What Is New in Spring Boot 4

Spring Boot 4.0 (November 2025) is the first new major version since Spring Boot 3 moved the ecosystem to Jakarta EE in 2022. It is built on Spring Framework 7 and brings a modular code base, Jackson 3, null-safety annotations across the portfolio, built-in API versioning, declarative HTTP clients, new test tooling and first-class OpenTelemetry. Spring Boot 4.1 followed with gRPC support and many refinements.

Whether you are starting a new project or maintaining a Boot 3 application, you will meet these changes in documentation, starters and error messages. This lesson summarises what changed, why, and what it means for your code — and gives a practical upgrade checklist.

Platform Baseline

The versions underneath Spring Boot 4 moved forward together:

• Java 17 minimum (Java 21 or 25 recommended for virtual threads and the latest language features).
• Spring Framework 7, Spring Security 7, Spring Data 2025.1, Spring Batch 6, Spring Cloud 2025.1 "Oakwood", Spring AI 2.0.
• Jakarta EE 11 with Servlet 6.1 — Tomcat 11 and Jetty 12.1. Undertow is no longer supported because it does not yet implement Servlet 6.1.
• Hibernate 7, Jackson 3, Micrometer 1.16, Kotlin 2.2, JUnit Jupiter 6 and Testcontainers 2.

Modular Spring Boot and Renamed Starters

The big spring-boot-autoconfigure jar was split into focused modules, one per technology. Starters now map to those modules, which means smaller applications and clearer dependencies. Some starters were renamed: spring-boot-starter-web → spring-boot-starter-webmvc, spring-boot-starter-aop → spring-boot-starter-aspectj, and the OAuth2 starters gained a security- prefix. Flyway and Liquibase now need their own starters, and every technology has a matching test starter such as spring-boot-starter-webmvc-test. The old names still work but are deprecated; spring-boot-starter-classic restores the Boot 3 layout as a temporary migration bridge.

Developer-Facing Features

Several features you will use daily arrived in this generation:

• Jackson 3 — new tools.jackson packages, immutable JsonMapper, unchecked exceptions (see the Jackson lesson).
• API versioning — @GetMapping(version = "2") and spring.mvc.apiversion.*.
• HTTP service clients — @ImportHttpServices plus spring.http.serviceclient.* for declarative interface clients.
• Resilience in core Spring — @Retryable and @ConcurrencyLimit with @EnableResilientMethods, no extra library.
• JSpecify null-safety — @Nullable/@NonNull semantics across Spring APIs, understood by IDEs and Kotlin.
• Testing — RestTestClient, MockMvcTester, @MockitoBean (the old @MockBean is removed).
• Observability — spring-boot-starter-opentelemetry for OTLP metrics and traces.
• Spring Boot 4.1 — gRPC server and client support, InetAddressFilter against SSRF, lazy JDBC connection fetching, Redis listener support and more.

Upgrade Checklist from Spring Boot 3

Upgrade to the latest Spring Boot 3.5.x first and fix all deprecation warnings — Boot 4 removes APIs deprecated in 3.x. Then move to 4.0 and work through: starter renames (or temporarily add the classic starters); Jackson 3 imports and customizers; @MockBean → @MockitoBean; explicit @AutoConfigureMockMvc/@AutoConfigureRestTestClient; Spring Security 7 changes (lambda DSL only, PathPatternRequestMatcher); Hibernate 7 and Undertow removal. The OpenRewrite recipe UpgradeSpringBoot_4_0 automates much of this.

Examples

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>4.1.1</version>
</parent>

<properties>
    <java.version>21</java.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webmvc</artifactId>      <!-- was spring-boot-starter-web -->
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-flyway</artifactId>      <!-- new: needed for Flyway -->
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-security-oauth2-resource-server</artifactId>
    </dependency>

    <!-- per-technology test starters -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webmvc-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

mvn dependency:tree | findstr autoconfigure
[INFO] +- org.springframework.boot:spring-boot-webmvc:jar:4.1.1
[INFO] +- org.springframework.boot:spring-boot-data-jpa:jar:4.1.1
[INFO] +- org.springframework.boot:spring-boot-flyway:jar:4.1.1
(focused modules instead of one large spring-boot-autoconfigure jar)

// 1. Tests: @MockBean is gone
- @MockBean ProductService service;
+ @MockitoBean ProductService service;

// 2. Jackson: new package for core classes (annotations unchanged)
- import com.fasterxml.jackson.databind.ObjectMapper;
+ import tools.jackson.databind.json.JsonMapper;

- @Bean Jackson2ObjectMapperBuilderCustomizer custom() { ... }
+ @Bean JsonMapperBuilderCustomizer custom() { ... }

// 3. Full-context tests must opt in to MockMvc
  @SpringBootTest
+ @AutoConfigureMockMvc
  class CheckoutIT { ... }

// 4. Retry without the spring-retry library
+ @EnableResilientMethods
  @SpringBootApplication
  public class ShopApplication { ... }

  @Retryable(maxRetries = 3, delay = 200)
  public Rates fetchRates() { ... }

(Run the OpenRewrite recipe to apply most of these automatically: mvn -U org.openrewrite.maven:rewrite-maven-plugin:run -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-spring:RELEASE -Drewrite.activeRecipes=org.openrewrite.java.spring.boot4.UpgradeSpringBoot_4_0)

<!-- Restores a Boot 3-like classpath so you can upgrade in small steps.
     Remove it once you have switched to the new focused starters. -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-classic</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test-classic</artifactId>
    <scope>test</scope>
</dependency>

Started ShopApplication in 2.4 seconds (Spring Boot 4.1.1, classic layout)