intermediate~2h

Spring Boot Actuator

"Is the service healthy?" is a question your infrastructure asks constantly, automatically. This module is the built-in feature that answers it, without you writing a single health-check line yourself.

Learning objectives

Beginner: Add the Actuator starter and hit the built-in /actuator/health endpoint.
Intermediate: Expose and secure specific Actuator endpoints for a production deployment.
Advanced: Write a custom HealthIndicator that reports on an application-specific dependency (e.g. a downstream API).

◆ The problem

Kubernetes needs to know if your pod is actually healthy before routing traffic to it (DevOps Mastery's Kubernetes book covers liveness/readiness probes in depth — they need SOMETHING to call), and an on-call engineer at 3am needs to see memory usage, active threads, and configuration without SSH-ing into a container.

Adding spring-boot-starter-actuator exposes a set of production-monitoring endpoints automatically — /actuator/health, /actuator/metrics, /actuator/info — without you writing a single line of monitoring code yourself.

management.endpoints.web.exposure.include=health,metrics,info

/actuator/health isn't one check — it's an aggregate of every HealthIndicator bean Spring Boot auto-detects, including built-in ones for your database connection pool, disk space, and Redis connection if those dependencies are on the classpath. If ANY indicator reports DOWN, the whole endpoint reports DOWN.

Status	Meaning
UP	Everything this service depends on is reachable and healthy.
DOWN	At least one critical dependency (usually the database) is unreachable.
OUT_OF_SERVICE	Deliberately marked unavailable (e.g. during a graceful shutdown).

This exact endpoint is what a Kubernetes readiness probe (DevOps Mastery's Kubernetes book) calls repeatedly — a DOWN response there means Kubernetes stops routing traffic to this pod instantly.

The built-in indicators check generic infrastructure (database, disk); they know nothing about YOUR application's specific dependencies, like a third-party payment API your checkout flow can't function without.

@Component
public class PaymentGatewayHealthIndicator implements HealthIndicator {
    @Override
    public Health health() {
        boolean reachable = paymentClient.ping();
        return reachable
            ? Health.up().withDetail("gateway", "reachable").build()
            : Health.down().withDetail("gateway", "unreachable").build();
    }
}

Once registered as a bean, Spring Boot automatically folds this into the aggregate /actuator/health response alongside the built-in checks — no extra wiring needed.

💻 Code example

@Component
public class PaymentGatewayHealthIndicator implements HealthIndicator {
    @Override
    public Health health() {
        boolean reachable = paymentClient.ping();
        return reachable
            ? Health.up().withDetail("gateway", "reachable").build()
            : Health.down().withDetail("gateway", "unreachable").build();
    }
}

▲ Pitfall

Exposing /actuator/env or /actuator/heapdump to the public internet is a real, commonly-exploited misconfiguration — /actuator/env can leak configuration values (sometimes including secrets), and /actuator/heapdump hands out a full memory dump an attacker can mine for credentials and session tokens.

Production configuration should expose only what's genuinely needed (health, info, metrics) and require authentication for anything sensitive, using the exact Spring Security patterns covered in that category:

.requestMatchers("/actuator/health").permitAll()
.requestMatchers("/actuator/**").hasRole("ADMIN")

✓ Quick recap

Actuator exposes production-monitoring endpoints automatically once the starter is added.
/actuator/health aggregates every HealthIndicator bean — one DOWN makes the whole endpoint DOWN.
Write a custom HealthIndicator for application-specific dependencies the built-in checks don't know about.
Never expose sensitive Actuator endpoints (env, heapdump) without authentication in production.

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