Merge pull request #5572 from Quinten De Swaef

* gh-5572:
  Polish contribution
  Allow Tomcat's minimum threads to be configured via the environment

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

@@ -72,6 +72,7 @@ import org.springframework.util.StringUtils;
  * @author Ivan Sopov
  * @author Marcos Barbero
  * @author Eddú Meléndez
+ * @author Quinten De Swaef
  */
 @ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)
 public class ServerProperties
@@ -580,6 +581,11 @@ public class ServerProperties
 		 */
 		private int maxThreads = 0; // Number of threads in protocol handler
 
+		/**
+		 * Minimum amount of worker threads.
+		 */
+		private int minSpareThreads = 0; // Minimum spare threads in protocol handler
+
 		/**
 		 * Maximum size in bytes of the HTTP message header.
 		 */
@@ -598,6 +604,14 @@ public class ServerProperties
 			this.maxThreads = maxThreads;
 		}
 
+		public int getMinSpareThreads() {
+			return this.minSpareThreads;
+		}
+
+		public void setMinSpareThreads(int minSpareThreads) {
+			this.minSpareThreads = minSpareThreads;
+		}
+
 		public int getMaxHttpHeaderSize() {
 			return this.maxHttpHeaderSize;
 		}
@@ -684,6 +698,9 @@ public class ServerProperties
 			if (this.maxThreads > 0) {
 				customizeMaxThreads(factory);
 			}
+			if (this.minSpareThreads > 0) {
+				customizeMinThreads(factory);
+			}
 			if (this.maxHttpHeaderSize > 0) {
 				customizeMaxHttpHeaderSize(factory);
 			}
@@ -747,6 +764,22 @@ 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

@@ -62,6 +62,7 @@ import static org.mockito.Mockito.verify;
  * @author Andy Wilkinson
  * @author Phillip Webb
  * @author Eddú Meléndez
+ * @author Quinten De Swaef
  */
 public class ServerPropertiesTests {
 
@@ -258,6 +259,14 @@ 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>();

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -204,6 +204,7 @@ content into your application; rather pick only the properties that you need.
 			172\\.3[0-1]{1}\\.\\d{1,3}\\.\\d{1,3} # regular expression matching trusted IP addresses.
 	server.tomcat.max-http-header-size=0 # Maximum size in bytes of the HTTP message header.
 	server.tomcat.max-threads=0 # Maximum amount of worker threads.
+	server.tomcat.min-spare-threads=0 # Minimum amount of worker threads.
 	server.tomcat.port-header=X-Forwarded-Port # Name of the HTTP header used to override the original port value.
 	server.tomcat.protocol-header= # Header that holds the incoming protocol, usually named "X-Forwarded-Proto".
 	server.tomcat.protocol-header-https-value=https # Value of the protocol header that indicates that the incoming request uses SSL.