Ensure that Jetty fails to start when its thread pool is misconfigured

Previously, if Jetty's thread pool was misconfigured, the Server would
still start successfully.

When it is started, the Server examines its Connectors to determine
how many threads are required. If its thread pool does not meet the
Connectors' requirements, an IllegalStateException is thrown and the
Server fails to start. However, JettyEmbeddedServletContainer
temporarily removes the Server's Connectors while it is being started
so that requests will not be accepted until the application is ready.
This has the unwanted side-effect of causing a misconfigured thread
pool to go undetected as the Connectors' thread requirements are not
taken into consideration.

The verification of the thread pool configuration and the starting of
the Connectors is done in the Server's doStart() method which has
three steps that are of interest:

1. Verify the Server's thread pool configuration
2. Start any managed beans that have been added to the Server
3. Start the Server's Connectors.

To allow the thread pool configuration to be verified while still
preventing the Connectors from being started, the Connectors need to
be removed in step 2. This is achieved by registering a managed bean
with the Server that nulls out the Server's Connectors as part of its
doStart() method.

Closes gh-8917

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

@@ -27,6 +27,7 @@ import org.eclipse.jetty.server.NetworkConnector;
 import org.eclipse.jetty.server.Server;
 import org.eclipse.jetty.server.handler.HandlerCollection;
 import org.eclipse.jetty.server.handler.HandlerWrapper;
+import org.eclipse.jetty.util.component.AbstractLifeCycle;
 
 import org.springframework.boot.context.embedded.EmbeddedServletContainer;
 import org.springframework.boot.context.embedded.EmbeddedServletContainerException;
@@ -84,11 +85,21 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 	private void initialize() {
 		synchronized (this.monitor) {
 			try {
-				// Cache and clear the connectors to prevent requests being handled before
-				// the application context is ready
+				// Cache the connectors and then remove them to prevent requests being
+				// handled before the application context is ready.
 				this.connectors = this.server.getConnectors();
-				this.server.setConnectors(null);
+				this.server.addBean(new AbstractLifeCycle() {
 
+					@Override
+					protected void doStart() throws Exception {
+						for (Connector connector : JettyEmbeddedServletContainer.this.connectors) {
+							Assert.state(connector.isStopped(), "Connector " + connector
+									+ " has been started prematurely");
+						}
+						JettyEmbeddedServletContainer.this.server.setConnectors(null);
+					}
+
+				});
 				// Start the server so that the ServletContext is available
 				this.server.start();
 				this.server.setStopAtShutdown(false);

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

@@ -42,6 +42,7 @@ import org.eclipse.jetty.server.SslConnectionFactory;
 import org.eclipse.jetty.server.handler.HandlerCollection;
 import org.eclipse.jetty.server.handler.HandlerWrapper;
 import org.eclipse.jetty.servlet.ServletHolder;
+import org.eclipse.jetty.util.thread.QueuedThreadPool;
 import org.eclipse.jetty.util.thread.ThreadPool;
 import org.eclipse.jetty.webapp.Configuration;
 import org.eclipse.jetty.webapp.WebAppContext;
@@ -59,6 +60,7 @@ import org.springframework.boot.web.servlet.ServletRegistrationBean;
 import org.springframework.http.HttpHeaders;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.hamcrest.CoreMatchers.instanceOf;
 import static org.mockito.Matchers.anyObject;
 import static org.mockito.Mockito.inOrder;
 import static org.mockito.Mockito.mock;
@@ -322,6 +324,22 @@ public class JettyEmbeddedServletContainerFactoryTests
 		factory.getEmbeddedServletContainer().start();
 	}
 
+	@Test
+	public void startFailsWhenThreadPoolIsTooSmall() throws Exception {
+		JettyEmbeddedServletContainerFactory factory = getFactory();
+		factory.addServerCustomizers(new JettyServerCustomizer() {
+
+			@Override
+			public void customize(Server server) {
+				QueuedThreadPool threadPool = server.getBean(QueuedThreadPool.class);
+				threadPool.setMaxThreads(2);
+				threadPool.setMinThreads(2);
+			}
+		});
+		this.thrown.expectCause(instanceOf(IllegalStateException.class));
+		factory.getEmbeddedServletContainer().start();
+	}
+
 	@Override
 	@SuppressWarnings("serial")
 	// Workaround for Jetty issue - https://bugs.eclipse.org/bugs/show_bug.cgi?id=470646