spring-boot-demo/docs/02-boot-4-changes.md

← 01 What Actuator exposes · 02 · What changed in Spring Boot 4 · 03 The endpoint catalogue →

02 — What changed in Spring Boot 4

Every claim below was checked against the 4.1.1 jars with javap, or against the 3.5.16 jars for the "before" side. None of it came from a migration blog.

HealthIndicator moved module and package

This is the one that breaks every custom health indicator ever written.

Boot 3.5.16   org.springframework.boot.actuate.health.HealthIndicator     (spring-boot-actuator)
Boot 4.1.1    org.springframework.boot.health.contributor.HealthIndicator (spring-boot-health)

org.springframework.boot.actuate.health does not exist in Boot 4.1.1 at all — not deprecated, absent. Health, Status, AbstractHealthIndicator, HealthContributor and the composites all moved with it, into a new spring-boot-health module.

Two neighbouring packages in the same new module are worth knowing:

• org.springframework.boot.health.application — DiskSpaceHealthIndicator, LivenessStateHealthIndicator, ReadinessStateHealthIndicator, SslHealthIndicator
• org.springframework.boot.health.registry — HealthContributorRegistry, which is how DiagnosticsEndpoint enumerates contributors at runtime

The interface method was renamed

// Boot 3.5.16
public interface HealthIndicator extends HealthContributor {
    default Health getHealth(boolean includeDetails);
    Health health();
}

// Boot 4.1.1
public interface HealthIndicator extends HealthContributor {
    default Health health(boolean includeDetails);
    Health health();
}

If you overrode getHealth(boolean) — which people do to control detail rendering — it now silently stops being an override. Add @Override and let the compiler find it.

Health also stopped extending HealthComponent; that class is gone from Boot 4.1.1 entirely.

EndpointRequest moved too

Boot 3.5.16   org.springframework.boot.actuate.autoconfigure.security.servlet.EndpointRequest
Boot 4.1.1    org.springframework.boot.security.autoconfigure.actuate.web.servlet.EndpointRequest

Same class, same static methods (toAnyEndpoint(), to(Class...), to(String...), toLinks(), toAdditionalPaths(...)), new package, and it now ships in spring-boot-security rather than spring-boot-actuator-autoconfigure. The reactive variant made the equivalent move.

RestClient is not in the web starter any more

Boot 4 split the framework into fine-grained modules, and RestClient auto-configuration went with it. Adding spring-boot-starter-webmvc and injecting RestClient.Builder fails at startup:

Parameter 0 of constructor in com.ankurm.actuator.health.ExternalApiHealthIndicator
required a bean of type 'org.springframework.web.client.RestClient$Builder' that could not be found.

The fix is one dependency:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-restclient</artifactId>
</dependency>

This cost a startup cycle while building this repository, which is why it is documented here rather than glossed over.

Liveness and readiness probes are on by default

From the 4.0 migration guide, and confirmed by running the app with no configuration at all — /actuator/health reports "groups":["liveness","readiness"] out of the box. Turn them off with management.endpoint.health.probes.enabled=false if you genuinely do not want them.

Nullability annotations

Actuator endpoint parameters can no longer use org.springframework.lang.Nullable to mark a parameter optional. Migrate to org.jspecify.annotations.Nullable.

There is no Micrometer 2

Several migration write-ups claim Boot 4 replaces "legacy Actuator endpoints" with a "Micrometer 2 observability stack". Spring Boot 4.1.1 resolves Micrometer 1.17.1 (docs/output/00-versions.txt), and maven-metadata.xml for micrometer-core shows 1.17.1 as the newest GA with 1.18.0-M1 as a milestone. Metrics did move to their own module (spring-boot-micrometer-metrics, which spring-boot-starter-actuator still pulls in), and there is a new spring-boot-starter-opentelemetry. Neither of those is a major Micrometer release.

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