What Surprising Changes Await You When Upgrading to Jackson 3 in Spring Boot 4?

Jackson 3 Breaking Changes in Spring Boot 4

After upgrading to Spring Boot 4, the Jackson libraries are upgraded to version 3 and several breaking changes appear. The following points describe the required code adjustments.

1. Package reorganization

The core implementation classes moved from com.fasterxml to tools.jackson. Annotation classes remain in the old package, so a global replace of com.fasterxml would break annotations.

// Jackson 2 (old)
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

// Jackson 3 (new)
import tools.jackson.core.JacksonException;
import tools.jackson.core.type.TypeReference;
import tools.jackson.databind.ObjectMapper;

2. ObjectMapper replaced by JsonMapper

Jackson 2’s ObjectMapper is mutable; Jackson 3 enforces an immutable, builder‑based API via JsonMapper. The mapper is thread‑safe after build().

// Recommended Jackson 3 usage
JsonMapper mapper = JsonMapper.builder()
    .enable(SerializationFeature.INDENT_OUTPUT)
    .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
    .build();

Instantiating new ObjectMapper() yields a default mutable mapper; custom configuration must use the builder.

3. Date serialization format

Jackson 3 serializes dates as ISO‑8601 strings instead of numeric timestamps.

{"now": 1767588151648}   // Jackson 2 (timestamp)
{"now": "2026-01-05T02:02:31Z"}   // Jackson 3 (ISO‑8601)

If existing code or tests expect a long timestamp, they will fail. A temporary workaround is to enable Jackson 2 defaults in application.yml:

spring:
  jackson:
    use-jackson2-defaults: true   # forces old timestamp handling (not recommended)

4. Checked exceptions removed

Methods such as readValue now throw RuntimeException instead of checked IOException. This removes the need for explicit try‑catch blocks and simplifies Stream/Lambda usage.

// Jackson 3: no need to catch IOException
list.stream()
    .map(o -> jsonMapper.writeValueAsString(o))
    .toList();

Migration checklist

Update import statements for core classes to the tools.jackson namespace.

Replace direct new ObjectMapper() constructions with the JsonMapper.builder() pattern.

Verify date handling in JSON payloads; adjust frontend or tests, or use the temporary use-jackson2-defaults flag.

Remove unnecessary catch IOException clauses.