeserializer() { super(String.class); }
#Override
public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
String value = StringDeserializer.instance.deserialize(p, ctxt);
LOG.trace("in deserialize for value: " + value);
String encodedValue = StringEscapeUtils.escapeHtml4(value);
return encodedValue;
}
#Override
public String deserializeWithType(JsonParser jp, DeserializationContext ctxt, TypeDeserializer typeDeserializer) throws IOException {
return StringDeserializer.instance.deserializeWithType(jp, ctxt, typeDeserializer);
}
#Override
public boolean isCachable() { return StringDeserializer.instance.isCachable(); }
}
In your case you can register your class deserializer, call super method of the object deserializer. Then instead of returning DeepThirdPartyClass, create object of MyClass, set field one from DeepThirdPartyClass and add second field. See StringDeserializer and others for implementation details and available properties.
Let me know if that helps.
I reworked #olga-khylkouskaya's solution to fit my problem:
#Test
public void newDeserializer() throws Exception {
ObjectMapper objectMapper = new ObjectMapper();
SimpleModule module = new SimpleModule("DeepThirdPartyClass subclass override", new Version(1, 0, 0, "FINAL", "com.example", "deep-third-party-class-override"));
module.addDeserializer(DeepThirdPartyClass.class, new JsonDeserializer<DeepThirdPartyClass>() {
#Override
public DeepThirdPartyClass deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
return p.readValueAs(MyClass.class);
}
});
objectMapper.registerModule(module);
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
String json = "{\n" +
" \"middle\": {\n" +
" \"fieldOne\": \"valueOne\",\n" +
" \"fieldTwo\": {\n" +
" \"fieldThree\": \"valueThree\"\n" +
" }\n" +
" }\n" +
"}\n";
ThirdPartyClass thirdPartyClass = objectMapper.readValue(json, ThirdPartyClass.class);
}
public class ThirdPartyClass {
public DeepThirdPartyClass middle;
}
public class InnerClass {
public String fieldThree;
}
I am using Jackson fasterxml for unmarshalling JSON. In my object there are two kinds of properties:Input properties and Calculated properties. In the input JSON, I get only input values.
The calculated values are actually dependent on input values. I have to populate these values before the object gets referred. So I am just checking if there are any hooks provided by Jackson so that I can do my calculations there. For example JAXB provides afterUnmarshal method to customize the unmarshaling behavior:
void afterUnmarshal(Unmarshaller u, Object parent)
But I could not find similar information about customizing Jackson. Are any such framework hooks provided by Jackson to customize the unmarshaling behavior?
I'd rather recommend to keep your model objects immutable by using constructor creators. That is, all the JSON values are passed to a constructor which would initialize the other calculated properties.
Anyway, if you want to customize an object after deserialization (without writing a deserializer for every type) you can modify the deserializer in a way that at the end it calls a special method(s) of a newly constructed instance. Here is an example which would work for all the classes that implements a special interface (one can consider using an annotation to mark the post construct methods).
public class JacksonPostConstruct {
public static interface PostConstructor {
void postConstruct();
}
public static class Bean implements PostConstructor {
private final String field;
#JsonCreator
public Bean(#JsonProperty("field") String field) {
this.field = field;
}
public void postConstruct() {
System.out.println("Post construct: " + toString());
}
#Override
public String toString() {
return "Bean{" +
"field='" + field + '\'' +
'}';
}
}
private static class PostConstructDeserializer extends DelegatingDeserializer {
private final JsonDeserializer<?> deserializer;
public PostConstructDeserializer(JsonDeserializer<?> deserializer) {
super(deserializer);
this.deserializer = deserializer;
}
#Override
protected JsonDeserializer<?> newDelegatingInstance(JsonDeserializer<?> newDelegatee) {
return deserializer;
}
#Override
public Object deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
Object result = _delegatee.deserialize(jp, ctxt);
if (result instanceof PostConstructor) {
((PostConstructor) result).postConstruct();
}
return result;
}
}
public static void main(String[] args) throws IOException {
ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new BeanDeserializerModifier() {
#Override
public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config,
BeanDescription beanDesc,
final JsonDeserializer<?> deserializer) {
return new PostConstructDeserializer(deserializer);
}
});
mapper.registerModule(module);
String json = "{\"field\":\"value\"}";
System.out.println(mapper.readValue(json, Bean.class));
}
}
Output:
Post construct: Bean{field='value'}
Bean{field='value'}
Let's assume that your JSON looks like this:
{
"input1" : "Input value",
"input2" : 3
}
And your POJO class looks like this:
class Entity {
private String input1;
private int input2;
private String calculated1;
private long calculated2;
...
}
In this case you can write a custom deserializer for your Entity class:
class EntityJsonDeserializer extends JsonDeserializer<Entity> {
#Override
public Entity deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException,
JsonProcessingException {
InnerEntity innerEntity = jp.readValueAs(InnerEntity.class);
Entity entity = new Entity();
entity.setInput1(innerEntity.input1);
entity.setInput2(innerEntity.input2);
entity.recalculate();
return entity;
}
public static class InnerEntity {
public String input1;
public int input2;
}
}
In above class you can see that Entity has a recalculate method. It could look like this:
public void recalculate() {
calculated1 = input1 + input2;
calculated2 = input1.length() + input2;
}
You can also move this logic to your deserializer class.
Now, you have to inform Jackson that you want to use your custom deserializer:
#JsonDeserialize(using = EntityJsonDeserializer.class)
class Entity {
...
}
The example below shows how to use these classes:
ObjectMapper mapper = new ObjectMapper();
System.out.println(mapper.readValue(json, Entity.class));
This program prints:
Entity [input1=Input value, input2=3, calculated1=Input value3, calculated2=14]