Create Interface for HealthCheck Operations Providers

[jwcarman]

We are using an OSGi environment and we have created a "wrapper" interface for HealthCheck objects, so that they can be exposed as OSGi services and picked up using the "whiteboard" pattern. It would be great if there were a native interface bundled with this library that folks could implement in order to provide health check results.

[jillesvangurp]

We use Java 8 and it would be nice if HealthCheck was a functional interface instead of an abstract class. That way we could use lambda functions to quickly create health checks without a lot of boiler plate for the inner class.

The execute method in the class could move elsewhere probably. Unfortunately it is public and non-final, which suggest that somebody might be overriding this. Or if you want to keep the abstract class, it could be renamed to AbstractHealthCheck and implement a new HealthCheck interface. This would be a backwards incompatible change. Alternatively, the interface could have a different name.

Another interface that would be nice is something similar to the MetricSet interface to indicate that a class provides a HealthCheck. I actually created one for our spring setup along with a spring context listener that registers all health checks.

public interface HealthCheckProvider {
    HealthCheck healthCheck();
}

public class HealthCheckContextListener implements ApplicationListener<ContextRefreshedEvent> {
    private static final Logger LOG = LoggerFactory.getLogger(HealthCheckContextListener.class);

    private final HealthCheckRegistry healthCheckRegistry;

    public HealthCheckContextListener(HealthCheckRegistry healthCheckRegistry) {
        this.healthCheckRegistry = healthCheckRegistry;
    }

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        // register health checks provided by other beans
        Map<String, HealthCheckProvider> healthCheckProvidingBeans = event.getApplicationContext().getBeansOfType(HealthCheckProvider.class);
        for(Entry<String, HealthCheckProvider> e: healthCheckProvidingBeans.entrySet()) {
            LOG.info("health check " + e.getKey() + " " + e.getValue().getClass().getName());
            HealthCheck check = e.getValue().healthCheck();
            healthCheckRegistry.register(e.getKey(), check);
        }
        // register any health check beans registered directly
        Map<String, HealthCheck> healthCheckBeans = event.getApplicationContext().getBeansOfType(HealthCheck.class);
        for(Entry<String, HealthCheck> e: healthCheckBeans.entrySet()) {
            LOG.info("health check " + e.getKey() + " " + e.getValue().getClass().getName());
            HealthCheck check = e.getValue();
            healthCheckRegistry.register(e.getKey(), check);
        }
    }
}

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[jillesvangurp]

It seems this was resolved by #1289

[joschi]

@jillesvangurp Thanks for finding the PR! 😄