Add tests for parallel execution and usage of Spring threads

spring-core-lab/src/main/java/pl/mperor/lab/spring/AsyncService.java

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+package pl.mperor.lab.spring;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.scheduling.annotation.Async;
+import org.springframework.stereotype.Service;
+
+import java.time.Duration;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executor;
+
+@Service
+public class AsyncService {
+
+    private static final Logger log = LoggerFactory.getLogger(AsyncService.class);
+
+    public CompletableFuture<String> delayedMethod(Duration duration, Executor executor) {
+        return CompletableFuture.supplyAsync(() -> task(duration), executor);
+    }
+
+    @Async
+    public CompletableFuture<String> delayedMethod(Duration duration) {
+        return CompletableFuture.completedFuture(task(duration));
+    }
+
+    private static String task(Duration duration) {
+        try {
+            var threadName = Thread.currentThread().getName();
+            log.info("Time to sleep 😴 [{}] ...", threadName);
+            Thread.sleep(duration);
+            return threadName;
+        } catch (InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

spring-core-lab/src/main/resources/application.yaml

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+#spring: # Spring ThreadPoolTaskExecutor customization
+#  task:
+#    execution:
+#      pool: # Doesn't have an effect if virtual threads are enabled.
+#        core-size: 5 # (default = 8)
+#        max-size: 10 # Ignored if the queue is unbounded.
+#        queue-capacity: 100
+#      thread-name-prefix: async-

spring-core-lab/src/test/java/pl/mperor/lab/spring/SpringAsyncConfigTest.java

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+package pl.mperor.lab.spring;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.scheduling.annotation.EnableAsync;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
+
+import java.time.Duration;
+import java.util.concurrent.CompletableFuture;
+import java.util.stream.IntStream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@EnableAsync
+@SpringBootTest
+class SpringAsyncConfigTest {
+
+    @Autowired
+    private ThreadPoolTaskExecutor executor;
+    @Autowired
+    private AsyncService asyncService;
+
+    @Test
+    void shouldHaveDefaultExecutorConfiguration() {
+        assertThat(executor).isNotNull();
+        assertThat(executor.getCorePoolSize()).isEqualTo(8);
+        assertThat(executor.getMaxPoolSize()).isEqualTo(Integer.MAX_VALUE);
+        assertThat(executor.getQueueCapacity()).isEqualTo(Integer.MAX_VALUE);
+        assertThat(executor.getKeepAliveSeconds()).isEqualTo(60);
+        assertThat(executor.getThreadNamePrefix()).isEqualTo("task-");
+    }
+
+    @Test
+    void shouldExecuteTasksInParallel() {
+        long start = System.currentTimeMillis();
+
+        var completableFutures = IntStream.range(0, 4)
+                .mapToObj(_ -> asyncService.delayedMethod(Duration.ofMillis(200)))
+                .toList();
+        var tasks = completableFutures.stream()
+                .map(CompletableFuture::join)
+                .toList();
+
+        long duration = System.currentTimeMillis() - start;
+
+        // 4 x 200ms sequentially = ~800ms
+        // In parallel = ~200ms, so we assert under 400ms
+        assertThat(duration).isLessThan(400);
+        assertThat(tasks).allMatch(task -> task.startsWith("task-"));
+    }
+
+    @Test
+    void shouldToRunAsynchronouslyWithoutAsyncAnnotation() {
+        var done = asyncService.delayedMethod(Duration.ofMillis(10), executor)
+                .join();
+        assertThat(done).startsWith("task-");
+    }
+}