Reduce GC in SpringConfigurationPropertySource

Update `SpringConfigurationPropertySource` so that the
`DelegatingPropertyMapper` accepts a maximum of two values and does
not need to wrap arguments in an array. Also optimize the merge
operation to return a single result directly rather than always using
a new `List`.

Closes gh-14563

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/SpringConfigurationPropertySource.java

@@ -16,9 +16,6 @@
 
 package org.springframework.boot.context.properties.source;
 
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.List;
 import java.util.Map;
 import java.util.Random;
 import java.util.function.Function;
@@ -30,6 +27,7 @@ import org.springframework.core.env.PropertySource;
 import org.springframework.core.env.StandardEnvironment;
 import org.springframework.core.env.SystemEnvironmentPropertySource;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
 /**
  * {@link ConfigurationPropertySource} backed by a non-enumerable Spring
@@ -217,34 +215,67 @@ class SpringConfigurationPropertySource implements ConfigurationPropertySource {
 	 */
 	private static class DelegatingPropertyMapper implements PropertyMapper {
 
-		private final PropertyMapper[] mappers;
+		private static final PropertyMapping[] NONE = {};
 
-		DelegatingPropertyMapper(PropertyMapper... mappers) {
-			this.mappers = mappers;
+		private final PropertyMapper first;
+
+		private final PropertyMapper second;
+
+		DelegatingPropertyMapper(PropertyMapper first) {
+			this(first, null);
+		}
+
+		DelegatingPropertyMapper(PropertyMapper first, PropertyMapper second) {
+			this.first = first;
+			this.second = second;
 		}
 
 		@Override
 		public PropertyMapping[] map(
 				ConfigurationPropertyName configurationPropertyName) {
-			return callMappers((mapper) -> mapper.map(configurationPropertyName));
+			PropertyMapping[] first = map(this.first, configurationPropertyName);
+			PropertyMapping[] second = map(this.second, configurationPropertyName);
+			return merge(first, second);
+		}
+
+		private PropertyMapping[] map(PropertyMapper mapper,
+				ConfigurationPropertyName configurationPropertyName) {
+			try {
+				return (mapper != null) ? mapper.map(configurationPropertyName) : NONE;
+			}
+			catch (Exception ex) {
+				return NONE;
+			}
 		}
 
 		@Override
 		public PropertyMapping[] map(String propertySourceName) {
-			return callMappers((mapper) -> mapper.map(propertySourceName));
+			PropertyMapping[] first = map(this.first, propertySourceName);
+			PropertyMapping[] second = map(this.second, propertySourceName);
+			return merge(first, second);
 		}
 
-		private PropertyMapping[] callMappers(
-				Function<PropertyMapper, PropertyMapping[]> function) {
-			List<PropertyMapping> mappings = new ArrayList<>();
-			for (PropertyMapper mapper : this.mappers) {
-				try {
-					mappings.addAll(Arrays.asList(function.apply(mapper)));
-				}
-				catch (Exception ex) {
-				}
+		private PropertyMapping[] map(PropertyMapper mapper, String propertySourceName) {
+			try {
+				return (mapper != null) ? mapper.map(propertySourceName) : NONE;
 			}
-			return mappings.toArray(new PropertyMapping[] {});
+			catch (Exception ex) {
+				return NONE;
+			}
+		}
+
+		private PropertyMapping[] merge(PropertyMapping[] first,
+				PropertyMapping[] second) {
+			if (ObjectUtils.isEmpty(second)) {
+				return first;
+			}
+			if (ObjectUtils.isEmpty(first)) {
+				return second;
+			}
+			PropertyMapping[] merged = new PropertyMapping[first.length + second.length];
+			System.arraycopy(first, 0, merged, 0, first.length);
+			System.arraycopy(second, 0, merged, first.length, second.length);
+			return merged;
 		}
 
 	}