spring-boot-demo/boot42-preview/src/test/java/com/ankurm/boot42/BootRun.java

package com.ankurm.boot42;

import java.util.Map;
import org.springframework.boot.WebApplicationType;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.env.Environment;
import org.springframework.web.client.RestClient;

/** Starts the real application with the given properties on a random port. */
final class BootRun {

	private BootRun() {
	}

	static ConfigurableApplicationContext start(String... props) {
		return new SpringApplicationBuilder(Boot42PreviewApplication.class)
				.web(WebApplicationType.SERVLET)
				.properties(props)
				.properties("server.port=0", "spring.main.banner-mode=off", "logging.level.root=WARN")
				.run();
	}

	static RestClient client(ConfigurableApplicationContext ctx) {
		Environment env = ctx.getEnvironment();
		return RestClient.create("http://localhost:" + env.getProperty("local.server.port"));
	}

	static Map<?, ?> whoAmI(ConfigurableApplicationContext ctx, Map<String, String> headers) {
		return client(ctx).get().uri("/whoami").headers(h -> headers.forEach(h::add)).retrieve().body(Map.class);
	}
}