Add parallel item creation to TestDataCreator

Opt-in per call site via parallel(), opt-out via sequential(). The
it.aboutbits.testing.testdata.parallel-by-default system property flips
the default, so a whole suite can run parallel without touching call
sites. The result list keeps index order; worker failures are rethrown
unwrapped.

Author: SirCotare

src/main/java/it/aboutbits/springboot/testing/testdata/base/ModifiableTestDataCreator.java

@@ -1,11 +1,11 @@
 package it.aboutbits.springboot.testing.testdata.base;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.errorprone.annotations.CheckReturnValue;
 import lombok.extern.slf4j.Slf4j;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 
-import java.util.ArrayList;
 import java.util.List;
 import java.util.function.BiFunction;
 import java.util.function.Consumer;
@@ -68,22 +68,34 @@ public CREATOR modifyResult(Consumer<ITEM> resultMutator) {
     }
 
     @Override
-    protected List<ITEM> create() {
-        var result = new ArrayList<ITEM>();
+    @SuppressWarnings("unchecked")
+    @CanIgnoreReturnValue
+    public CREATOR parallel() {
+        super.parallel();
+        return (CREATOR) this;
+    }
 
-        for (var index = 0; index < numberOfItems; index++) {
+    @Override
+    @SuppressWarnings("unchecked")
+    @CanIgnoreReturnValue
+    public CREATOR sequential() {
+        super.sequential();
+        return (CREATOR) this;
+    }
+
+    @Override
+    protected List<ITEM> create() {
+        var result = createItems(index -> {
             var item = create(index);
 
             if (resultMutator != null) {
                 resultMutator.accept(item, index);
 
-                result.add(
-                        saveMutation(item)
-                );
-            } else {
-                result.add(item);
+                return saveMutation(item);
             }
-        }
+
+            return item;
+        });
 
         if (mutatorSet && !mutatorCalled) {
             log.error("Parameter-mutation is defined but was never called.");

src/main/java/it/aboutbits/springboot/testing/testdata/base/TestDataCreator.java

@@ -1,5 +1,6 @@
 package it.aboutbits.springboot.testing.testdata.base;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import it.aboutbits.springboot.testing.testdata.FakerExtended;
 import org.jspecify.annotations.NullMarked;
 
@@ -8,20 +9,54 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.function.Function;
+import java.util.function.IntFunction;
 
 @SuppressWarnings("java:S119")
 @NullMarked
 public abstract class TestDataCreator<ITEM> {
+    /// System property that flips the default creation mode of every creator to parallel;
+    /// [#sequential()] then opts a single call site back out.
+    public static final String PARALLEL_BY_DEFAULT_PROPERTY =
+            "it.aboutbits.testing.testdata.parallel-by-default";
+
     @SuppressWarnings("unused")
     protected static final FakerExtended FAKER = new FakerExtended();
 
     protected final int numberOfItems;
 
+    private boolean parallel = Boolean.getBoolean(PARALLEL_BY_DEFAULT_PROPERTY);
+
     protected TestDataCreator(int numberOfItems) {
         this.numberOfItems = numberOfItems;
     }
 
+    /// Creates the items concurrently, one thread per item, instead of sequentially. The returned
+    /// list keeps index order, but database side effects (e.g. sequence-assigned ids) interleave
+    /// arbitrarily across items.
+    ///
+    /// Only safe for creators whose per-item creation is independent: a creator that shares lazily
+    /// created state across items (e.g. `sameXyz()` memoization) must resolve that state before
+    /// creation fans out.
+    @SuppressWarnings("unused")
+    @CanIgnoreReturnValue
+    public TestDataCreator<ITEM> parallel() {
+        this.parallel = true;
+        return this;
+    }
+
+    /// Creates the items sequentially — the default, unless [#PARALLEL_BY_DEFAULT_PROPERTY]
+    /// flipped it; then this is the per-call opt-out.
+    @SuppressWarnings("unused")
+    @CanIgnoreReturnValue
+    public TestDataCreator<ITEM> sequential() {
+        this.parallel = false;
+        return this;
+    }
+
     @SuppressWarnings("unused")
     public void commit() {
         create();
@@ -73,15 +108,55 @@ public Set<ITEM> returnSet() {
     }
 
     protected List<ITEM> create() {
-        var result = new ArrayList<ITEM>();
+        return createItems(this::create);
+    }
+
+    /// Runs one full item creation per index and returns the items in index order — sequentially
+    /// by default, concurrently after [#parallel()].
+    protected final List<ITEM> createItems(IntFunction<ITEM> itemForIndex) {
+        if (!parallel || numberOfItems <= 1) {
+            var result = new ArrayList<ITEM>();
 
-        for (var index = 0; index < numberOfItems; index++) {
-            result.add(
-                    create(index)
-            );
+            for (var index = 0; index < numberOfItems; index++) {
+                result.add(
+                        itemForIndex.apply(index)
+                );
+            }
+
+            return result;
         }
 
-        return result;
+        try (var executor = Executors.newFixedThreadPool(numberOfItems)) {
+            var futures = new ArrayList<Future<ITEM>>();
+            for (var index = 0; index < numberOfItems; index++) {
+                var itemIndex = index;
+                futures.add(
+                        executor.submit(() -> itemForIndex.apply(itemIndex))
+                );
+            }
+
+            var result = new ArrayList<ITEM>();
+            for (var future : futures) {
+                result.add(awaitItem(future));
+            }
+
+            return result;
+        }
+    }
+
+    private ITEM awaitItem(Future<ITEM> future) {
+        try {
+            return future.get();
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new IllegalStateException("Parallel test data creation was interrupted", e);
+        } catch (ExecutionException e) {
+            switch (e.getCause()) {
+                case RuntimeException runtimeException -> throw runtimeException;
+                case Error error -> throw error;
+                case null, default -> throw new IllegalStateException("Parallel test data creation failed", e.getCause());
+            }
+        }
     }
 
     protected abstract ITEM create(int index);

src/test/java/it/aboutbits/springboot/testing/testdata/base/TestDataCreatorTest.java

@@ -0,0 +1,128 @@
+package it.aboutbits.springboot.testing.testdata.base;
+
+import org.jspecify.annotations.NullMarked;
+import org.junit.jupiter.api.Test;
+
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.function.IntFunction;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+@NullMarked
+class TestDataCreatorTest {
+    @Test
+    void sequentialByDefault() {
+        var creator = new ThreadRecordingCreator(5, index -> "item-" + index);
+
+        var result = creator.returnAll();
+
+        assertThat(result).containsExactly("item-0", "item-1", "item-2", "item-3", "item-4");
+        assertThat(creator.threads).hasSize(1);
+    }
+
+    @Test
+    void parallelKeepsIndexOrder() {
+        var creator = new ThreadRecordingCreator(5, index -> "item-" + index);
+
+        var result = creator.parallel().returnAll();
+
+        assertThat(result).containsExactly("item-0", "item-1", "item-2", "item-3", "item-4");
+    }
+
+    @Test
+    void parallelRunsItemsConcurrently() {
+        var allItemsStarted = new CountDownLatch(3);
+        var creator = new ThreadRecordingCreator(3, index -> {
+            allItemsStarted.countDown();
+            try {
+                // Only finishes if all items run at the same time.
+                if (!allItemsStarted.await(5, TimeUnit.SECONDS)) {
+                    throw new IllegalStateException("Items did not run concurrently");
+                }
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                throw new IllegalStateException(e);
+            }
+            return "item-" + index;
+        });
+
+        var result = creator.parallel().returnAll();
+
+        assertThat(result).hasSize(3);
+        assertThat(creator.threads).hasSize(3);
+    }
+
+    @Test
+    void parallelPropagatesItemFailure() {
+        var creator = new ThreadRecordingCreator(3, index -> {
+            if (index == 1) {
+                throw new IllegalArgumentException("item 1 failed");
+            }
+            return "item-" + index;
+        });
+
+        assertThatExceptionOfType(IllegalArgumentException.class)
+                .isThrownBy(() -> creator.parallel().returnAll())
+                .withMessage("item 1 failed");
+    }
+
+    @Test
+    void parallelWithSingleItemStaysSequential() {
+        var creator = new ThreadRecordingCreator(1, index -> "item-" + index);
+
+        var result = creator.parallel().returnAll();
+
+        assertThat(result).containsExactly("item-0");
+        assertThat(creator.threads).containsExactly(Thread.currentThread().getName());
+    }
+
+    @Test
+    void parallelByDefaultProperty_flipsTheDefault() {
+        System.setProperty(TestDataCreator.PARALLEL_BY_DEFAULT_PROPERTY, "true");
+        try {
+            var creator = new ThreadRecordingCreator(3, index -> "item-" + index);
+
+            var result = creator.returnAll();
+
+            assertThat(result).containsExactly("item-0", "item-1", "item-2");
+            assertThat(creator.threads).doesNotContain(Thread.currentThread().getName());
+        } finally {
+            System.clearProperty(TestDataCreator.PARALLEL_BY_DEFAULT_PROPERTY);
+        }
+    }
+
+    @Test
+    void sequential_optsOutOfTheParallelDefault() {
+        System.setProperty(TestDataCreator.PARALLEL_BY_DEFAULT_PROPERTY, "true");
+        try {
+            var creator = new ThreadRecordingCreator(3, index -> "item-" + index);
+
+            var result = creator.sequential().returnAll();
+
+            assertThat(result).containsExactly("item-0", "item-1", "item-2");
+            assertThat(creator.threads).containsExactly(Thread.currentThread().getName());
+        } finally {
+            System.clearProperty(TestDataCreator.PARALLEL_BY_DEFAULT_PROPERTY);
+        }
+    }
+
+    private static final class ThreadRecordingCreator extends TestDataCreator<String> {
+        private final IntFunction<String> itemForIndex;
+        private final Set<String> threads = ConcurrentHashMap.newKeySet();
+
+        private ThreadRecordingCreator(int numberOfItems, IntFunction<String> itemForIndex) {
+            super(numberOfItems);
+            this.itemForIndex = itemForIndex;
+        }
+
+        @Override
+        protected String create(int index) {
+            threads.add(Thread.currentThread().getName());
+            return itemForIndex.apply(index);
+        }
+    }
+}