implementation 'org.springframework.boot:spring-boot-starter' implementation 'org.springframework.boot:spring-boot-starter-web' implementation 'org.springframework.boot:spring-boot-starter-webflux' testImplementation 'org.springframework.boot:spring-boot-starter-test' 

@SpringBootTest(webEnvironment = RANDOM_PORT) class DemoApplicationTests { @LocalServerPort private int randomServerPort; private RestClient restClient; @BeforeEach void setup() { restClient = RestClient.builder() .requestFactory(new ReactorNettyClientRequestFactory()) // depending on this line, the test below fails/succeeds .baseUrl("http://localhost:" + randomServerPort + "/fakeServer") .build(); } @Test void failure() { for (int i = 0; i < 70000; i++) { try { restClient.get() .uri("/") .retrieve() .body(String.class); } catch (Exception e) { throw new RuntimeException("failed at " + i, e); } } } } 

@RestController @RequestMapping("/fakeServer") public class FakeServer { @GetMapping("/") public String getTest() { return "test"; } } 

java.lang.RuntimeException: failed at 15566	at com.example.demo.DemoApplicationTests.failure(DemoApplicationTests.java:37)	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)	at java.base/java.lang.reflect.Method.invoke(Method.java:580)	at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:728)	at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)	at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:156)	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:147)	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:86)	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(InterceptingExecutableInvoker.java:103)	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.lambda$invoke$0(InterceptingExecutableInvoker.java:93)	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:92)	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:86)	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$7(TestMethodTestDescriptor.java:218)	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:214)	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:139)	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:69)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:151)	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:41)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:155)	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:41)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:155)	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:35)	at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)	at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:54)	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:107)	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88)	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54)	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67)	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52)	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:114)	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:86)	at org.junit.platform.launcher.core.DefaultLauncherSession$DelegatingLauncher.execute(DefaultLauncherSession.java:86)	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.processAllTestClasses(JUnitPlatformTestClassProcessor.java:119)	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.access$000(JUnitPlatformTestClassProcessor.java:94)	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor.stop(JUnitPlatformTestClassProcessor.java:89)	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.stop(SuiteTestClassProcessor.java:62)	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)	at java.base/java.lang.reflect.Method.invoke(Method.java:580)	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)	at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)	at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:94)	at jdk.proxy1/jdk.proxy1.$Proxy2.stop(Unknown Source)	at org.gradle.api.internal.tasks.testing.worker.TestWorker$3.run(TestWorker.java:193)	at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:129)	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:100)	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:60)	at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:113)	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:65)	at worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)	at worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74) Caused by: org.springframework.web.client.ResourceAccessException: I/O error on GET request for "http://localhost:50951/fakeServer/": Address already in use: getsockopt: localhost/127.0.0.1:50951	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.createResourceAccessException(DefaultRestClient.java:557)	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.exchangeInternal(DefaultRestClient.java:482)	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.retrieve(DefaultRestClient.java:444)	at com.example.demo.DemoApplicationTests.failure(DemoApplicationTests.java:34)	... 80 more Caused by: io.netty.channel.AbstractChannel$AnnotatedSocketException: Address already in use: getsockopt: localhost/127.0.0.1:50951 Caused by: java.net.BindException: Address already in use: getsockopt	at java.base/sun.nio.ch.Net.pollConnect(Native Method)	at java.base/sun.nio.ch.Net.pollConnectNow(Net.java:682)	at java.base/sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:973)	at io.netty.channel.socket.nio.NioSocketChannel.doFinishConnect(NioSocketChannel.java:337)	at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.finishConnect(AbstractNioChannel.java:335)	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:776)	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724)	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650)	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562)	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)	at java.base/java.lang.Thread.run(Thread.java:1583)