Load Testing with JUnit and External Tools (Gatling, JMeter)

Performance testing is no longer optional in today’s fast-paced microservices and cloud-native ecosystems. While JUnit 5 is excellent for unit and integration testing, it is not designed to simulate thousands of concurrent users or heavy throughput. That’s where external tools like Gatling and JMeter come into play. When combined with JUnit, these tools provide a seamless pipeline for functional and performance testing.

In this tutorial, we’ll explore how to perform load testing with JUnit, Gatling, and JMeter, integrating them into your existing Java project and CI/CD pipelines.

What is Load Testing?

Load testing validates how a system behaves under expected or extreme traffic. It answers questions like:

Can the API handle 10,000 requests per second?
What is the latency under 500 concurrent users?
Where are the performance bottlenecks?

Key Metrics

Throughput (TPS/RPS): Number of requests per second handled.
Response Time (Latency): Time taken per request.
Error Rate: Percentage of failed requests.
Resource Utilization: CPU, memory, and I/O usage.

Using JUnit for Baseline Performance Tests

JUnit is not a load testing tool, but you can create baseline performance assertions with timeouts.

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertTimeout;
import java.time.Duration;

public class BaselinePerformanceTest {

    @Test
    void testApiResponseTime() {
        assertTimeout(Duration.ofMillis(200), () -> {
            // Simulate calling a REST API
            Thread.sleep(150); 
        });
    }
}

🔹 This test ensures that a given operation completes within 200ms.

Load Testing with Gatling

Gatling is a developer-friendly load testing tool built in Scala, but it integrates seamlessly with Java/Maven/Gradle projects.

Example Simulation

import io.gatling.core.Predef._
import io.gatling.http.Predef._
import scala.concurrent.duration._

class ApiLoadTest extends Simulation {

  val httpProtocol = http.baseUrl("http://localhost:8080")

  val scn = scenario("API Load Test")
    .exec(http("Get Users").get("/api/users"))
    .pause(1)

  setUp(
    scn.inject(rampUsers(1000) during (30.seconds))
  ).protocols(httpProtocol)
}

🔹 This simulates 1000 users ramping up over 30 seconds against /api/users.

Running Gatling

mvn gatling:test

gradle gatlingRun

Load Testing with JMeter

Apache JMeter is another powerful tool for load testing, with GUI and CLI support.

Steps:

Download JMeter from jmeter.apache.org.
Create a Thread Group with concurrent users.
Add an HTTP Request Sampler for your endpoint.
Add Listeners (Graph Results, Summary Report).

CLI Execution for CI/CD

jmeter -n -t test-plan.jmx -l results.jtl -e -o ./report

🔹 This runs the test headlessly and outputs an HTML report.

Integrating Load Tests into CI/CD

Jenkins

stage('Load Test') {
    steps {
        sh 'mvn gatling:test'
        sh 'jmeter -n -t test-plan.jmx -l results.jtl'
    }
}

GitHub Actions

jobs:
  load-test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Run Gatling
        run: mvn gatling:test
      - name: Run JMeter
        run: jmeter -n -t test-plan.jmx -l results.jtl

Best Practices for Load Testing

Start with baseline JUnit performance tests.
Use Testcontainers to spin up real dependencies (e.g., PostgreSQL, Kafka).
Run Gatling/JMeter in headless mode in CI/CD.
Monitor CPU, memory, and logs during load tests.
Automate regression checks for performance over time.

Version Tracker

JUnit 4 → JUnit 5: Modern annotations, better extensions, parallel tests.
Mockito Updates: Static/final method mocking available.
Testcontainers: Support for Docker Compose and LocalStack.
Gatling & JMeter: Improved CI/CD integration in latest versions.

Conclusion & Key Takeaways

JUnit provides baseline performance checks.
Gatling is developer-focused with expressive DSL for HTTP load testing.
JMeter is versatile, GUI-friendly, and integrates well in CI/CD.
Combining them ensures both correctness and scalability of your applications.

FAQ

1. What’s the difference between unit and load testing?
Unit tests validate correctness of individual components, while load tests validate scalability under heavy traffic.

2. Can I use JUnit alone for load testing?
JUnit can set performance thresholds, but for realistic load testing, you need tools like Gatling or JMeter.

3. Which is better, Gatling or JMeter?
Gatling is code-driven (great for developers), JMeter has GUI and plugin ecosystem (great for QA teams).

4. How do I integrate load testing into CI/CD?
Run Gatling/JMeter in headless mode and publish HTML reports in Jenkins or GitHub Actions.

5. Can I use Testcontainers with load testing?
Yes. Spin up real databases or message brokers with Testcontainers before running Gatling/JMeter.

6. What’s ramp-up in load testing?
Ramp-up defines how users are gradually introduced (e.g., 1000 users over 30 seconds).

7. Can Gatling simulate WebSocket traffic?
Yes, Gatling supports WebSocket, SSE, and gRPC protocols.

8. Is JMeter still relevant?
Yes, it’s widely used, especially for legacy systems and GUI-driven test setup.

9. How do I analyze results?
Look for throughput, response time percentiles (p95, p99), and error rates.

10. How do I prevent flaky load tests?
Ensure test environments are isolated, use dedicated hardware/containers, and monitor resource bottlenecks.