Add support for 'server.jetty.max-form-key' property

Add a new 'server.jetty.max-form-key' property that can be used to
configure Jetty's Handler.setMaxFormKeys(...).

See gh-42448

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

@@ -1148,6 +1148,11 @@ public class ServerProperties {
 		 */
 		private DataSize maxHttpFormPostSize = DataSize.ofBytes(200000);
 
+		/**
+		 * Maximum number of form keys.
+		 */
+		private int maxFormKeys = 1000;
+
 		/**
 		 * Time that the connection can be idle before it is closed.
 		 */
@@ -1180,6 +1185,14 @@ public class ServerProperties {
 			this.maxHttpFormPostSize = maxHttpFormPostSize;
 		}
 
+		public int getMaxFormKeys() {
+			return this.maxFormKeys;
+		}
+
+		public void setMaxFormKeys(int maxFormKeys) {
+			this.maxFormKeys = maxFormKeys;
+		}
+
 		public Duration getConnectionIdleTimeout() {
 			return this.connectionIdleTimeout;
 		}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/JettyWebServerFactoryCustomizer.java

@@ -19,6 +19,7 @@ package org.springframework.boot.autoconfigure.web.embedded;
 import java.time.Duration;
 import java.util.Arrays;
 import java.util.List;
+import java.util.function.Consumer;
 
 import org.eclipse.jetty.ee10.servlet.ServletContextHandler;
 import org.eclipse.jetty.server.AbstractConnector;
@@ -93,7 +94,11 @@ public class JettyWebServerFactoryCustomizer
 		map.from(properties::getMaxHttpFormPostSize)
 			.asInt(DataSize::toBytes)
 			.when(this::isPositive)
-			.to((maxHttpFormPostSize) -> customizeMaxHttpFormPostSize(factory, maxHttpFormPostSize));
+			.to((maxHttpFormPostSize) -> customizeServletContextHandler(factory, contextHandler -> contextHandler.setMaxFormContentSize(maxHttpFormPostSize)));
+		map.from(properties::getMaxFormKeys)
+				.when(this::isPositive)
+				.to((maxFormKeys) -> customizeServletContextHandler(factory, contextHandler -> contextHandler.setMaxFormKeys(maxFormKeys)));
+
 		map.from(properties::getConnectionIdleTimeout).to((idleTimeout) -> customizeIdleTimeout(factory, idleTimeout));
 		map.from(properties::getAccesslog)
 			.when(ServerProperties.Jetty.Accesslog::isEnabled)
@@ -122,29 +127,29 @@ public class JettyWebServerFactoryCustomizer
 		});
 	}
 
-	private void customizeMaxHttpFormPostSize(ConfigurableJettyWebServerFactory factory, int maxHttpFormPostSize) {
+	private void customizeServletContextHandler(ConfigurableJettyWebServerFactory factory, Consumer<ServletContextHandler> customFunc) {
 		factory.addServerCustomizers(new JettyServerCustomizer() {
 
 			@Override
 			public void customize(Server server) {
-				setHandlerMaxHttpFormPostSize(server.getHandlers());
+				acceptCustomizeServletContextHandler(server.getHandlers());
 			}
 
-			private void setHandlerMaxHttpFormPostSize(List<Handler> handlers) {
+			private void acceptCustomizeServletContextHandler(List<Handler> handlers) {
 				for (Handler handler : handlers) {
-					setHandlerMaxHttpFormPostSize(handler);
+					acceptCustomizeServletContextHandler(handler);
 				}
 			}
 
-			private void setHandlerMaxHttpFormPostSize(Handler handler) {
+			private void acceptCustomizeServletContextHandler(Handler handler) {
 				if (handler instanceof ServletContextHandler contextHandler) {
-					contextHandler.setMaxFormContentSize(maxHttpFormPostSize);
+					customFunc.accept(contextHandler);
 				}
 				else if (handler instanceof Handler.Wrapper wrapper) {
-					setHandlerMaxHttpFormPostSize(wrapper.getHandler());
+					acceptCustomizeServletContextHandler(wrapper.getHandler());
 				}
 				else if (handler instanceof Handler.Collection collection) {
-					setHandlerMaxHttpFormPostSize(collection.getHandlers());
+					acceptCustomizeServletContextHandler(collection.getHandlers());
 				}
 			}
 

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

@@ -465,6 +465,15 @@ class ServerPropertiesTests {
 			.isEqualTo(((ServletContextHandler) server.getHandler()).getMaxFormContentSize());
 	}
 
+	@Test
+	void jettyMaxFormKeysMatchesDefault() {
+		JettyServletWebServerFactory jettyFactory = new JettyServletWebServerFactory(0);
+		JettyWebServer jetty = (JettyWebServer) jettyFactory.getWebServer();
+		Server server = jetty.getServer();
+		assertThat(this.properties.getJetty().getMaxFormKeys())
+				.isEqualTo(((ServletContextHandler) server.getHandler()).getMaxFormKeys());
+	}
+
 	@Test
 	void undertowMaxHttpPostSizeMatchesDefault() {
 		assertThat(this.properties.getUndertow().getMaxHttpPostSize().toBytes())

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/embedded/JettyWebServerFactoryCustomizerTests.java

@@ -26,6 +26,7 @@ import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.SynchronousQueue;
 import java.util.function.Function;
 
+import org.eclipse.jetty.ee10.servlet.ServletContextHandler;
 import org.eclipse.jetty.server.AbstractConnector;
 import org.eclipse.jetty.server.Connector;
 import org.eclipse.jetty.server.CustomRequestLog;
@@ -324,6 +325,23 @@ class JettyWebServerFactoryCustomizerTests {
 		assertThat(timeouts).containsOnly(60000L);
 	}
 
+	@Test
+	void customMaxFormKeys() {
+		bind("server.jetty.max-form-keys=2048");
+		JettyWebServer server = customizeAndGetServer();
+		List<Integer> maxFormKeys = getMaxFormKeys(server);
+		assertThat(maxFormKeys).containsOnly(2048);
+	}
+
+	private List<Integer> getMaxFormKeys(JettyWebServer server) {
+		server.start();
+		server.stop();
+		return server.getServer().getHandlers().stream()
+				.filter(handler -> handler instanceof ServletContextHandler)
+				.map(handler -> ((ServletContextHandler) handler).getMaxFormKeys())
+				.toList();
+	}
+
 	private List<Long> connectorsIdleTimeouts(JettyWebServer server) {
 		// Start (and directly stop) server to have connectors available
 		server.start();