Ensure embedded start can be called multiple times

Update all `EmbeddedServletContainer` implementations to ensure that
the `start()` method can be called multiple times, with the second call
being ignored.

Fixes gh-8036

spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyEmbeddedServletContainer.java

@@ -59,6 +59,8 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 
 	private Connector[] connectors;
 
+	private volatile boolean started;
+
 	/**
 	 * Create a new {@link JettyEmbeddedServletContainer} instance.
 	 * @param server the underlying Jetty server
@@ -111,6 +113,10 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 
 	@Override
 	public void start() throws EmbeddedServletContainerException {
+		synchronized (this.monitor) {
+			if (this.started) {
+				return;
+			}
 			this.server.setConnectors(this.connectors);
 			if (!this.autoStart) {
 				return;
@@ -133,6 +139,7 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 						throw ex;
 					}
 				}
+				this.started = true;
 				JettyEmbeddedServletContainer.logger
 						.info("Jetty started on port(s) " + getActualPortsDescription());
 			}
@@ -144,6 +151,7 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 						"Unable to start embedded Jetty servlet container", ex);
 			}
 		}
+	}
 
 	private String getActualPortsDescription() {
 		StringBuilder ports = new StringBuilder();
@@ -197,6 +205,10 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 	@Override
 	public void stop() {
 		synchronized (this.monitor) {
+			if (!this.started) {
+				return;
+			}
+			this.started = false;
 			try {
 				this.server.stop();
 			}

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainer.java

@@ -62,6 +62,8 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 
 	private final boolean autoStart;
 
+	private volatile boolean started;
+
 	/**
 	 * Create a new {@link TomcatEmbeddedServletContainer} instance.
 	 * @param tomcat the underlying Tomcat server
@@ -174,6 +176,10 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 
 	@Override
 	public void start() throws EmbeddedServletContainerException {
+		synchronized (this.monitor) {
+			if (this.started) {
+				return;
+			}
 			try {
 				addPreviouslyRemovedConnectors();
 				Connector connector = this.tomcat.getConnector();
@@ -181,6 +187,7 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 					startConnector(connector);
 				}
 				checkThatConnectorsHaveStarted();
+				this.started = true;
 				TomcatEmbeddedServletContainer.logger
 						.info("Tomcat started on port(s): " + getPortsDescription(true));
 			}
@@ -198,6 +205,7 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 						getClass().getClassLoader());
 			}
 		}
+	}
 
 	private void checkThatConnectorsHaveStarted() {
 		for (Connector connector : this.tomcat.getService().findConnectors()) {
@@ -271,7 +279,11 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 	@Override
 	public void stop() throws EmbeddedServletContainerException {
 		synchronized (this.monitor) {
+			if (!this.started) {
+				return;
+			}
 			try {
+				this.started = false;
 				try {
 					stopTomcat();
 					this.tomcat.destroy();

spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowEmbeddedServletContainer.java

@@ -88,7 +88,7 @@ public class UndertowEmbeddedServletContainer implements EmbeddedServletContaine
 
 	private Undertow undertow;
 
-	private boolean started = false;
+	private volatile boolean started = false;
 
 	/**
 	 * Create a new {@link UndertowEmbeddedServletContainer} instance.
@@ -201,6 +201,9 @@ public class UndertowEmbeddedServletContainer implements EmbeddedServletContaine
 	@Override
 	public void start() throws EmbeddedServletContainerException {
 		synchronized (this.monitor) {
+			if (this.started) {
+				return;
+			}
 			try {
 				if (!this.autoStart) {
 					return;
@@ -362,9 +365,11 @@ public class UndertowEmbeddedServletContainer implements EmbeddedServletContaine
 	@Override
 	public void stop() throws EmbeddedServletContainerException {
 		synchronized (this.monitor) {
-			if (this.started) {
+			if (!this.started) {
-				try {
+				return;
+			}
 			this.started = false;
+			try {
 				this.manager.stop();
 				this.undertow.stop();
 			}
@@ -374,7 +379,6 @@ public class UndertowEmbeddedServletContainer implements EmbeddedServletContaine
 			}
 		}
 	}
-	}
 
 	@Override
 	public int getPort() {

spring-boot/src/test/java/org/springframework/boot/context/embedded/AbstractEmbeddedServletContainerFactoryTests.java

@@ -79,6 +79,7 @@ import org.mockito.InOrder;
 import org.springframework.boot.ApplicationHome;
 import org.springframework.boot.ApplicationTemp;
 import org.springframework.boot.context.embedded.Ssl.ClientAuth;
+import org.springframework.boot.testutil.InternalOutputCapture;
 import org.springframework.boot.web.servlet.ErrorPage;
 import org.springframework.boot.web.servlet.FilterRegistrationBean;
 import org.springframework.boot.web.servlet.ServletContextInitializer;
@@ -120,6 +121,9 @@ public abstract class AbstractEmbeddedServletContainerFactoryTests {
 	@Rule
 	public TemporaryFolder temporaryFolder = new TemporaryFolder();
 
+	@Rule
+	public InternalOutputCapture output = new InternalOutputCapture();
+
 	protected EmbeddedServletContainer container;
 
 	private final HttpClientContext httpClientContext = HttpClientContext.create();
@@ -153,6 +157,19 @@ public abstract class AbstractEmbeddedServletContainerFactoryTests {
 		assertThat(getResponse(getLocalUrl("/hello"))).isEqualTo("Hello World");
 	}
 
+	@Test
+	public void startCalledTwice() throws Exception {
+		AbstractEmbeddedServletContainerFactory factory = getFactory();
+		this.container = factory
+				.getEmbeddedServletContainer(exampleServletRegistration());
+		this.container.start();
+		int port = this.container.getPort();
+		this.container.start();
+		assertThat(this.container.getPort()).isEqualTo(port);
+		assertThat(getResponse(getLocalUrl("/hello"))).isEqualTo("Hello World");
+		assertThat(this.output.toString()).containsOnlyOnce("started on port");
+	}
+
 	@Test
 	public void emptyServerWhenPortIsMinusOne() throws Exception {
 		AbstractEmbeddedServletContainerFactory factory = getFactory();