Sınıf Yükleme ve JIT — Uygulama Neden Isınır?

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public interface StatementFormat {
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    String name();                               // "csv", "mt940", "camt053"
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    byte[] export(List<Movement> movements);
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}

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public class CsvFormat implements StatementFormat {
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    @Override
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    public String name() {
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        return "csv";
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    }
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    @Override
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    public byte[] export(List<Movement> movements) {
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        String body = movements.stream()
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                .map(m -> m.valueDate() + ";" + m.amount().toPlainString() + ";" + m.balanceAfter().toPlainString())
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                .collect(Collectors.joining("\n", "valor;tutar;bakiye\n", "\n"));
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        return body.getBytes(StandardCharsets.UTF_8);
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    }
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}
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// File: src/main/resources/META-INF/services/statement.StatementFormat
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// Content (one implementation per line):
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//   statement.csv.CsvFormat
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// An MT940 or camt.053 module adds its own line in its own jar: no change here.

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public class Formats {
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    // ServiceLoader reads META-INF/services files on the classpath. Each class is
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    // loaded and instantiated only when the iteration reaches it.
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    public static Map<String, StatementFormat> discover() {
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        Map<String, StatementFormat> formats = new TreeMap<>();
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        for (StatementFormat format : ServiceLoader.load(StatementFormat.class)) {
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            formats.put(format.name(), format);
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        }
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        return formats;
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    }
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}

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public class InitOrder {
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    // Loads the whole SWIFT BIC directory in its static initializer: slow, so do it only when needed.
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    static class BicDirectory {
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        static {
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            System.out.println("BicDirectory initialized");   // runs on first active use, once
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        }
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        static void lookup() {}
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    }
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    public static void main(String[] args) {
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        System.out.println("main started");
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        BicDirectory[] array = new BicDirectory[10];          // does NOT initialize the class
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        System.out.println("array created");
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        BicDirectory.lookup();                                // this does
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        BicDirectory.lookup();                                // and not again
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        // main started
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        // array created
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        // BicDirectory initialized
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    }
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}

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@BenchmarkMode(Mode.AverageTime)
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@OutputTimeUnit(TimeUnit.MICROSECONDS)
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@Warmup(iterations = 5, time = 1)        // interpreted -> C1 -> C2 happens here, not in the results
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@Measurement(iterations = 5, time = 1)
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@Fork(2)                                 // fresh JVMs: one lucky or unlucky run does not decide
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@State(Scope.Benchmark)
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public class CsvExportBenchmark {
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    List<Movement> movements;
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    CsvFormat format;
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    @Setup
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    public void setUp() {
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        movements = Movements.sample(1_000);   // one busy month of a corporate account
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        format = new CsvFormat();
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    }
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    @Benchmark
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    public byte[] export() {
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        return format.export(movements);  // returned, so the JIT cannot remove the work as dead code
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    }
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}

# Which methods get compiled, at which tier, and when (noisy: for experiments only)
java -XX:+PrintCompilation -jar app.jar | grep CsvFormat

# Inlining decisions of the C2 compiler
java -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining -jar app.jar

# Run the benchmark; read the error column, not just the score
./gradlew jmh