Add examples using Virtual Threads.

Closes #665

jpa/example/src/main/java/example/springdata/jpa/simple/SimpleConfiguration.java

@@ -16,9 +16,11 @@
 package example.springdata.jpa.simple;
 
 import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.scheduling.annotation.EnableAsync;
 
 /**
  * @author Oliver Gierke
  */
 @SpringBootApplication
+@EnableAsync
 class SimpleConfiguration {}

jpa/example/src/main/java/example/springdata/jpa/simple/SimpleUserRepository.java

@@ -24,7 +24,7 @@ import org.springframework.data.domain.Pageable;
 import org.springframework.data.domain.Slice;
 import org.springframework.data.domain.Sort;
 import org.springframework.data.jpa.repository.Query;
-import org.springframework.data.repository.CrudRepository;
+import org.springframework.data.repository.ListCrudRepository;
 import org.springframework.scheduling.annotation.Async;
 
 /**
@@ -35,7 +35,7 @@ import org.springframework.scheduling.annotation.Async;
  * @author Thomas Darimont
  * @author Christoph Strobl
  */
-public interface SimpleUserRepository extends CrudRepository<User, Long> {
+public interface SimpleUserRepository extends ListCrudRepository<User, Long> {
 
 	/**
 	 * Find the user with the given username. This method will be translated into a query using the

jpa/example/src/test/java/example/springdata/jpa/simple/VirtualThreadsTests.java

@@ -0,0 +1,114 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.springdata.jpa.simple;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.util.List;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.TimeUnit;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledOnJre;
+import org.junit.jupiter.api.condition.JRE;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.core.task.SimpleAsyncTaskExecutor;
+import org.springframework.transaction.annotation.Propagation;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * Integration test showing the basic usage of {@link SimpleUserRepository} with Virtual Threads.
+ *
+ * @author Mark Paluch
+ */
+@Transactional
+@SpringBootTest(properties = "spring.threads.virtual.enabled=true")
+@EnabledOnJre(JRE.JAVA_21)
+class VirtualThreadsTests {
+
+	@Autowired SimpleUserRepository repository;
+	private User user;
+
+	@BeforeEach
+	void setUp() {
+
+		user = new User();
+		user.setUsername("foobar");
+		user.setFirstname("firstname");
+		user.setLastname("lastname");
+	}
+
+	/**
+	 * This repository invocation runs on a dedicated virtual thread.
+	 */
+	@Test
+	@Transactional(propagation = Propagation.NOT_SUPPORTED)
+	void supportsVirtualThreads() throws Exception {
+
+		BlockingQueue<String> thread = new LinkedBlockingQueue<>();
+		repository.save(new User("Customer1", "Foo"));
+		repository.save(new User("Customer2", "Bar"));
+
+		try (SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor()) {
+			executor.setVirtualThreads(true);
+
+			var future = executor.submit(() -> {
+				thread.add(Thread.currentThread().toString());
+				return repository.findAll();
+			});
+
+			List<User> users = future.get();
+			String threadName = thread.poll(1, TimeUnit.SECONDS);
+
+			assertThat(threadName).contains("VirtualThread");
+			assertThat(users).hasSize(2);
+		}
+
+		repository.deleteAll();
+	}
+
+	/**
+	 * Here we demonstrate the usage of {@link CompletableFuture} as a result wrapper for asynchronous repository query
+	 * methods running on Virtual Threads. Note, that we need to disable the surrounding transaction to be able to
+	 * asynchronously read the written data from another thread within the same test method.
+	 */
+	@Test
+	@Transactional(propagation = Propagation.NOT_SUPPORTED)
+	void asyncUsesVirtualThreads() throws Exception {
+
+		BlockingQueue<String> thread = new LinkedBlockingQueue<>();
+		repository.save(new User("Customer1", "Foo"));
+		repository.save(new User("Customer2", "Bar"));
+
+		var future = repository.readAllBy().thenAccept(users -> {
+
+			assertThat(users).hasSize(2);
+			thread.add(Thread.currentThread().toString());
+		});
+
+		future.join();
+		String threadName = thread.poll(1, TimeUnit.SECONDS);
+
+		assertThat(threadName).contains("VirtualThread");
+
+		repository.deleteAll();
+	}
+}