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Kafka's JsonDeserializer not working for java.util.Map

By : guest
Date : October 16 2020, 06:10 PM
With these it helps My strong suspicion would be that it is the polymorphic nature of your value rather than Maps per-se.
Spring is using Jackson underneath the hood for it's serialisation/deserialisation - and by default Jackson (in serialisation) when handling Object instances does not encode what class it is serialising.
code :
public class JacksonConfiguration {

 public ObjectMapper objectMapper() {
    ObjectMapper mapper = new ObjectMapper();
    #Your Configuration Here for example  mapper.configure(DeserializationFeature.something, true);

    return mapper;

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how to create a custom JsonDeserializer in Java?

By : Paddy
Date : March 29 2020, 07:55 AM
this will help I am a complete newbie with Jackson, but the following works for me.
First I add a JsonCreator method to A:
code :
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; }

    public static A fromJSON(String val) throws JsonParseException, JsonMappingException, IOException {
        ObjectMapper mapper = new ObjectMapper();
        A a = mapper.readValue(val,A.class);
        return a;
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) {
        for(Class<?> cl:objectKeys) {

    public JsonNode getSchema(SerializerProvider provider, Type typeHint) throws JsonMappingException {
        return DEFAULT.getSchema(provider, typeHint);

    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);
        } else {
            DEFAULT.serialize(value, jgen, provider);
    ObjectMapper mapper = new ObjectMapper();
    StdSerializerProvider provider = new StdSerializerProvider();
    provider.setKeySerializer(new KeySerializer(A.class));

    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);
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"}}}

Calling custom JsonDeserializer from another JsonDeserializer

By : Olaf Johannes
Date : March 29 2020, 07:55 AM
Does that help In general, when writing a custom deserializer you need to make sure anything you can serialize with the ObjectMapper instead of actually parsing every field by itself. Once you do that, the ObjectMapper will automatically call the deserializer of an object if exists.
And the code:
code :
@JsonDeserialize(using = BobDeserializer.class)
class Bob {

    private Integer number;

    public Bob() {

    public Integer getNumber() {
        return number;

    public void setNumber(Integer number) {
        this.number = number;

@JsonDeserialize(using = AliceDeserializer.class)
public class Alice {
    private Bob bob;

    public Alice() {

    public Bob getBob() {
        return bob;

    public void setBob(Bob bob) {
        this.bob = bob;

class BobDeserializer extends StdDeserializer<Bob> {

    protected BobDeserializer() {

    public Bob deserialize(JsonParser parser, DeserializationContext context) throws IOException {
        JsonNode node = parser.getCodec().readTree(parser);

        Integer number = node.get("number").asInt();

        Bob bob = new Bob();

        return bob;

class AliceDeserializer extends StdDeserializer<Alice> {

    protected AliceDeserializer() {

    public Alice deserialize(JsonParser parser, DeserializationContext context) throws IOException {
        JsonNode node = parser.getCodec().readTree(parser);

        ObjectMapper mapper = new ObjectMapper();
        Bob bob = mapper.readValue(node.get("bob").toString(), Bob.class);

        Alice alice = new Alice();

        return alice;
Alice alice = new Alice();
Bob bob = new Bob();

ObjectMapper mapper = new ObjectMapper();

Alice alice1 = mapper.readValue(mapper.writeValueAsString(alice), Alice.class);

Spring cloud stream: Two different Kafkas in same app

By : Segrest
Date : March 29 2020, 07:55 AM
this will help You question is too general so I'd suggest to start by looking at Multibinder samples which include several variations including Kafka. There is more information and details on multiple binder support in reference manual.

I am seeing the error: A message body writer for Java class java.util.ArrayList, and Java type java.util.List<java.la

By : Jitendra Kumar
Date : March 29 2020, 07:55 AM
I wish did fix the issue. I see the JSON providers in the error message, so it seems you have the jersey-json dependency. But it seems (from what I tested) that the internal Jersey providers don't know how to handle lists of scalar values. For that you would need to make sure that Jackson is enabled. You can do that simply configuring the POJOMappingFeature in your web.xml

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.Iterator java.util.ArrayList

By : Konstantin Fedorov
Date : March 29 2020, 07:55 AM
help you fix your problem It looks like String s parameter is null, use logging to confirm. In such case Gson.fromJson() returns null as a result. Then you're trying to iterate through the list postList which is null and this results to NullPointerException.
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