Allow processor cache to be configured to be unlimited

Closes gh-16415

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

@@ -368,7 +368,8 @@ public class ServerProperties {
 
 		/**
 		 * Maximum number of idle processors that will be retained in the cache and reused
-		 * with a subsequent request.
+		 * with a subsequent request. When set to -1 the cache will be unlimited with a
+		 * theoretical maximum size equal to the maximum number of connections.
 		 */
 		private int processorCache = 200;
 

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

@@ -110,7 +110,7 @@ public class TomcatWebServerFactoryCustomizer implements
 				.to((maxConnections) -> customizeMaxConnections(factory, maxConnections));
 		propertyMapper.from(tomcatProperties::getAcceptCount).when(this::isPositive)
 				.to((acceptCount) -> customizeAcceptCount(factory, acceptCount));
-		propertyMapper.from(tomcatProperties::getProcessorCache).when(this::isPositive)
+		propertyMapper.from(tomcatProperties::getProcessorCache)
 				.to((processorCache) -> customizeProcessorCache(factory, processorCache));
 		customizeStaticResources(factory);
 		customizeErrorReportValve(properties.getError(), factory);

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

@@ -89,7 +89,17 @@ public class TomcatWebServerFactoryCustomizerTests {
 	@Test
 	public void customProcessorCache() {
 		bind("server.tomcat.processor-cache=100");
-		assertThat(this.serverProperties.getTomcat().getProcessorCache()).isEqualTo(100);
+		customizeAndRunServer((server) -> assertThat(((AbstractProtocol<?>) server
+				.getTomcat().getConnector().getProtocolHandler()).getProcessorCache())
+						.isEqualTo(100));
+	}
+
+	@Test
+	public void unlimitedProcessorCache() {
+		bind("server.tomcat.processor-cache=-1");
+		customizeAndRunServer((server) -> assertThat(((AbstractProtocol<?>) server
+				.getTomcat().getConnector().getProtocolHandler()).getProcessorCache())
+						.isEqualTo(-1));
 	}
 
 	@Test