spring-boot4-guide

name	spring-boot4-guide
description	Guide for developing with Spring Boot 4 and Spring Framework 7. Use this skill whenever the user is working on a Spring Boot 4 project, asking about Spring Boot 4 or Spring Framework 7 changes, migrating from Boot 3 to 4, or encountering issues related to Boot 4 breaking changes. Trigger on "Spring Boot 4", "Spring 7", "Boot 4 migration", "Spring Boot 4 변경사항", "Boot 4로 업그레이드", "Spring Boot 4 프로젝트", "spring-boot 4.x", "Spring Framework 7 변경", "Boot 4 에러", "Jackson 3 마이그레이션". Also trigger when the user creates a new Spring Boot project and the version is 4.x, or when build errors suggest Boot 4 compatibility issues (e.g., missing starters, Jakarta EE 11 issues, Jackson 3 package errors, HttpHeaders API changes). Do NOT trigger for Spring Boot 3.x or earlier questions that don't involve upgrading.
version	0.1.0

Spring Boot 4 / Spring Framework 7 Guide

Spring Boot 4 (November 2025) is built on Spring Framework 7 and introduces significant structural changes. This skill provides the essential knowledge for developing with or migrating to Boot 4.

Baseline Requirements

Dependency	Minimum	Recommended
Java	17	25 (LTS)
Jakarta EE	11	—
Servlet	6.1 (Tomcat 11+, Jetty 12.1+)	—
JPA	3.2 (Hibernate 7.1/7.2)	—
Jackson	3.x (2.x deprecated)	—
Kotlin	2.2+	—
GraalVM	25+	—
JUnit	6 (JUnit 4 deprecated)	—

Critical Breaking Changes

These are the changes most likely to cause immediate build or runtime failures.

1. Modularization — Starter POM overhaul

Boot 4's biggest structural change: the monolithic spring-boot-autoconfigure has been split into technology-specific modules. Starter POMs are renamed and restructured.

Immediate impact:

<!-- BEFORE (Boot 3) -->
<artifactId>spring-boot-starter-web</artifactId>

<!-- AFTER (Boot 4) -->
<artifactId>spring-boot-starter-webmvc</artifactId>

Key renames:

Boot 3	Boot 4
spring-boot-starter-web	spring-boot-starter-webmvc
spring-boot-starter-aop	spring-boot-starter-aspectj
spring-boot-starter-oauth2-client	spring-boot-starter-security-oauth2-client
spring-boot-starter-oauth2-resource-server	spring-boot-starter-security-oauth2-resource-server

Technologies that previously only needed a 3rd-party dependency (e.g., Flyway, Liquibase) now require a dedicated starter:

• spring-boot-starter-flyway
• spring-boot-starter-liquibase

Test starters are also separated: spring-boot-starter-<tech>-test. For example, @WithMockUser requires spring-boot-starter-security-test.

Quick migration path: Use spring-boot-starter-classic / spring-boot-starter-test-classic temporarily, then migrate incrementally.

For the full starter mapping table, see references/modularization.md.

2. Jackson 3.x

Boot 4 defaults to Jackson 3. The package name changed from com.fasterxml.jackson to tools.jackson (except annotations which stay in com.fasterxml.jackson.annotation).

Key changes:

• Jackson2ObjectMapperBuilder removed → use JsonMapper.builder()
• @JsonComponent → @JacksonComponent
• @JsonMixin → @JacksonMixin
• Properties: spring.jackson.read.* → spring.jackson.json.read.*

For Jackson 2 compatibility during migration, add spring-boot-jackson2 module and use spring.jackson.use-jackson2-defaults=true.

For detailed migration steps, see references/jackson3-migration.md.

3. javax.* completely removed

javax.annotation and javax.inject annotations are no longer supported. All code must use jakarta.annotation / jakarta.inject equivalents. This is not new from Jakarta EE 9 migration, but Spring 7 removes any remaining fallback support.

4. HttpHeaders no longer extends MultiValueMap

This is a subtle but widespread breaking change. Code that treats HttpHeaders as a MultiValueMap will fail. Use HttpHeaders#asMultiValueMap() as a temporary bridge (itself deprecated).

5. Null Safety — JSpecify

Spring's own @Nullable / @NonNull annotations (org.springframework.lang) are deprecated in favor of JSpecify (org.jspecify.annotations). Kotlin projects may see compilation errors from changed nullability.

6. Undertow removed

Undertow doesn't support Servlet 6.1 yet. Use Tomcat 11+ or Jetty 12.1+.

7. RestTemplate deprecated

RestTemplate is officially deprecated. Use RestClient (introduced in 6.1) for imperative HTTP calls.

New Features Worth Knowing

API Versioning (built-in)

Spring MVC and WebFlux now support API versioning natively:

@GetMapping("/users")
@ApiVersion("1")
public List<UserV1> getUsersV1() { ... }

@GetMapping("/users")
@ApiVersion("2")
public List<UserV2> getUsersV2() { ... }

Configurable version resolution (header, path, query param), deprecation marking, and client-side support in RestClient/WebClient.

Resilience — Built-in Retry & Concurrency

Spring Retry has been absorbed into Spring Framework core:

@EnableResilientMethods
@Configuration
public class AppConfig { }

@Retryable(maxAttempts = 3)
public String callApi() { ... }

@ConcurrencyLimit(limit = 10)
public String heavy() { ... }

Works with reactive methods too (auto-adapts to Reactor retry).

RestTestClient — Non-reactive test client

The community-requested replacement for WebTestClient that doesn't require reactive dependencies:

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
@AutoConfigureRestTestClient
class MyTest {
    @Autowired
    RestTestClient restTestClient;
}

Programmatic Bean Registration

New BeanRegistrar contract for flexible, multi-bean registration beyond @Bean methods:

public class MyRegistrar implements BeanRegistrar {
    @Override
    public void register(BeanRegistry registry, Environment env) {
        registry.registerBean("myService", MyService.class);
    }
}

@ImportHttpServices — Simplified HTTP client config

@Configuration
@ImportHttpServices(group = "weather", types = {WeatherApi.class})
static class Config extends AbstractHttpServiceRegistrar { }

JmsClient

Fluent JMS client following the JdbcClient/RestClient pattern.

JPA 3.2 Improvements

• EntityManager / EntityManagerFactory directly injectable via @Autowired
• Hibernate StatelessSession DI support
• PersistenceConfiguration for XML-free JPA setup

For comprehensive feature details, see references/new-features.md.

Testing Changes

These changes will affect almost every test class:

Change	Action Required
@SpringBootTest no longer auto-configures MockMVC	Add @AutoConfigureMockMvc
@SpringBootTest no longer provides TestRestTemplate	Add @AutoConfigureTestRestTemplate
@MockBean / @SpyBean	Deprecated → use @MockitoBean / @MockitoSpyBean
JUnit 4 support	Deprecated → use JUnit Jupiter SpringExtension
@Mock / @Captor fields	Require MockitoExtension from Mockito directly

For full testing migration details, see references/testing-changes.md.

Migration Strategy

1. Upgrade to latest 3.5.x first — fix all deprecation warnings
2. Switch to Boot 4 with spring-boot-starter-classic for quick compatibility
3. Fix javax.* → jakarta.* if any remain
4. Update starter POMs to new names
5. Migrate Jackson 2 → 3 (or use spring-boot-jackson2 temporarily)
6. Update test annotations (@AutoConfigureMockMvc, @MockitoBean, etc.)
7. Replace spring-boot-starter-classic with individual technology starters
8. Adopt JSpecify nullability annotations
9. Replace RestTemplate with RestClient

Property Changes

Some commonly used properties have been renamed:

Boot 3	Boot 4
spring.data.mongodb.* (some)	spring.mongodb.*
spring.session.redis.*	spring.session.data.redis.*
spring.jackson.read.*	spring.jackson.json.read.*
spring.jackson.write.*	spring.jackson.json.write.*
spring.dao.exceptiontranslation.enabled	spring.persistence.exceptiontranslation.enabled

Package Changes

Boot 3	Boot 4
org.springframework.boot.env.EnvironmentPostProcessor	org.springframework.boot.EnvironmentPostProcessor
org.springframework.boot.BootstrapRegistry	org.springframework.boot.bootstrap.BootstrapRegistry
org.springframework.boot.autoconfigure.orm.jpa.EntityScan (concept)	org.springframework.boot.persistence.autoconfigure.EntityScan
org.springframework.boot.test.autoconfigure.properties.PropertyMapping	org.springframework.boot.test.context.PropertyMapping

Reference Files

• references/modularization.md — Full starter/module mapping tables and migration strategy
• references/jackson3-migration.md — Jackson 2→3 detailed migration guide
• references/new-features.md — All new features with examples
• references/testing-changes.md — Test code migration details