Allow Tomcat's minimum threads to be configured via the environment

Closes gh-5572

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

@@ -580,6 +580,11 @@ public class ServerProperties
 		 */
 		private int maxThreads = 0; // Number of threads in protocol handler
 
+		/**
+		 * Minimum amount of worker threads.
+		 */
+		private int minSpareThreads = 0; //Number of minimum spare threads in protocol handler
+
 		/**
 		 * Maximum size in bytes of the HTTP message header.
 		 */
@@ -598,6 +603,14 @@ public class ServerProperties
 			this.maxThreads = maxThreads;
 		}
 
+		public int getMinSpareThreads() {
+			return minSpareThreads;
+		}
+
+		public void setMinSpareThreads(int minSpareThreads) {
+			this.minSpareThreads = minSpareThreads;
+		}
+
 		public int getMaxHttpHeaderSize() {
 			return this.maxHttpHeaderSize;
 		}
@@ -684,6 +697,9 @@ public class ServerProperties
 			if (this.maxThreads > 0) {
 				customizeMaxThreads(factory);
 			}
+			if (this.minSpareThreads > 0) {
+				customizeMinThreads(factory);
+			}
 			if (this.maxHttpHeaderSize > 0) {
 				customizeMaxHttpHeaderSize(factory);
 			}
@@ -747,6 +763,22 @@ public class ServerProperties
 			});
 		}
 
+		@SuppressWarnings("rawtypes")
+		private void customizeMinThreads(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.setMinSpareThreads(Tomcat.this.minSpareThreads);
+					}
+
+				}
+			});
+		}
+
 		@SuppressWarnings("rawtypes")
 		private void customizeMaxHttpHeaderSize(
 				TomcatEmbeddedServletContainerFactory factory) {

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

@@ -258,6 +258,14 @@ public class ServerPropertiesTests {
 		assertThat(this.properties.getTomcat().getMaxHttpHeaderSize()).isEqualTo(9999);
 	}
 
+	@Test
+	public void testCustomizeTomcatMinSpareThreads() throws Exception {
+		Map<String, String> map = new HashMap<String, String>();
+		map.put("server.tomcat.min-spare-threads", "10");
+		bindProperties(map);
+		assertThat(this.properties.getTomcat().getMinSpareThreads()).isEqualTo(10);
+	}
+
 	@Test
 	public void customizeTomcatDisplayName() throws Exception {
 		Map<String, String> map = new HashMap<String, String>();