Manage Tomcat queued connections

Adding two tomcat server properties:
- server.tomcat.accept-count
- server.tomcat.max-connections

Closes gh-6433

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java

@@ -86,6 +86,7 @@ import org.springframework.util.StringUtils;
  * @author Eddú Meléndez
  * @author Quinten De Swaef
  * @author Venil Noronha
+ * @author Aurélien Leboulanger
  */
 @ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)
 public class ServerProperties
@@ -656,6 +657,19 @@ public class ServerProperties
 		 */
 		private Charset uriEncoding;
 
+		/**
+		 * Maximum amount of connections accept and process.
+		 * <p>Once the limit has been reached,
+		 * the operating system may still accept connections based on the @link{acceptCount} setting.</p>
+		 */
+		private int maxConnections = 0;
+
+		/**
+		 * Maximum queue length for incoming connection requests when all possible request processing threads are in use.
+		 * Any requests received when the queue is full will be refused.
+		 */
+		private int acceptCount = 0;
+
 		public int getMaxThreads() {
 			return this.maxThreads;
 		}
@@ -748,6 +762,22 @@ public class ServerProperties
 			this.uriEncoding = uriEncoding;
 		}
 
+		public int getMaxConnections() {
+			return this.maxConnections;
+		}
+
+		public void setMaxConnections(int maxConnections) {
+			this.maxConnections = maxConnections;
+		}
+
+		public int getAcceptCount() {
+			return this.acceptCount;
+		}
+
+		public void setAcceptCount(int acceptCount) {
+			this.acceptCount = acceptCount;
+		}
+
 		void customizeTomcat(ServerProperties serverProperties,
 				TomcatEmbeddedServletContainerFactory factory) {
 			if (getBasedir() != null) {
@@ -782,6 +812,40 @@ public class ServerProperties
 			if (this.redirectContextRoot != null) {
 				customizeRedirectContextRoot(factory, this.redirectContextRoot);
 			}
+			if (this.maxConnections > 0) {
+				customizeMaxConnections(factory);
+			}
+			if (this.acceptCount > 0) {
+				customizeAcceptCount(factory);
+			}
+		}
+
+		private void customizeAcceptCount(TomcatEmbeddedServletContainerFactory factory) {
+			factory.addConnectorCustomizers(new TomcatConnectorCustomizer() {
+
+				@Override
+				public void customize(Connector connector) {
+					ProtocolHandler handler = connector.getProtocolHandler();
+					if (handler instanceof AbstractProtocol) {
+						AbstractProtocol protocol = (AbstractProtocol) handler;
+						protocol.setBacklog(Tomcat.this.acceptCount);
+					}
+				}
+			});
+		}
+
+		private void customizeMaxConnections(TomcatEmbeddedServletContainerFactory factory) {
+			factory.addConnectorCustomizers(new TomcatConnectorCustomizer() {
+
+				@Override
+				public void customize(Connector connector) {
+					ProtocolHandler handler = connector.getProtocolHandler();
+					if (handler instanceof AbstractProtocol) {
+						AbstractProtocol protocol = (AbstractProtocol) handler;
+						protocol.setMaxConnections(Tomcat.this.maxConnections);
+					}
+				}
+			});
 		}
 
 		private void customizeConnectionTimeout(

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/ServerPropertiesTests.java

@@ -324,6 +324,22 @@ public class ServerPropertiesTests {
 		assertThat(this.properties.getTomcat().getMinSpareThreads()).isEqualTo(10);
 	}
 
+	@Test
+	public void testCustomizeTomcatAcceptCount() throws Exception {
+		Map<String, String> map = new HashMap<String, String>();
+		map.put("server.tomcat.accept-count", "10");
+		bindProperties(map);
+		assertThat(this.properties.getTomcat().getAcceptCount()).isEqualTo(10);
+	}
+
+	@Test
+	public void testCustomizeTomcatMaxConnections() throws Exception {
+		Map<String, String> map = new HashMap<String, String>();
+		map.put("server.tomcat.max-connections", "5");
+		bindProperties(map);
+		assertThat(this.properties.getTomcat().getMaxConnections()).isEqualTo(5);
+	}
+
 	@Test
 	public void customizeTomcatDisplayName() throws Exception {
 		Map<String, String> map = new HashMap<String, String>();