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deserialize to static List<T> field get double repeat value

See original GitHub issue


public class Test {
    private static final ObjectMapper MAPPER = new ObjectMapper();

    static {
        MAPPER.registerModule(new JodaModule());
        MAPPER.registerModule(new JodaTimeModule());
        MAPPER.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        MAPPER.configure(DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT, true);
        MAPPER.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, true);
        MAPPER.configure(com.fasterxml.jackson.databind.SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);

    protected static List<String> list = new ArrayList();

    protected static String str = "str";

    public List<String> getList() {
        return list;

    public String getStr() {
        return str;

    public static void main(String[] args) {
        Test test = new Test();
        String jsonString = MAPPER.writeValueAsString(test);
        Test test1 = MAPPER.readValue(jsonString, Test.class)

Result: {“str”:“str”,“list”:[“1”]} [1, 1] str

I am very confused why “list” value change to double repeat value. if execute “writeValueAsString” and “readValue” multiple times, each will double for the result

Issue Analytics

  • State:closed
  • Created 3 years ago
  • Comments:8 (4 by maintainers)

github_iconTop GitHub Comments

medlyingcommented, Dec 4, 2020

@medlying This looks accidental:

    protected static List<String> list = new ArrayList();

since that does not make much sense. If it is intentional, you may want to read a Java tutorial explaining how static members work. @cowtowncoder my jackson-databind version is 2.10.5, in debug model , to execute MAPPER.readValue(jsonString, Test.class) will first go to method : com.fasterxml.jackson.databind.deser.impl.SetterlessProperty#deserializeAndSet it will do this: toModify = _getter.invoke(instance, (Object[]) null); this step will get the ‘list’ value because in my case contains getter method. the second go to com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer#deserialize(com.fasterxml.jackson.core.JsonParser, com.fasterxml.jackson.databind.DeserializationContext, java.util.Collection<java.lang.String>){ … String value = p.nextTextValue(); if (value != null) { result.add(value); continue; } } then the list will add the text value. so the result is double.So there is no way to avoid this problem except delete getter method?

medlyingcommented, Dec 9, 2020

@cowtowncoder cool!! I think this is what I want, Thanks!!

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