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Java

This guide demonstrates how to instrument a Java application with OpenTelemetry to send traces, metrics, and logs to FusionReactor Cloud.

Prerequisites

  • FusionReactor API Key: Obtain this from Account Settings > API Keys in FusionReactor Cloud.
  • Java: Java 8 or later installed on your system.
  • Telemetry Pipeline: You must have either an OpenTelemetry Collector or Grafana Alloy configured and running to receive data from your Java application.

Set up your telemetry pipeline first

Before instrumenting your Java application, ensure you have completed either the Collector setup guide or Grafana Alloy setup guide so your telemetry data has a destination.

Step 1: Download the OpenTelemetry Java Agent

The OpenTelemetry Java Agent provides automatic instrumentation without code changes. Download the latest version:

curl -L -O https://github.com/open-telemetry/opentelemetry-java-instrumentation/releases/latest/download/opentelemetry-javaagent.jar

This agent automatically instruments popular frameworks like Spring Boot, Tomcat, JDBC, and more.

Step 2: Create your Java application

Create a simple Java application that demonstrates telemetry collection. Create a file named FibonacciApp.java:

import java.util.concurrent.TimeUnit;

public class FibonacciApp {

    public static void main(String[] args) throws InterruptedException {
        if (args.length < 1) {
            System.out.println("Usage: java FibonacciApp <iterations>");
            System.exit(1);
        }

        int iterations = Integer.parseInt(args[0]);
        System.out.println("Starting Fibonacci calculator");
        System.out.println("Fibonacci by Iteration - " + iterations + " rounds");

        calculateFibonacci(iterations);

        System.out.println("Fibonacci complete");
    }

    public static void calculateFibonacci(int iterations) throws InterruptedException {
        long prev = 0;
        long current = 1;

        for (int i = 0; i < iterations; i++) {
            System.out.println("Iteration " + (i + 1) + ": " + current);

            long next = prev + current;
            prev = current;
            current = next;

            // Simulate work
            TimeUnit.MILLISECONDS.sleep(100);
        }
    }
}

Compile the application:

javac FibonacciApp.java

How automatic instrumentation works

The OpenTelemetry Java Agent automatically: - Creates spans for HTTP requests, database calls, and messaging operations - Captures JVM metrics (CPU, memory, garbage collection) - Instruments HTTP clients/servers, database calls, and messaging - Propagates trace context across service boundaries

Best suited for web frameworks

The Java agent works by auto-instrumenting supported frameworks such as Spring Boot, Tomcat, JDBC, and gRPC. A plain command-line application like FibonacciApp will not produce traces since it contains no instrumented code paths. For full trace, metric, and log output, attach the agent to a web service (e.g. Spring Boot) where HTTP requests are the natural instrumentation entry point.

Step 3: Run with OpenTelemetry Agent

Run your Java application with the OpenTelemetry agent attached:

java -javaagent:opentelemetry-javaagent.jar \
  -Dotel.service.name=fibonacci-service \
  -Dotel.traces.exporter=otlp \
  -Dotel.metrics.exporter=otlp \
  -Dotel.logs.exporter=otlp \
  -Dotel.exporter.otlp.endpoint=http://localhost:4318 \
  -Dotel.exporter.otlp.protocol=http/protobuf \
  FibonacciApp 20

Configuration options explained

  • -javaagent:opentelemetry-javaagent.jar: Attaches the OpenTelemetry agent
  • -Dotel.service.name: Sets the service name (appears as job in FusionReactor Cloud)
  • -Dotel.exporter.otlp.endpoint: Points to your local collector
  • -Dotel.exporter.otlp.protocol: Uses HTTP protocol (port 4318)

Cannot connect to collector?

If you see: Failed to export spans or Connection refused Fix: Your collector is not running. Start it first using the Collector setup guide.

Step 4: Verify in FusionReactor Cloud

  1. Log in to FusionReactor Cloud
  2. Navigate to Explore:
  3. Traces: Select Resource Service Name = fibonacci-service
  4. Metrics: Search for JVM metrics like process.runtime.jvm.memory.usage{job="fibonacci-service"}
  5. Logs: Filter by job = fibonacci-service

You should see: - Trace spans showing method executions - JVM metrics (heap usage, GC activity, thread counts) - Log entries from your application

Next steps

  • Use the agent with Spring Boot applications for automatic REST endpoint instrumentation
  • Configure agent extensions for custom instrumentation
  • Add manual instrumentation for business-specific tracing using the Java SDK
  • Create custom dashboards in FusionReactor Cloud to visualize your Java application metrics

Learn more

OpenTelemetry Java Documentation

Need more help?

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