JMH
- Java course
- Expert
- Lesson 31/46
- 5 min
Learn to benchmark with JMH.
Microbenchmarking with JMH
JMH (Java Microbenchmark Harness) is a framework for measuring small code pieces accurately. Warmup rounds trigger the JIT compiler and measurement rounds collect the result. This avoids misleading measurements.
JMH measures methods marked with @Benchmark. @State carries state and Blackhole prevents dead code elimination. For correct benchmarks warmup and iteration counts are given explicitly.
Warmup and measurement rounds
JMH runs in two phases: first warmup rounds let the JIT compile hot methods; their result is not counted. Then measurement rounds collect timings. Without warmup the measurement includes cold compilation and is wrong.
Round counts are set with @Warmup(iterations=3) and @Measurement(iterations=5). The result is presented statistically: average, error margin.
Practice Write the BenchmarkRunner class
Define class BenchmarkRunner with static int iterations = 5; and static int warmup = 3; fields and an empty void measure() {} method. In main print System.out.println("warmup=" + warmup + " iters=" + iterations);.
Starter code
// Define the BenchmarkRunner class: iterations and warmup fields, measure method
// main: print warmup and iterations valuesSample solution
class BenchmarkRunner {
static int iterations = 5;
static int warmup = 3;
void measure() {}
public static void main(String[] args) {
System.out.println("warmup=" + warmup + " iters=" + iterations);
}
}Expected output
warmup=3 iters=5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is JMH used for?
- Database management
- Graphics rendering
- Web server
- Microbenchmarking Correct answer
Explanation: JMH measures small code pieces accurately.
2 Complete the benchmark annotation
@____
public void measure() {}@Benchmark
public void measure() {}Explanation: @Benchmark marks the measured method.
3 Match the JMH concepts
| Term | Meaning |
|---|---|
@Benchmark |
Measured method |
@State |
State carrier |
Blackhole |
Prevents dead code |
@Warmup |
Warmup round |