Spring Boot 4 Switches Default JSON Library from Jackson 2 to Jackson 3 – What Changes to Expect

Spring Boot 4 switches its default JSON library from Jackson 2 to Jackson 3. The dependency tree looks like:

spring-boot-starter-jackson (4.x)
├── tools.jackson.core:jackson-core:3.x   ← core library
└── com.fasterxml.jackson.annotation:jackson-annotations:2.x ← annotations

Both Jackson 2 and Jackson 3 artifacts appear because the Jackson team kept annotation packages unchanged to allow a gradual migration.

Key migration changes

1. Package name change

Core classes moved from com.fasterxml.jackson to tools.jackson. Example:

// Jackson 2
import com.fasterxml.jackson.databind.ObjectMapper;
// Jackson 3
import tools.jackson.databind.ObjectMapper;

Only core APIs change packages; annotation classes remain under com.fasterxml.jackson.annotation.

2. ObjectMapper → JsonMapper

Jackson 2’s ObjectMapper is mutable and not thread‑safe after configuration:

ObjectMapper mapper = new ObjectMapper();
mapper.enable(SerializationFeature.INDENT_OUTPUT);

Jackson 3 introduces an immutable builder pattern:

JsonMapper mapper = JsonMapper.builder()
    .enable(SerializationFeature.INDENT_OUTPUT)
    .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
    .build();

After build() the configuration is locked, the instance can be safely shared, and it is thread‑safe.

3. Default date serialization

Jackson 2 serializes dates as timestamps, while Jackson 3 uses ISO‑8601 strings:

// Jackson 2
{"nowDate":1767588151648}
// Jackson 3
{"nowDate":"2026-01-05T02:02:31.648Z"}

This may break tests or API contracts that expect timestamps. A temporary compatibility setting is:

spring:
  jackson:
    use-jackson2-defaults: true

4. Checked exceptions removed

Jackson 2 throws checked IOException for parsing errors:

try {
    objectMapper.readValue(json, MyClass.class);
} catch (IOException e) {
    // handle
}

Jackson 3 consolidates all errors under the unchecked JacksonException (a subclass of RuntimeException), allowing cleaner code:

jsonMapper.readValue(json, MyClass.class);

Consequently, stream operations that previously failed with lambda expressions now work:

// Jackson 2 – lambda may crash
list.stream()
    .map(o -> objectMapper.writeValueAsString(o))
    .toList();
// Jackson 3 – works fine
list.stream()
    .map(o -> jsonMapper.writeValueAsString(o))
    .toList();

These changes modernize JSON handling in Spring Boot 4, providing immutable, thread‑safe mappers, more developer‑friendly date formats, and a simplified exception model.