Why Custom Health Indicators?

Spring Boot provides several built-in health checks (e.g., diskSpace, db, ping), but what if:

• Your system depends on a third-party API, and if that service is down, your app is practically unusable?
• You need to verify if a certain table in your database has expected values?
• You want to check if a certificate is valid or a cache is filled?

This is where custom health indicators come into play.

How to Create a Custom Health Indicator?

You can create a custom health indicator by doing one of the following:

Option 1: Implement HealthIndicator Interface

import org.springframework.boot.actuate.health.Health;
import org.springframework.boot.actuate.health.HealthIndicator;
import org.springframework.stereotype.Component;

@Component
public class MyApiHealthIndicator implements HealthIndicator {

    @Override
    public Health health() {
        // Simulate a failing check
        boolean apiIsDown = true;

        if (apiIsDown) {
            return Health.down()
                    .withDetail("API Status", "External API is down")
                    .withDetail("Reason", "SSL Certificate expired")
                    .build();
        }

        return Health.up()
                .withDetail("API Status", "Running smoothly")
                .build();
    }
}

Option 2: Extend AbstractHealthIndicator

import org.springframework.boot.actuate.health.AbstractHealthIndicator;
import org.springframework.boot.actuate.health.Health;
import org.springframework.stereotype.Component;

@Component
public class ExternalServiceHealthIndicator extends AbstractHealthIndicator {

    @Override
    protected void doHealthCheck(Health.Builder builder) throws Exception {
        // Custom logic
        boolean serviceIsOk = true;

        if (serviceIsOk) {
            builder.up().withDetail("External Service", "Available");
        } else {
            builder.down().withDetail("External Service", "Unavailable");
        }
    }
}

Both classes must be annotated with @Component so Spring can detect and register them as beans.

What Happens Internally?

When Spring Boot sees a bean of type HealthIndicator, it automatically aggregates it into the /actuator/health endpoint.

By default, the overall health status is composite, meaning:

• If all indicators return UP, the status is UP.
• If any returns DOWN, the overall status is DOWN.
• Some statuses like UNKNOWN do not affect the overall health.

Testing the Custom Indicator

Once your custom indicator is added, start your app and hit:

GET https://localhost:8080/actuator/health

Example Response

{
  "status": "DOWN",
  "components": {
    "db": { "status": "UP" },
    "diskSpace": { "status": "UP" },
    "ping": { "status": "UP" },
    "myApiHealthIndicator": {
      "status": "DOWN",
      "details": {
        "API Status": "External API is down",
        "Reason": "SSL Certificate expired"
      }
    }
  }
}

As you can see, one failing component (myApiHealthIndicator) caused the overall health to become DOWN.