How can you use Spring Boot Actuator to expose custom metrics or endpoints?

Spring Boot Actuator is a powerful feature of Spring Boot that provides a set of production-ready features that can help developers monitor and manage their applications. One of the key features of Actuator is the ability to expose custom metrics and endpoints that can provide additional insights into the health and performance of the application.

To expose custom metrics or endpoints in a Spring Boot application using Actuator, you can follow these steps:

Define the custom metric or endpoint: Define the custom metric or endpoint in a Spring bean, using a naming convention that starts with custom (e.g., customMetric). The bean should implement the appropriate interface for the type of metric or endpoint you want to expose. For example, to expose a custom counter metric, you can define a bean that implements the Counter interface.

    @Bean
    public Counter customMetric() {
        return new SimpleCounter();
    }

Configure the Actuator endpoint: Use the @Endpoint annotation to expose the custom metric or endpoint through the Actuator management endpoint. You can also use the @ReadOperation or @WriteOperation annotations to specify whether the endpoint is read-only or writeable.

    @Endpoint(id = "customEndpoint")
    public class CustomEndpoint {
        private final Counter customMetric;
        
        public CustomEndpoint(Counter customMetric) {
            this.customMetric = customMetric;
        }
        
        @ReadOperation
        public Map<String, Long> getCustomMetric() {
            Map<String, Long> result = new LinkedHashMap<>();
            result.put("customMetric", customMetric.count());
            return result;
        }
    }

In this example, we have defined a custom counter metric using a Spring bean that implements the Counter interface. We have also defined a custom endpoint using the @Endpoint annotation, with a @ReadOperation method that returns a map of custom metrics. The @Endpoint annotation specifies the ID of the endpoint, which is used to access it through the Actuator management endpoint.

Enable Actuator endpoints: Enable the Actuator endpoints in the application by adding the appropriate dependencies and configuration. You can use the management.endpoints.web.exposure.include property to specify which endpoints to include in the Actuator management endpoint.

management:
  endpoints:
    web:
      exposure:
        include: customEndpoint

In this example, we have enabled the Actuator management endpoint and included our custom endpoint using the management.endpoints.web.exposure.include property in the application.yml file.

Access the custom metric or endpoint: Access the custom metric or endpoint by making a request to the Actuator management endpoint. You can use a tool such as curl or a web browser to make the request.

    $ curl http://localhost:8080/actuator/customEndpoint
    {"customMetric":0}

In this example, we have accessed the custom endpoint by making a GET request to http://localhost:8080/actuator/customEndpoint. The response is a JSON object containing the custom metric value.

Overall, Spring Boot Actuator provides a powerful and flexible way to expose custom metrics and endpoints in a Spring Boot application. By defining custom beans and endpoints and configuring Actuator, developers can gain valuable insights into the health and performance of their applications.