Refine semantics of server.port=0

If the server port is 0 it doesn't mean you don't want *any* container
because there might still be servlet context aware components in the
classpath. Better to try and stop the server from actually starting.

This change is a step in the right direction: it prevents the server
from starting in the start() method (even though for technical reasons
it may have already been started and stopped in initialize()).

N.B. Tomcat at least uses port=0 as a signal it should pick a random
port. Pertty useful feature that we may want to capitalize on at some
point (on the other hand having our own port scanner would be useful
generally too).

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

@@ -33,12 +33,22 @@ import org.springframework.util.Assert;
 public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 
 	private final Server server;
+	private boolean autoStart;
 
 	/**
 	 * Create a new {@link JettyEmbeddedServletContainer} instance.
 	 * @param server the underlying Jetty server
 	 */
 	public JettyEmbeddedServletContainer(Server server) {
+		this(server, true);
+	}
+
+	/**
+	 * Create a new {@link JettyEmbeddedServletContainer} instance.
+	 * @param server the underlying Jetty server
+	 */
+	public JettyEmbeddedServletContainer(Server server, boolean autoStart) {
+		this.autoStart = autoStart;
 		Assert.notNull(server, "Jetty Server must not be null");
 		this.server = server;
 		initialize();
@@ -63,6 +73,9 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 
 	@Override
 	public void start() throws EmbeddedServletContainerException {
+		if (!this.autoStart) {
+			return;
+		}
 		try {
 			this.server.start();
 			Connector[] connectors = this.server.getConnectors();

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

@@ -97,9 +97,6 @@ public class JettyEmbeddedServletContainerFactory extends
 	public EmbeddedServletContainer getEmbeddedServletContainer(
 			ServletContextInitializer... initializers) {
 		WebAppContext context = new WebAppContext();
-		if (getPort() == 0) {
-			return EmbeddedServletContainer.NONE;
-		}
 		Server server = new Server(new InetSocketAddress(getAddress(), getPort()));
 
 		if (this.resourceLoader != null) {
@@ -243,7 +240,7 @@ public class JettyEmbeddedServletContainerFactory extends
 	 * @return a new {@link JettyEmbeddedServletContainer} instance
 	 */
 	protected JettyEmbeddedServletContainer getJettyEmbeddedServletContainer(Server server) {
-		return new JettyEmbeddedServletContainer(server);
+		return new JettyEmbeddedServletContainer(server, getPort() > 0);
 	}
 
 	@Override

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

@@ -43,11 +43,22 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 
 	private final Tomcat tomcat;
 
+	private boolean autoStart;
+
 	/**
 	 * Create a new {@link TomcatEmbeddedServletContainer} instance.
 	 * @param tomcat the underlying Tomcat server
 	 */
 	public TomcatEmbeddedServletContainer(Tomcat tomcat) {
+		this(tomcat, true);
+	}
+
+	/**
+	 * Create a new {@link TomcatEmbeddedServletContainer} instance.
+	 * @param tomcat the underlying Tomcat server
+	 */
+	public TomcatEmbeddedServletContainer(Tomcat tomcat, boolean autoStart) {
+		this.autoStart = autoStart;
 		Assert.notNull(tomcat, "Tomcat Server must not be null");
 		this.tomcat = tomcat;
 		initialize();
@@ -90,7 +101,7 @@ public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer
 	@Override
 	public void start() throws EmbeddedServletContainerException {
 		Connector connector = this.tomcat.getConnector();
-		if (connector != null) {
+		if (connector != null && this.autoStart) {
 			try {
 				connector.getProtocolHandler().start();
 			}

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

@@ -123,9 +123,6 @@ public class TomcatEmbeddedServletContainerFactory extends
 		Connector connector;
 		Tomcat tomcat = new Tomcat();
 
-		if (getPort() == 0) {
-			return EmbeddedServletContainer.NONE;
-		}
 		File baseDir = (this.baseDirectory != null ? this.baseDirectory
 				: createTempDir("tomcat"));
 		tomcat.setBaseDir(baseDir.getAbsolutePath());
@@ -195,7 +192,12 @@ public class TomcatEmbeddedServletContainerFactory extends
 
 	// Needs to be protected so it can be used by subclasses
 	protected void customizeConnector(Connector connector) {
-		connector.setPort(getPort());
+		if (getPort() > 0) {
+			connector.setPort(getPort());
+		}
+		else {
+			connector.setPort(8080);
+		}
 		if (connector.getProtocolHandler() instanceof AbstractProtocol) {
 			if (getAddress() != null) {
 				((AbstractProtocol) connector.getProtocolHandler())
@@ -256,7 +258,7 @@ public class TomcatEmbeddedServletContainerFactory extends
 	 */
 	protected TomcatEmbeddedServletContainer getTomcatEmbeddedServletContainer(
 			Tomcat tomcat) {
-		return new TomcatEmbeddedServletContainer(tomcat);
+		return new TomcatEmbeddedServletContainer(tomcat, getPort() > 0);
 	}
 
 	private File createTempDir(String prefix) {