Because of spring-projects/spring-framework#33949

Author: edeandrea

chapter-4/chapter-4-spring-data-jpa/pom.xml

@@ -38,6 +38,11 @@
 			<artifactId>postgresql</artifactId>
 			<scope>runtime</scope>
 		</dependency>
+		<dependency>
+			<groupId>io.projectreactor</groupId>
+			<artifactId>reactor-core</artifactId>
+			<scope>runtime</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>