Add Jetty acceptors and selectors support

Add support for Jetty `acceptors` and `selectors` configuration, either
directly on the `JettyEmbeddedServletContainerFactory` or via
`server.jetty.acceptors`/`server.jetty.selectors` server properties.

Fixes gh-5380
Closes gh-5649

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

@@ -402,7 +402,7 @@ public class ServerProperties
 		return this.tomcat;
 	}
 
-	private Jetty getJetty() {
+	public Jetty getJetty() {
 		return this.jetty;
 	}
 
@@ -950,11 +950,43 @@ public class ServerProperties
 
 	}
 
-	private static class Jetty {
+	public static class Jetty {
+
+		/**
+		 * Number of acceptor threads to use.
+		 */
+		private Integer acceptors;
+
+		/**
+		 * Number of selector threads to use.
+		 */
+		private Integer selectors;
+
+		public Integer getAcceptors() {
+			return this.acceptors;
+		}
+
+		public void setAcceptors(Integer acceptors) {
+			this.acceptors = acceptors;
+		}
+
+		public Integer getSelectors() {
+			return this.selectors;
+		}
+
+		public void setSelectors(Integer selectors) {
+			this.selectors = selectors;
+		}
 
 		void customizeJetty(ServerProperties serverProperties,
 				JettyEmbeddedServletContainerFactory factory) {
 			factory.setUseForwardHeaders(serverProperties.getOrDeduceUseForwardHeaders());
+			if (this.acceptors != null) {
+				factory.setAcceptors(this.acceptors);
+			}
+			if (this.selectors != null) {
+				factory.setSelectors(this.selectors);
+			}
 			if (serverProperties.getMaxHttpHeaderSize() > 0) {
 				customizeMaxHttpHeaderSize(factory,
 						serverProperties.getMaxHttpHeaderSize());

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

@@ -64,6 +64,7 @@ import static org.mockito.Mockito.verify;
  * @author Phillip Webb
  * @author Eddú Meléndez
  * @author Quinten De Swaef
+ * @author Venil Noronha
  */
 public class ServerPropertiesTests {
 
@@ -260,6 +261,30 @@ public class ServerPropertiesTests {
 		assertThat(this.properties.getMaxHttpHeaderSize()).isEqualTo(9999);
 	}
 
+	@Test
+	public void testCustomizePostSize() throws Exception {
+		Map<String, String> map = new HashMap<String, String>();
+		map.put("server.maxHttpPostSize", "9999");
+		bindProperties(map);
+		assertThat(this.properties.getMaxHttpPostSize()).isEqualTo(9999);
+	}
+
+	@Test
+	public void testCustomizeJettyAcceptors() throws Exception {
+		Map<String, String> map = new HashMap<String, String>();
+		map.put("server.jetty.acceptors", "10");
+		bindProperties(map);
+		assertThat(this.properties.getJetty().getAcceptors()).isEqualTo(10);
+	}
+
+	@Test
+	public void testCustomizeJettySelectors() throws Exception {
+		Map<String, String> map = new HashMap<String, String>();
+		map.put("server.jetty.selectors", "10");
+		bindProperties(map);
+		assertThat(this.properties.getJetty().getSelectors()).isEqualTo(10);
+	}
+
 	@Test
 	public void testCustomizeTomcatMinSpareThreads() throws Exception {
 		Map<String, String> map = new HashMap<String, String>();