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;
}
}
Related
I'm trying to create a generic Jackon polymorphic serializer that is able to serialize and deserialize to and from JSON with this format including the fqcn of the class of the object:
{
"fqcn": "full qualified class name of the object",
"data": "serialized object"
}
This wrapper should be applied to any object, so for example this will be the JSON representation of a HashMap> object:
{
"fqcn": "java.util.HashMap",
"data": {
"key1": {
"fqcn": "java.util.ArrayList",
"data": [
{
"fqcn": "java.lang.String",
"data": "value1"
},
{
"fqcn": "java.lang.String",
"data": "value2"
}
]
},
"key2": {
...
}
}
}
I could use a MixIn annotation all objects with #JsonTypeInfo
#JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.WRAPPER_OBJECT)
public interface ObjMixin {
}
---
ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(Object.class, ObjMixin.class);
However, the format does not match with the required format: {"fqcn": ..., "data": ...}
I've also tried to register a StdConverter to convert any objects to a wrapper like this:
public class ObjectWrapper {
private String fqcn;
private Object data;
public ObjectWrapper(Object obj) {
this.fqcn = obj.getClass.getCanonicalName();
this.data = obj;
}
}
However it is not possible to create a StdDelegatingSerializer for Object.class.
With a custom StdSerializer like the following I am getting StackOverflowError:
#Override
public void serialize(Object obj, JsonGenerator jsonGen, SerializerProvider serializerProvider) throws IOException {
jsonGen.writeStartObject();
jsonGen.writeStringField("fqcn", obj.getClass().getCanonicalName());
jsonGen.writeFieldName("data");
if (obj instanceof Iterable) {
jsonGen.writeStartArray();
// Recursive serialization of all elements in the iterable
jsonGen.writeEndArray();
} else if (obj instanceof Map) {
jsonGen.writeStartObject();
// Recursive serialization of all elements in the map
jsonGen.writeEndObject();
} else {
// Infinite recursion here because I'm defining this serializer for Object.class
serializerProvider.defaultSerializeValue(obj, jsonGen);
}
}
Does anyone know any other solution to be able to achieve this?
You could use a custom serializer and custom serializer provider to wrap every object you want to serialize into this wrapper object (EDIT: that did not work recusrively, updated the code to not use the wrapper object but write the fields instead):
public class FQCNTest {
#Test
public void doTest() throws JsonProcessingException {
final ObjectMapper om = getObjectMapper();
final Object obj = getTestObject();
final String json = om.writeValueAsString(obj);
System.out.println(json); // {"fqcn":"java.util.HashMap","data":{"k":{"fqcn":"java.lang.String","data":"v"}}}
final Object obj2 = getTestValue();
final String json2 = om.writeValueAsString(obj2);
System.out.println(json2); // {"fcqn":"java.lang.String","data":"hello"}
final Object obj3 = null;
final String json3 = om.writeValueAsString(obj3);
System.out.println(json3); // null
}
private ObjectMapper getObjectMapper() {
final ObjectMapper mapper = new ObjectMapper();
final SerializerProvider sp = mapper.getSerializerProviderInstance();
mapper.setSerializerProvider(new CustomSerializerProvider(sp, mapper.getSerializerFactory()));
return mapper;
}
private Object getTestObject() {
final HashMap<Object, Object> hashMap = new HashMap<>();
hashMap.put("k", "v");
return hashMap;
}
private Object getTestValue() {
return "hello";
}
}
class CustomSerializerProvider extends DefaultSerializerProvider {
private final SerializerProvider defaultInstance;
protected CustomSerializerProvider(final SerializerProvider defaultInstance, final SerializerFactory f) {
super(defaultInstance, defaultInstance.getConfig(), f);
this.defaultInstance = defaultInstance;
}
#Override
public WritableObjectId findObjectId(final Object forPojo, final ObjectIdGenerator<?> generatorType) {
return defaultInstance.findObjectId(forPojo, generatorType);
}
#Override
public JsonSerializer<Object> serializerInstance(final Annotated annotated, final Object serDef) throws JsonMappingException {
return new CustomSerializer();
}
#Override
public Object includeFilterInstance(final BeanPropertyDefinition forProperty, final Class<?> filterClass) {
try {
return defaultInstance.includeFilterInstance(forProperty, filterClass);
} catch (final JsonMappingException e) {
throw new RuntimeException(e);
}
}
#Override
public boolean includeFilterSuppressNulls(final Object filter) throws JsonMappingException {
return defaultInstance.includeFilterSuppressNulls(filter);
}
#Override
public DefaultSerializerProvider createInstance(final SerializationConfig config, final SerializerFactory jsf) {
return this;
}
#Override
public void serializeValue(final JsonGenerator gen, final Object value) throws IOException {
new CustomSerializer().serialize(value, gen, this);
}
#Override
public void serializeValue(final JsonGenerator gen, final Object value, final JavaType rootType) throws IOException {
super.serializeValue(gen, value, rootType);
}
#Override
public void serializeValue(final JsonGenerator gen, final Object value, final JavaType rootType, final JsonSerializer<Object> ser) throws IOException {
super.serializeValue(gen, value, rootType, ser);
}
}
class CustomSerializer extends StdSerializer<Object> {
protected CustomSerializer() {
super(Object.class);
}
#Override
public void serialize(final Object value, final JsonGenerator gen, final SerializerProvider provider) throws IOException {
if (value == null) {
provider.defaultSerializeValue(value, gen);
return;
}
final Class<?> clazz = value.getClass();
final JsonSerializer<Object> serForClazz = provider.findValueSerializer(clazz);
gen.writeStartObject();
gen.writeStringField("fqcn", clazz.getCanonicalName());
gen.writeFieldName("data");
if (value instanceof Iterable) {
gen.writeStartArray();
for (final Object e : ((Iterable<?>) value)) {
final JsonSerializer<Object> ser = new CustomSerializer();
ser.serialize(e, gen, provider);
}
gen.writeEndArray();
} else if (value instanceof Map) {
gen.writeStartObject();
// Recursive serialization of all elements in the map
for (final Map.Entry<?, ?> e : ((Map<?, ?>) value).entrySet()) {
final String key = e.getKey().toString(); // need to handle keys better
final Object mapValue = e.getValue();
gen.writeFieldName(key);
final JsonSerializer<Object> ser = new CustomSerializer();
ser.serialize(mapValue, gen, provider);
}
gen.writeEndObject();
} else {
serForClazz.serialize(value, gen, provider);
}
gen.writeEndObject();
}
}
Note: this code may contain too much stuff that is not necessary, I just took it far enough to make it work for the specific example. (and did not test deserialization, that may be a totally different thing)
I am trying to customise the serialisation of strings to avoid null values in the YAML file.
The code I have so far:
YAMLFactory yamlFactory = new YAMLFactory();
ObjectMapper mapper = new ObjectMapper(yamlFactory);
DefaultSerializerProvider sp = new DefaultSerializerProvider.Impl();
sp.setNullValueSerializer(new NullSerializer());
ObjectMapper m = new ObjectMapper();
mapper.setSerializerProvider(sp);
Map<String, Object> data = new HashMap<>();
data.put("aString", "test");
data.put("aNullObject", null);
data.put("anEmptyString", "");
String output = mapper.writeValueAsString(data);
System.out.println(output);
NullSerializer:
public class NullSerializer extends JsonSerializer<Object> {
public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider)
throws IOException, JsonProcessingException {
jgen.writeString("");
}
}
Result:
---
aNullObject: ""
aString: "test"
anEmptyString: ""
The problem is that writeString is writing an empty string, and I'm trying to have an empty value entirely.
Desired result:
---
aNullObject:
aString: "test"
anEmptyString: ""
I tried to use jgen.writeRaw(""); but I get this error:
Caused by: java.lang.UnsupportedOperationException: Operation not supported by generator of type com.fasterxml.jackson.dataformat.yaml.YAMLGenerator
at com.fasterxml.jackson.core.JsonGenerator._reportUnsupportedOperation(JsonGenerator.java:1967)
at com.fasterxml.jackson.dataformat.yaml.YAMLGenerator.writeRaw(YAMLGenerator.java:590)
at com.example.jackson.NullSerializer.serialize(NullSerializer.java:13)
at com.fasterxml.jackson.databind.SerializerProvider.defaultSerializeNull(SerializerProvider.java:1127)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:711)
... 7 more
For me disabling of "MINIMIZE_QUOTES" feature didn't work, still, an empty string is written. The only solution I found was to override ObjectMapper and YamlGenerator and to allow YamlGenerator to write empty raw value. And also you have to provide a custom null serializer that writes this raw value.
class YamlObjectMapper(yamlFactory: YAMLFactory) : ObjectMapper(yamlFactory) {
init {
val dS = DefaultSerializerProvider.Impl()
dS.setNullValueSerializer(NullSerializer)
setSerializerProvider(dS)
}}
class RawYAMLFactory : YAMLFactory() {
override fun _createGenerator(out: Writer?, ctxt: IOContext?): YAMLGenerator? {
val feats = _yamlGeneratorFeatures
return RawYamlGenerator(ctxt, _generatorFeatures, feats, _objectCodec, out, _version)
}}
private object NullSerializer : JsonSerializer<Any?>() {
override fun serialize(value: Any?, jgen: JsonGenerator, provider: SerializerProvider?) {
jgen.writeRaw("")
}}
private class RawYamlGenerator(ctxt: IOContext?, jsonFeatures: Int, yamlFeatures: Int,
codec: ObjectCodec, out: Writer?, version: DumperOptions.Version?)
: YAMLGenerator(ctxt, jsonFeatures, yamlFeatures, codec, out, version) {
override fun writeRaw(c: String) {
_writeContext.writeValue()
_emit(_scalarEvent("", DumperOptions.ScalarStyle.PLAIN))
}}
Based on the #Yuliia Liubchyk solution - I rewrote his code into Java.
Instance:
var yamlObjectMapper = new YamlObjectMapper(new RawYamlFactory().
enable(YAMLGenerator.Feature.MINIMIZE_QUOTES)).
findAndRegisterModules();
class NullValueSerializer extends JsonSerializer<Object> {
#Override
public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
gen.writeRaw("");
}
}
public class RawYamlFactory extends YAMLFactory {
#Override
protected YAMLGenerator _createGenerator(Writer out, IOContext ctxt) throws IOException {
var feats = _yamlGeneratorFeatures;
return new RawYamlGenerator(ctxt, _generatorFeatures, feats, _objectCodec, out, _version);
}
}
public class RawYamlGenerator extends YAMLGenerator {
public RawYamlGenerator(IOContext ctxt, int jsonFeatures, int yamlFeatures,
ObjectCodec codec, Writer out, DumperOptions.Version version) throws IOException {
super(ctxt, jsonFeatures, yamlFeatures, codec, out, version);
}
#Override
public void writeRaw(String text) throws IOException {
_writeContext.writeValue();
_emit(_scalarEvent("", DumperOptions.ScalarStyle.PLAIN));
}
}
public class YamlObjectMapper extends ObjectMapper {
public YamlObjectMapper(YAMLFactory jf) {
super(jf);
final DefaultSerializerProvider.Impl ds = new DefaultSerializerProvider.Impl();
ds.setNullValueSerializer(new NullValueSerializer());
setSerializerProvider(ds);
}
}
I had a similar problem but using an enum (or object) and not a string directly you can implement a serializer and decide to add or remove the """ before to serialize the information.
For example
public class Data {
private String aString;
private MyData aNullObject;
private String anEmptyString;
...
}
public class MyData {
private String value;
....
}
public class MySerializer {
#Override
public void serialize(
MyData data, JsonGenerator jgen, SerializerProvider provider)
throws IOException {
YAMLGenerator yamlGenerator = (YAMLGenerator) jgen;
//DISABLE QUOTES
yamlGenerator.enable(YAMLGenerator.Feature.MINIMIZE_QUOTES);
yamlGenerator.writeString(data.getValue());
//ENABLE QUOTES AGAIN
yamlGenerator.disable(YAMLGenerator.Feature.MINIMIZE_QUOTES);
}
}
ObjectMapper mapper = new ObjectMapper(new YAMLFactory());
SimpleModule module = new SimpleModule();
module.addSerializer(MyData.class, new MySerializer());
mapper.registerModule(module);
I hope to explain correctly...
I'm using Jackson XML 2.8.9 and unfortunately I cannot find any way to serialize empty/null collections as empty nodes.
Method responsible for serializing to XML:
protected byte[] toXml(final Collection<ReportView> reports) throws IOException
{
final XmlMapper mapper = new XmlMapper();
// place for code which will solve my problem
return mapper.writerWithDefaultPrettyPrinter().withRootName("report").writeValueAsBytes(reports);
}
I tried to use:
serialization inclusion:
mapper.setSerializationInclusion(JsonInclude.Include.ALWAYS);
serialization provider:
final XmlSerializerProvider provider = new XmlSerializerProvider(new XmlRootNameLookup());
provider.setNullValueSerializer(new JsonSerializer<Object>()
{
#Override
public void serialize(final Object value, final JsonGenerator jgen, final SerializerProvider provider) throws IOException
{
jgen.writeString("");
}
});
mapper.setSerializerProvider(provider);
Jackson 2.9.0 EMPTY_ELEMENT_AS_NULL feature:
mapper.configure(FromXmlParser.Feature.EMPTY_ELEMENT_AS_NULL, false);
Unfortunately nothing works. Does anybody know how to achieve it?
Test method:
#Test
public void testToXml() throws IOException
{
final Map<String, Object> payload = new LinkedHashMap<>();
payload.put("amp", "&");
payload.put("empty", Collections.emptyList());
final Date date = new Date();
final ReportView reportView = new ReportView(payload, date, "system");
// when
final byte[] xmlBytes = reportService.toXml(Arrays.asList(reportView));
// then
final StringBuilder expected = new StringBuilder();
expected.append("<report>");
expected.append(" <item>");
expected.append(" <payload>");
expected.append(" <amp>&</amp>");
expected.append(" <empty></empty>");
expected.append(" </payload>");
expected.append(" <timestamp>" + date.getTime() + "</timestamp>");
expected.append(" <changingUser>system</changingUser>");
expected.append(" </item>");
expected.append("</report>");
final String xmlText = new String(xmlBytes).replace("\n", "").replace("\r", "");
assertThat(xmlText).isEqualTo(expected.toString());
}
ReportView class:
public class ReportView {
private final Map<String, Object> payload;
private final Date timestamp;
private final String changingUser;
public ReportView(Map<String, Object> payload, Date timestamp, String changingUser) {
this.payload = payload;
this.timestamp= timestamp;
this.changingUser = changingUser;
}
public String getChangingUser() {
return changingUser;
}
public Date getTimestamp() {
return timestamp;
}
public Map<String, Object> getPayload() {
return payload;
}
}
I prepared a repository with example code: https://github.com/agabrys/bugs-reports/tree/master/jackson-xml/empty-elements-serialization
EDIT:
I extended the test toXml method and did some code cleanup.
I also tried to create a solution based on Module and SerializerModifier. Unfortunately both ended with failure. I created an issue in jackson-dataformat-xml backlog:
NPE after overriding map serializer with custom implementation (XmlBeanSerializerModifier.modifyMapSerializer)
EDIT:
I've got a hint how to solve problem with exception (see NPE after overriding map serializer with custom implementation (XmlBeanSerializerModifier.modifyMapSerializer)) but still it does not solve problem with missing empty/null values.
I needed to tackle the same issue, and here's how I got it working:
First I create a serializer that serializes nulls as empty string:
public class NullAsEmptyStringSerializer extends JsonSerializer<Object> {
static final JsonSerializer<Object> INSTANCE = new NullAsEmptyStringSerializer();
private static final String EMPTY_STRING = "";
private final StringSerializer stringSerializer = new StringSerializer();
#Override
public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers)
throws IOException {
stringSerializer.serialize(EMPTY_STRING, gen, serializers);
}
}
Then I create a serializer modifier, that overwrites the null serializer of the bean properties with my new serializer:
public class NullToEmptyPropertySerializerModifier extends BeanSerializerModifier {
#Override
public List<BeanPropertyWriter> changeProperties(SerializationConfig config,
BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
for (BeanPropertyWriter beanProperty : beanProperties) {
beanProperty.assignNullSerializer(NullAsEmptyStringSerializer.INSTANCE);
}
return beanProperties;
}
}
And finally, I configure the xml mapper to use my modifier:
NullToEmptyPropertySerializerModifier modifier = new NullToEmptyPropertySerializerModifier();
SerializerFactory serializerFactory = BeanSerializerFactory.instance.withSerializerModifier(modifier);
XmlMapper xmlMapper = new XmlMapper();
xmlMapper.setSerializerFactory(serializerFactory);
Trying to see if it's working for strings and objects (Person and Dog are dummy data holder objects):
Dog dog = new Dog("bobby");
Person person = new Person("utku", null, 29, null);
String serialized = xmlMapper.writeValueAsString(person);
System.out.println(serialized);
Gives the following output:
<Person><name>utku</name><address></address><age>29</age><dog></dog></Person>
Now i'm working with Jackson and i have some questions about it.
First of all. I have two services, first is data collecting and sending service and second receive this data and, for example, log it into a file.
So, first service has class hierarchy like this:
+----ConcreteC
|
Base ----+----ConcreteA
|
+----ConcreteB
And second service has class hierarchy like this:
ConcreteAAdapter extends ConcreteA implements Adapter {}
ConcreteBAdapter extends ConcreteB implements Adapter {}
ConcreteCAdapter extends ConcreteC implements Adapter {}
The first service knows nothing about ConcreteXAdapter.
The way i'm sending the data on the first service:
Collection<Base> data = new LinkedBlockingQueue<Base>()
JacksonUtils utils = new JacksonUtils();
data.add(new ConcreteA());
data.add(new ConcreteB());
data.add(new ConcreteC());
...
send(utils.marshall(data));
...
public class JacksonUtils {
public byte[] marshall(Collection<Base> data) throws IOException {
ByteArrayOutputStream out = new ByteArrayOutputStream() {
#Override
public byte[] toByteArray() {
return buf;
}
};
getObjectMapper().writeValue(out, data);
return out.toByteArray();
}
protected ObjectMapper getObjectMapper() {
return new ObjectMapper();
}
public Object unmarshall(byte[] json) throws IOException {
return getObjectMapper().readValue(json, Object.class);
}
public <T> T unmarshall(InputStream source, TypeReference<T> typeReference) throws IOException {
return getObjectMapper().readValue(source, typeReference);
}
public <T> T unmarshall(byte[] json, TypeReference<T> typeReference) throws IOException {
return getObjectMapper().readValue(json, typeReference);
}
}
So, i want to desirialize json into Collection of ConcreteXAdapter, not into Collection of ConcreteX (ConcreteA -> ConcreteAAdapter, ConcreteB -> ConcreteBAdapter, ConcreteC -> ConcreteCAdapter). In the case i described i want to get:
Collection [ConcreteAAdapter, ConcreteBAdapter, ConcreteCAdapter]
How can i do this?
For this purpose you need to pass additional info in JSON:
#JsonTypeInfo(use=JsonTypeInfo.Id.NAME,
include=JsonTypeInfo.As.PROPERTY, property="#type")
class Base {
...
}
Then on serialization it will add #type field:
objectMapper.registerSubtypes(
new NamedType(ConcreteAAdapter.class, "ConcreteA"),
new NamedType(ConcreteBAdapter.class, "ConcreteB"),
new NamedType(ConcreteCAdapter.class, "ConcreteC")
);
// note, that for lists you need to pass TypeReference explicitly
objectMapper.writerWithType(new TypeReference<List<Base>>() {})
.writeValueAsString(someList);
{
"#type" : "ConcreteA",
...
}
on deserialization it will be:
objectMapper.registerSubtypes(
new NamedType(ConcreteA.class, "ConcreteA"),
new NamedType(ConcreteB.class, "ConcreteB"),
new NamedType(ConcreteC.class, "ConcreteC")
);
objectMapper.readValue(....)
More info here
How I solved this problem. Here is a class diagram for an example project:
So i want to get the ConcreteAAdapter form ConcreteA after deserialization.
My solution is to extend ClassNameIdResolver to add functionality to deserialize base class objects into subtype class objects (subtype classes adds no extra functionality and additional fields).
Here is a code which creates ObjectMapper for deserialization:
protected ObjectMapper getObjectMapperForDeserialization() {
ObjectMapper mapper = new ObjectMapper();
StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()) {
private HashMap<Class, Class> classes = new HashMap<Class, Class>() {
{
put(ConcreteA.class, ConcreteAAdapter.class);
put(ConcreteB.class, ConcreteBAdapter.class);
put(ConcreteC.class, ConcreteCAdapter.class);
}
};
#Override
public String idFromValue(Object value) {
return (classes.containsKey(value.getClass())) ? value.getClass().getName() : null;
}
#Override
public JavaType typeFromId(String id) {
try {
return classes.get(Class.forName(id)) == null ? super.typeFromId(id) : _typeFactory.constructSpecializedType(_baseType, classes.get(Class.forName(id)));
} catch (ClassNotFoundException e) {
// todo catch the e
}
return super.typeFromId(id);
}
});
mapper.setDefaultTyping(typeResolverBuilder);
return mapper;
}
And here is a code which create ObjectMapper for serialization:
protected ObjectMapper getObjectMapperForSerialization() {
ObjectMapper mapper = new ObjectMapper();
StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()));
mapper.setDefaultTyping(typeResolverBuilder);
return mapper;
}
Test code:
public static void main(String[] args) throws IOException {
JacksonUtils JacksonUtils = new JacksonUtilsImpl();
Collection<Base> data = new LinkedBlockingQueue<Base>();
data.add(new ConcreteA());
data.add(new ConcreteB());
data.add(new ConcreteC());
String json = JacksonUtils.marshallIntoString(data);
System.out.println(json);
Collection<? extends Adapter> adapters = JacksonUtils.unmarshall(json, new TypeReference<ArrayList<Adapter>>() {});
for (Adapter adapter : adapters) {
System.out.println(adapter.getClass().getName());
}
}
Full code of JacksonUtils class:
public class JacksonUtilsImpl implements JacksonUtils {
#Override
public byte[] marshall(Collection<Base> data) throws IOException {
ByteArrayOutputStream out = new ByteArrayOutputStream() {
#Override
public byte[] toByteArray() {
return buf;
}
};
getObjectMapperForSerialization().writerWithType(new TypeReference<Collection<Base>>() {}).writeValue(out, data);
return out.toByteArray();
}
#Override
public String marshallIntoString(Collection<Base> data) throws IOException {
return getObjectMapperForSerialization().writeValueAsString(data);
}
protected ObjectMapper getObjectMapperForSerialization() {
ObjectMapper mapper = new ObjectMapper();
StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()));
mapper.setDefaultTyping(typeResolverBuilder);
return mapper;
}
protected ObjectMapper getObjectMapperForDeserialization() {
ObjectMapper mapper = new ObjectMapper();
StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()) {
private HashMap<Class, Class> classes = new HashMap<Class, Class>() {
{
put(ConcreteA.class, ConcreteAAdapter.class);
put(ConcreteB.class, ConcreteBAdapter.class);
put(ConcreteC.class, ConcreteCAdapter.class);
}
};
#Override
public String idFromValue(Object value) {
return (classes.containsKey(value.getClass())) ? value.getClass().getName() : null;
}
#Override
public JavaType typeFromId(String id) {
try {
return classes.get(Class.forName(id)) == null ? super.typeFromId(id) : _typeFactory.constructSpecializedType(_baseType, classes.get(Class.forName(id)));
} catch (ClassNotFoundException e) {
// todo catch the e
}
return super.typeFromId(id);
}
});
mapper.setDefaultTyping(typeResolverBuilder);
return mapper;
}
#Override
public Object unmarshall(byte[] json) throws IOException {
return getObjectMapperForDeserialization().readValue(json, Object.class);
}
#Override
public <T> T unmarshall(InputStream source, TypeReference<T> typeReference) throws IOException {
return getObjectMapperForDeserialization().readValue(source, typeReference);
}
#Override
public <T> T unmarshall(byte[] json, TypeReference<T> typeReference) throws IOException {
return getObjectMapperForDeserialization().readValue(json, typeReference);
}
#Override
public <T> Collection<? extends T> unmarshall(String json, Class<? extends Collection<? extends T>> klass) throws IOException {
return getObjectMapperForDeserialization().readValue(json, klass);
}
#Override
public <T> Collection<? extends T> unmarshall(String json, TypeReference typeReference) throws IOException {
return getObjectMapperForDeserialization().readValue(json, typeReference);
}
}
I find programmerbruce's approach to be the most clear and easy to get working (example below).
I got the information from his answer to a related question:
https://stackoverflow.com/a/6339600/1148030
and the related blog post:
http://programmerbruce.blogspot.fi/2011/05/deserialize-json-with-jackson-into.html
Also check out this friendly wiki page (also mentioned in Eugene Retunsky's answer):
https://github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization
Another nice wiki page: https://github.com/FasterXML/jackson-docs/wiki/JacksonMixInAnnotations
Here is a short example to give you the idea:
Configure the ObjectMapper like this:
mapper.getDeserializationConfig().addMixInAnnotations(Base.class, BaseMixin.class);
mapper.getSerializationConfig().addMixInAnnotations(Base.class, BaseMixin.class);
Example BaseMixin class (easy to define as an inner class.)
#JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include=JsonTypeInfo.As.PROPERTY, property="type")
#JsonSubTypes({
#JsonSubTypes.Type(value=ConcreteA.class, name="ConcreteA"),
#JsonSubTypes.Type(value=ConcreteB.class, name="ConcreteB")
})
private static class BaseMixin {
}
On second service you could define the BaseMixin like this:
#JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include=JsonTypeInfo.As.PROPERTY, property="type")
#JsonSubTypes({
#JsonSubTypes.Type(value=ConcreteAAdapter.class, name="ConcreteA"),
#JsonSubTypes.Type(value=ConcreteBAdapter.class, name="ConcreteB")
})
private static class BaseMixin {
}
I have a Map<A,B> fieldOfC as a field of a class C. When I try to deserialize C with Jackson, an Exception is thrown because it can't find a Deserializer for Map's key A. So, I guess the solution is to extend StdJsonDeserializer and do it manually.
My problem is that I can't find an example on how to use the parser and the context of the method "deserialize" that I have to implement.
Can anyone write the code for this simple example so I can use it as a start to build my real deserializer?
public class A{
private String a1;
private Integer a2;
}
public class B{
private String b1;
}
public class C{
#JsonDeserialize(keyUsing=ADeserializer.class)
//also tried this: #JsonDeserialize(keyAs=A.class) without success
private Map<A,B> fieldOfC;
private String c1;
}
public class ADeserializer extends StdKeyDeserializer {
protected ADeserializer(Class<A> cls) {
super(cls);
}
protected Object _parse(String key, DeserializationContext ctxt) throws Exception {
ObjectMapper mapper = new ObjectMapper();
return mapper.readValue(key, A.class);
}
}
Thanks in advance
EDIT: googling, I found a test of the same problem I have. This is exactly my problem
EDIT: changed extended class from StdDeserializer to StdKeyDeserializer as I read here in method findKeyDeserializer(org.codehaus.jackson.map.DeserializationConfig, org.codehaus.jackson.type.JavaType, org.codehaus.jackson.map.BeanProperty)
EDIT: After solving this issue I got this one that is related.
I am a complete newbie with Jackson, but the following works for me.
First I add a JsonCreator method to A:
public class A {
private String a1;
private Integer a2;
public String getA1() { return a1; }
public Integer getA2() { return a2; }
public void setA1(String a1) { this.a1 = a1; }
public void setA2(Integer a2) { this.a2 = a2; }
#JsonCreator
public static A fromJSON(String val) throws JsonParseException, JsonMappingException, IOException {
ObjectMapper mapper = new ObjectMapper();
A a = mapper.readValue(val,A.class);
return a;
}
}
That alone solves the deserialization problem. The harder part for me was the correct serialization of the keys. What I did there was to define a key serializer that serializes named classes as there JSON serialization, like this:
public class KeySerializer extends SerializerBase<Object> {
private static final SerializerBase<Object> DEFAULT = new StdKeySerializer();
private Set<Class<?>> objectKeys_ = Collections.synchronizedSet(new HashSet<Class<?>>());
protected KeySerializer(Class<?>... objectKeys) {
super(Object.class);
for(Class<?> cl:objectKeys) {
objectKeys_.add(cl);
}
}
#Override
public JsonNode getSchema(SerializerProvider provider, Type typeHint) throws JsonMappingException {
return DEFAULT.getSchema(provider, typeHint);
}
#Override
public void serialize(Object value, JsonGenerator jgen,
SerializerProvider provider) throws IOException,
JsonGenerationException {
if (objectKeys_.contains(value.getClass())) {
ObjectMapper mapper = new ObjectMapper();
StringWriter writer = new StringWriter();
mapper.writeValue(writer, value);
jgen.writeFieldName(writer.toString());
} else {
DEFAULT.serialize(value, jgen, provider);
}
}
}
Then to prove it works, serializing and deserializing an instance of class C:
ObjectMapper mapper = new ObjectMapper();
StdSerializerProvider provider = new StdSerializerProvider();
provider.setKeySerializer(new KeySerializer(A.class));
mapper.setSerializerProvider(provider);
StringWriter out = new StringWriter();
mapper.writeValue(out, c);
String json = out.toString();
System.out.println("JSON= "+json);
C c2 = mapper.readValue(json, C.class);
System.out.print("C2= ");
StringWriter outC2 = new StringWriter();
mapper.writeValue(outC2, c2);
System.out.println(outC2.toString());
For me this produced the output:
JSON= {"c1":"goo","map":{"{\"a1\":\"1ccf\",\"a2\":7376}":{"b1":"5ox"},"{\"a1\":\"1cd2\",\"a2\":7379}":{"b1":"5p0"},"{\"a1\":\"1cd5\",\"a2\":7382}":{"b1":"5p3"},"{\"a1\":\"1cd8\",\"a2\":7385}":{"b1":"5p6"}}}
C2= {"c1":"goo","map":{"{\"a1\":\"1ccf\",\"a2\":7376}":{"b1":"5ox"},"{\"a1\":\"1cd2\",\"a2\":7379}":{"b1":"5p0"},"{\"a1\":\"1cd5\",\"a2\":7382}":{"b1":"5p3"},"{\"a1\":\"1cd8\",\"a2\":7385}":{"b1":"5p6"}}}
I feel there ought to have been a better way of doing saying how to serialize the key by using annotations, but I could not work it out.