test(concurrency): improve deadlock detection in BankAccount transfer stress test

src/test/java/banking/BankAccountTransferDeadlockTest.java

@@ -3,11 +3,10 @@ package banking;
 import dev.banking.model.BankAccount;
 import org.junit.jupiter.api.Test;
 
+import java.time.Duration;
 import java.util.concurrent.*;
 
-import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
-
-import java.time.Duration;
+import static org.junit.jupiter.api.Assertions.*;
 
 public class BankAccountTransferDeadlockTest {
 
@@ -19,17 +18,52 @@ public class BankAccountTransferDeadlockTest {
             BankAccount a = new BankAccount(1, 1_000_000);
             BankAccount b = new BankAccount(2, 1_000_000);
 
+            int taskCount = 50_000;
+
             ExecutorService executor = Executors.newFixedThreadPool(50);
 
-            for (int i = 0; i < 50_000; i++) {
+            CountDownLatch startGun = new CountDownLatch(1);
+            CountDownLatch finishLine = new CountDownLatch(taskCount * 2);
 
-                executor.submit(() -> a.transfer(b, 1));
-                executor.submit(() -> b.transfer(a, 1));
+            for (int i = 0; i < taskCount; i++) {
+
+                executor.submit(() -> {
+                    try {
+                        startGun.await();
+                        a.transfer(b, 1);
+                    } catch (InterruptedException ignored) {
+                        Thread.currentThread().interrupt();
+                    } finally {
+                        finishLine.countDown();
+                    }
+                });
+
+                executor.submit(() -> {
+                    try {
+                        startGun.await();
+                        b.transfer(a, 1);
+                    } catch (InterruptedException ignored) {
+                        Thread.currentThread().interrupt();
+                    } finally {
+                        finishLine.countDown();
+                    }
+                });
+            }
+
+            startGun.countDown();
+
+            boolean completed;
+
+            try {
+                completed = finishLine.await(5, TimeUnit.SECONDS);
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                throw new RuntimeException(e);
             }
 
             executor.shutdown();
 
-            executor.awaitTermination(5, TimeUnit.SECONDS);
+            assertTrue(completed, "Deadlock detected: tasks did not complete in time");
         });
     }
 }