Application Observability

Exposing Prometheus Metrics from Java

Marc Zottner

The Micrometer library is a popular way to expose application metrics to a service like Prometheus.

Adding Dependencies

To add the Micrometer dependency for Prometheus with Maven:

<dependencies>
    <dependency>
        <groupId>io.micrometer</groupId>
        <artifactId>micrometer-registry-prometheus</artifactId>
        <version>${micrometer.version}</version>
    </dependency>
</dependencies>

If you’re using Gradle instead:

dependencies {
    implementation 'io.micrometer:micrometer-registry-prometheus:latest.release'
}

With the correct dependencies in place, you’re ready to expose metrics.

Exposing Metrics

For exposing application metrics, you have two options:

• if you’re writing a Java application without using a framework, you must expose the endpoint yourself
• if you’re using Spring Boot, you can ad the Spring Boot Actuator dependency to your application

Using Vanilla Java

The following example uses the JDK’s com.sun.net.httpserver.HttpServer to expose a /prometheus endpoint. The PrometheusMeterRegistry contains a scrape() function, which can supply the metric data to Prometheus. All you need to do is wire the function to an endpoint.

PrometheusMeterRegistry prometheusRegistry =
    new PrometheusMeterRegistry(PrometheusConfig.DEFAULT);

try {
    HttpServer server = HttpServer.create(new InetSocketAddress(8080), 0);
    server.createContext("/prometheus", httpExchange -> {
        String response = prometheusRegistry.scrape(); (1)
        httpExchange.sendResponseHeaders(200, response.getBytes().length);
        try (OutputStream os = httpExchange.getResponseBody()) {
            os.write(response.getBytes());
        }
    });

    new Thread(server::start).start();
} catch (IOException e) {
    throw new RuntimeException(e);
}

Using Spring Boot

First, update the scope and remove the version of the micrometer-registry-prometheus dependency. Then add the spring-boot-starter-actuator dependency to your project.

Maven:

<dependencies>
    <dependency>
        <groupId>io.micrometer</groupId>
        <artifactId>micrometer-registry-prometheus</artifactId>
        <scope>runtime</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
</dependencies>

Gradle:

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-actuator'
    runtimeOnly 'io.micrometer:micrometer-registry-prometheus'
}

Next, enable the Prometheus actuator endpoint. At the time of writing, you must add prometheus to the list of exposed Spring Boot Actuator endpoints. Your endpoint configuration might look like the following:

management.endpoints.web.exposure.include: info, health, prometheus

This exposes metrics at the /actuator/prometheus endpoint.

References

• Spring Boot Actuator: Production-ready Features — Spring Boot reference documentation about Actuator
• Exposing Actuator Endpoints — provides more information about exposing Spring Boot actuator endpoints