Benchmark state

Our testMethod() function is a simple example, but most of the time our method with a critical piece of code depends on a state. State is a set of variables on which our code depends, but we don't want to initialize them in a benchmark method, and we need to maintain this state while the benchmark is running. Since the JMH is heavily used to build benchmarks with concurrent code, the concept of state-bearing objects was implemented. To implement the state class, you need to follow these rules:

• The class must be declared as public
• You can't use inner classes, but you can use nested classes that are declared with the static modifier
• The class must have a public constructor without arguments

A state class contains state variables, and if you specify an instance of this class as an argument it will be provided as a parameter to the @Benchmark method. In code, it might look as follows:

import org.openjdk.jmh.annotations.*;
import java.util.concurrent.TimeUnit;

public class MyBenchmark {

    @State(Scope.Thread)
    public static class MyState {
        int a = 3;
        int b = 4;
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)

    public void testMethod(MyState myState) {
        int c = myState.a + myState.b;
    }

}

We use a state in this example because the testMethod uses two constant values and the compiler can optimize this moment and just use the final result instead of calculations. If you look at the output, you'll see that the result time has doubled:

Result "org.sample.MyBenchmark.testMethod":
0.671 ±(99.9%) 0.071 ns/op [Average]
(min, avg, max) = (0.656, 0.671, 0.702), stdev = 0.018
CI (99.9%): [0.600, 0.743] (assumes normal distribution)
# Run complete. Total time: 00:00:10
Benchmark              Mode Cnt Score Error Units
MyBenchmark.testMethod avgt 5   0.671 ± 0.071 ns/op

The @State annotation takes an instance of the Scope enum, which defines how to share the instance of the MyState class between threads.