Jackson: Unrecognized field, not marked as ignorable

What the error means

The JSON has a property that matches no field, setter or constructor parameter in the target class, and the mapper is set to fail on unknown properties. Jackson 2 fails by default. Jackson 3 changed the default of DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES to false, and Spring's Jackson2ObjectMapperBuilder also turns it off, so the error mostly comes from a plain Jackson 2 ObjectMapper or a mapper that enables the check on purpose. Decide whether the key is a field you should map, a naming mismatch, or data you can ignore.

Exact messages by runtime

Messages depend on the input and runtime version. Node and Python rows were reproduced locally. Other rows cite official documentation or source. Rows marked as fragments or templates preserve the source wording and are not complete exception transcripts. Browsers and libraries may append location details.

Causes you can reproduce

record User(String name, String email) {}

new ObjectMapper().readValue(
    "{\"name\":\"Ada\",\"email\":\"ada@example.com\",\"fullName\":\"Ada Lovelace\"}", User.class);
// Unrecognized field "fullName" (class User), not marked as ignorable

record Person(String firstName) {}

new ObjectMapper().readValue("{\"first_name\":\"Ada\"}", Person.class);
// Unrecognized field "first_name" (class Person), not marked as ignorable

record Repo(String name) {}

new ObjectMapper().readValue("{\"name\":\"tools\",\"stars\":12}", Repo.class);
// Unrecognized field "stars" (class Repo), not marked as ignorable

How to fix it

1. Read the field name and the class. The known-properties list in the message shows what Jackson can map.
2. Compare it with your class: a new field to add, a naming mismatch to map, or extra data to ignore.
3. Prefer explicit mapping with @JsonProperty. Ignore unknown properties per class with @JsonIgnoreProperties(ignoreUnknown = true) when the class reads only part of an external document.
4. When you move to Jackson 3, unknown properties are ignored by default. Enable FAIL_ON_UNKNOWN_PROPERTIES where you rely on strict checking.

record Person(@JsonProperty("first_name") String firstName, String email) {}

@JsonIgnoreProperties(ignoreUnknown = true)
record Repo(String name) {}

ObjectMapper mapper = JsonMapper.builder()
    .propertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE)
    .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
    .build();

Compare the JSON shape with your types

1. Copy the exact JSON your code received, for example from a log or a failing test.
2. Paste it into the Tool Kumar JSON viewer. The document stays in your browser.
3. Open the tree at the property named in the error and check whether the value there is an object, an array or a single value. The JSON path finder copies the path to any value.
4. Generate types from the same document with a converter listed under related tools, then compare them with your own classes or structs.

Frequently asked questions

Related errors and tools

• Jackson: Cannot deserialize value of type from Array or Object value
• Jackson: Unrecognized token / Unexpected character ('<' (code 60))
• Go JSON: cannot unmarshal string, object or array into Go value
• All JSON error guides
• JSON syntax reference
• Generate Java records with @JsonProperty mappings from the JSON

Standards and diagnostic sources

RFC 8259 and ECMA-404 define JSON syntax. Diagnostic wording belongs to the parser. The table links each cited message to its source.

• Jackson 3 source: FAIL_ON_UNKNOWN_PROPERTIES defaults to false
• Spring source: Jackson2ObjectMapperBuilder disables FAIL_ON_UNKNOWN_PROPERTIES