javademos/lazy-constants/jmh/src/bench/AccessBench.java

package bench;

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

/**
 * Cost of READING an already-initialised lazily-built value, six ways. Every variant returns k + x, where x is an
 * ordinary mutable field, so the JIT cannot fold the whole method away, but can fold a constant k.
 */
@State(Scope.Benchmark)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class AccessBench {
    record Config(int k) {}

    // 1. Eager static final: the reference point, always initialised, always foldable.
    static final Config EAGER = new Config(7);

    // 2. Holder idiom.
    static class Holder { static final Config VALUE = new Config(7); }

    // 3. Double-checked locking with a volatile field.
    static volatile Config dclField;
    static Config dcl() {
        Config local = dclField;
        if (local == null) {
            synchronized (AccessBench.class) {
                local = dclField;
                if (local == null) dclField = local = new Config(7);
            }
        }
        return local;
    }

    // 4. LazyConstant in a static final field: the case the JEP is designed for.
    static final LazyConstant<Config> LAZY_STATIC = LazyConstant.of(() -> new Config(7));

    // 5. LazyConstant in a static field that is NOT final.
    static LazyConstant<Config> lazyNonFinalStatic = LazyConstant.of(() -> new Config(7));

    // 6. LazyConstant in an ordinary final instance field.
    final LazyConstant<Config> lazyInstance = LazyConstant.of(() -> new Config(7));

    int x = 3;

    @Benchmark public int eagerStaticFinal()   { return EAGER.k() + x; }
    @Benchmark public int holderIdiom()        { return Holder.VALUE.k() + x; }
    @Benchmark public int doubleCheckedLocking() { return dcl().k() + x; }
    @Benchmark public int lazyConstantStaticFinal() { return LAZY_STATIC.get().k() + x; }
    @Benchmark public int lazyConstantNonFinalStatic() { return lazyNonFinalStatic.get().k() + x; }
    @Benchmark public int lazyConstantInstanceField() { return lazyInstance.get().k() + x; }
}