JDK13Test.java

package com.example.demo;

import org.junit.Assert;
import org.junit.Test;
import reactor.blockhound.BlockHound;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

public class JDK13Test {

	@Test
	public void shouldWork() {
		Assert.assertTrue(System.getProperty("java.version").startsWith("13."));
		BlockHound.install();

		try {
			Mono.fromCallable(() -> {
				Thread.sleep(0);
				return null;
			}).hide().subscribeOn(Schedulers.parallel()).block();
			assert false : "should fail";
		} catch (Exception e) {
			Assert.assertTrue(e.getMessage().contains("Blocking call"));
		}
	}

}

output.txt

$ mvn test
[INFO] Scanning for projects...
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.example:demo:jar:0.0.1-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-surefire-plugin is missing. @ line 39, column 12
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]
[INFO]
[INFO] --------------------------< com.example:demo >--------------------------
[INFO] Building demo 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/bsideup/Downloads/demo 2/src/main/resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ demo ---
[INFO] No sources to compile
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/bsideup/Downloads/demo 2/src/test/resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ demo ---
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ demo ---
[INFO] Surefire report directory: /Users/bsideup/Downloads/demo 2/target/surefire-reports

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
OpenJDK 64-Bit Server VM warning: Option AllowRedefinitionToAddDeleteMethods was deprecated in version 13.0 and will likely be removed in a future release.
Running com.example.demo.JDK13Test
OpenJDK 64-Bit Server VM warning: Sharing is only supported for boot loader classes because bootstrap classpath has been appended
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.208 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  2.281 s
[INFO] Finished at: 2019-10-31T14:58:42+01:00
[INFO] ------------------------------------------------------------------------

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.example</groupId>
	<artifactId>demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<properties>
		<maven.compiler.source>13</maven.compiler.source>
		<maven.compiler.target>13</maven.compiler.target>
	</properties>

	<dependencies>
		<dependency>
			<groupId>io.projectreactor.tools</groupId>
			<artifactId>blockhound</artifactId>
			<version>1.0.1.RELEASE</version>
			<scope>test</scope>
		</dependency>

		<dependency>
			<groupId>io.projectreactor</groupId>
			<artifactId>reactor-core</artifactId>
			<version>3.3.0.RELEASE</version>
			<scope>test</scope>
		</dependency>

		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<scope>test</scope>
			<version>4.12</version>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<artifactId>maven-surefire-plugin</artifactId>
				<configuration>
					<argLine>-XX:+AllowRedefinitionToAddDeleteMethods</argLine>
				</configuration>
			</plugin>
		</plugins>
	</build>

</project>