Merge branch '2.4.x' into 2.5.x

Closes gh-28159

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/testing.adoc

@@ -487,7 +487,10 @@ include::{docs-java}/features/testing/springbootapplications/autoconfiguredsprin
 ----
 
 Data JPA tests may also inject a {spring-boot-test-autoconfigure-module-code}/orm/jpa/TestEntityManager.java[`TestEntityManager`] bean, which provides an alternative to the standard JPA `EntityManager` that is specifically designed for tests.
-If you want to use `TestEntityManager` outside of `@DataJpaTest` instances, you can also use the `@AutoConfigureTestEntityManager` annotation.
+
+TIP: `TestEntityManager` can also be auto-configured to any of your Spring-based test class by adding `@AutoConfigureTestEntityManager`.
+When doing so, make sure that your test is running in a transaction, for instance by adding  `@Transactional` on your test class or method.
+
 A `JdbcTemplate` is also available if you need that.
 The following example shows the `@DataJpaTest` annotation in use:
 

spring-boot-project/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/orm/jpa/TestEntityManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -234,7 +234,7 @@ public <T> T getId(Object entity, Class<T> idType) {
 	 */
 	public final EntityManager getEntityManager() {
 		EntityManager manager = EntityManagerFactoryUtils.getTransactionalEntityManager(this.entityManagerFactory);
-		Assert.state(manager != null, "No transactional EntityManager found");
+		Assert.state(manager != null, "No transactional EntityManager found, is your test running in a transactional?");
 		return manager;
 	}