Change property mapper to use array returns

Update the `PropertyMapper` interface to return arrays rather than
Lists. Since implementations are package-private it's possible for us
to control how they are used and it helps to save a little memory.

Fixes gh-11411

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

@@ -16,9 +16,6 @@
 
 package org.springframework.boot.context.properties.source;
 
-import java.util.Collections;
-import java.util.List;
-
 import org.springframework.util.ObjectUtils;
 
 /**
@@ -43,55 +40,53 @@ final class DefaultPropertyMapper implements PropertyMapper {
 	}
 
 	@Override
-	public List<PropertyMapping> map(
+	public PropertyMapping[] map(ConfigurationPropertyName configurationPropertyName) {
-			ConfigurationPropertyName configurationPropertyName) {
 		// Use a local copy in case another thread changes things
 		LastMapping<ConfigurationPropertyName> last = this.lastMappedConfigurationPropertyName;
 		if (last != null && last.isFrom(configurationPropertyName)) {
 			return last.getMapping();
 		}
 		String convertedName = configurationPropertyName.toString();
-		List<PropertyMapping> mapping = Collections.singletonList(
+		PropertyMapping[] mapping = {
-				new PropertyMapping(convertedName, configurationPropertyName));
+				new PropertyMapping(convertedName, configurationPropertyName) };
 		this.lastMappedConfigurationPropertyName = new LastMapping<>(
 				configurationPropertyName, mapping);
 		return mapping;
 	}
 
 	@Override
-	public List<PropertyMapping> map(String propertySourceName) {
+	public PropertyMapping[] map(String propertySourceName) {
 		// Use a local copy in case another thread changes things
 		LastMapping<String> last = this.lastMappedPropertyName;
 		if (last != null && last.isFrom(propertySourceName)) {
 			return last.getMapping();
 		}
-		List<PropertyMapping> mapping = tryMap(propertySourceName);
+		PropertyMapping[] mapping = tryMap(propertySourceName);
 		this.lastMappedPropertyName = new LastMapping<>(propertySourceName, mapping);
 		return mapping;
 	}
 
-	private List<PropertyMapping> tryMap(String propertySourceName) {
+	private PropertyMapping[] tryMap(String propertySourceName) {
 		try {
 			ConfigurationPropertyName convertedName = ConfigurationPropertyName
 					.adapt(propertySourceName, '.');
 			if (!convertedName.isEmpty()) {
-				PropertyMapping o = new PropertyMapping(propertySourceName,
+				return new PropertyMapping[] {
-						convertedName);
+						new PropertyMapping(propertySourceName, convertedName) };
-				return Collections.singletonList(o);
 			}
 		}
 		catch (Exception ex) {
 		}
-		return Collections.emptyList();
+		return NO_MAPPINGS;
 	}
 
 	private static class LastMapping<T> {
 
 		private final T from;
 
-		private final List<PropertyMapping> mapping;
+		private final PropertyMapping[] mapping;
 
-		LastMapping(T from, List<PropertyMapping> mapping) {
+		LastMapping(T from, PropertyMapping[] mapping) {
 			this.from = from;
 			this.mapping = mapping;
 		}
@@ -100,7 +95,7 @@ final class DefaultPropertyMapper implements PropertyMapper {
 			return ObjectUtils.nullSafeEquals(from, this.from);
 		}
 
-		public List<PropertyMapping> getMapping() {
+		public PropertyMapping[] getMapping() {
 			return this.mapping;
 		}
 

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

@@ -16,8 +16,6 @@
 
 package org.springframework.boot.context.properties.source;
 
-import java.util.List;
-
 import org.springframework.core.env.EnumerablePropertySource;
 import org.springframework.core.env.PropertySource;
 
@@ -40,19 +38,21 @@ import org.springframework.core.env.PropertySource;
  */
 interface PropertyMapper {
 
+	PropertyMapping[] NO_MAPPINGS = {};
+
 	/**
 	 * Provide mappings from a {@link ConfigurationPropertySource}
 	 * {@link ConfigurationPropertyName}.
 	 * @param configurationPropertyName the name to map
 	 * @return a stream of mappings or {@code Stream#empty()}
 	 */
-	List<PropertyMapping> map(ConfigurationPropertyName configurationPropertyName);
+	PropertyMapping[] map(ConfigurationPropertyName configurationPropertyName);
 
 	/**
 	 * Provide mappings from a {@link PropertySource} property name.
 	 * @param propertySourceName the name to map
 	 * @return a stream of mappings or {@code Stream#empty()}
 	 */
-	List<PropertyMapping> map(String propertySourceName);
+	PropertyMapping[] map(String propertySourceName);
 
 }

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

@@ -16,10 +16,7 @@
 
 package org.springframework.boot.context.properties.source;
 
-import java.util.Collections;
-import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 import java.util.Random;
 import java.util.function.Function;
 
@@ -85,7 +82,7 @@ class SpringConfigurationPropertySource implements ConfigurationPropertySource {
 	@Override
 	public ConfigurationProperty getConfigurationProperty(
 			ConfigurationPropertyName name) {
-		List<PropertyMapping> mappings = getMapper().map(name);
+		PropertyMapping[] mappings = getMapper().map(name);
 		return find(mappings, name);
 	}
 
@@ -100,10 +97,17 @@ class SpringConfigurationPropertySource implements ConfigurationPropertySource {
 		return this.propertySource;
 	}
 
-	protected final ConfigurationProperty find(List<PropertyMapping> mappings,
+	protected final ConfigurationProperty find(PropertyMapping[] mappings,
 			ConfigurationPropertyName name) {
-		return mappings.stream().filter((m) -> m.isApplicable(name)).map(this::find)
+		for (PropertyMapping candidate : mappings) {
-				.filter(Objects::nonNull).findFirst().orElse(null);
+			if (candidate.isApplicable(name)) {
+				ConfigurationProperty result = find(candidate);
+				if (result != null) {
+					return result;
+				}
+			}
+		}
+		return null;
 	}
 
 	private ConfigurationProperty find(PropertyMapping mapping) {
@@ -214,23 +218,23 @@ class SpringConfigurationPropertySource implements ConfigurationPropertySource {
 		}
 
 		@Override
-		public List<PropertyMapping> map(
+		public PropertyMapping[] map(
 				ConfigurationPropertyName configurationPropertyName) {
 			try {
 				return this.mapper.map(configurationPropertyName);
 			}
 			catch (Exception ex) {
-				return Collections.emptyList();
+				return NO_MAPPINGS;
 			}
 		}
 
 		@Override
-		public List<PropertyMapping> map(String propertySourceName) {
+		public PropertyMapping[] map(String propertySourceName) {
 			try {
 				return this.mapper.map(propertySourceName);
 			}
 			catch (Exception ex) {
-				return Collections.emptyList();
+				return NO_MAPPINGS;
 			}
 		}
 

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

@@ -71,7 +71,7 @@ class SpringIterableConfigurationPropertySource extends SpringConfigurationPrope
 		ConfigurationProperty configurationProperty = super.getConfigurationProperty(
 				name);
 		if (configurationProperty == null) {
-			configurationProperty = find(getPropertyMappings(), name);
+			configurationProperty = find(getPropertyMappings(getCache()), name);
 		}
 		return configurationProperty;
 	}
@@ -98,8 +98,8 @@ class SpringIterableConfigurationPropertySource extends SpringConfigurationPrope
 		if (names != null) {
 			return names;
 		}
-		List<PropertyMapping> mappings = getPropertyMappings();
+		PropertyMapping[] mappings = getPropertyMappings(cache);
-		names = new ArrayList<>(mappings.size());
+		names = new ArrayList<>(mappings.length);
 		for (PropertyMapping mapping : mappings) {
 			names.add(mapping.getConfigurationPropertyName());
 		}
@@ -110,22 +110,23 @@ class SpringIterableConfigurationPropertySource extends SpringConfigurationPrope
 		return names;
 	}
 
-	private List<PropertyMapping> getPropertyMappings() {
+	private PropertyMapping[] getPropertyMappings(Cache cache) {
-		Cache cache = getCache();
+		PropertyMapping[] result = (cache != null ? cache.getMappings() : null);
-		List<PropertyMapping> mappings = (cache != null ? cache.getMappings() : null);
+		if (result != null) {
-		if (mappings != null) {
+			return result;
-			return mappings;
 		}
 		String[] names = getPropertySource().getPropertyNames();
-		mappings = new ArrayList<>(names.length);
+		List<PropertyMapping> mappings = new ArrayList<>(names.length * 2);
 		for (String name : names) {
-			mappings.addAll(getMapper().map(name));
+			for (PropertyMapping mapping : getMapper().map(name)) {
+				mappings.add(mapping);
+			}
 		}
-		mappings = Collections.unmodifiableList(mappings);
+		result = mappings.toArray(new PropertyMapping[mappings.size()]);
 		if (cache != null) {
-			cache.setMappings(mappings);
+			cache.setMappings(result);
 		}
-		return mappings;
+		return result;
 	}
 
 	private Cache getCache() {
@@ -157,7 +158,7 @@ class SpringIterableConfigurationPropertySource extends SpringConfigurationPrope
 
 		private List<ConfigurationPropertyName> names;
 
-		private List<PropertyMapping> mappings;
+		private PropertyMapping[] mappings;
 
 		public List<ConfigurationPropertyName> getNames() {
 			return this.names;
@@ -167,11 +168,11 @@ class SpringIterableConfigurationPropertySource extends SpringConfigurationPrope
 			this.names = names;
 		}
 
-		public List<PropertyMapping> getMappings() {
+		public PropertyMapping[] getMappings() {
 			return this.mappings;
 		}
 
-		public void setMappings(List<PropertyMapping> mappings) {
+		public void setMappings(PropertyMapping[] mappings) {
 			this.mappings = mappings;
 		}
 

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

@@ -16,11 +16,6 @@
 
 package org.springframework.boot.context.properties.source;
 
-import java.util.ArrayList;
-import java.util.Collections;
-import java.util.LinkedHashSet;
-import java.util.List;
-import java.util.Set;
 import java.util.stream.IntStream;
 
 import org.springframework.boot.context.properties.source.ConfigurationPropertyName.Form;
@@ -31,10 +26,6 @@ import org.springframework.boot.context.properties.source.ConfigurationPropertyN
  * "{@code .}". For example, "{@code SERVER_PORT}" is mapped to "{@code server.port}". In
  * addition, numeric elements are mapped to indexes (e.g. "{@code HOST_0}" is mapped to
  * "{@code host[0]}").
- * <p>
- * List shortcuts (names that end with double underscore) are also supported by this
- * mapper. For example, "{@code MY_LIST__=a,b,c}" is mapped to "{@code my.list[0]=a}",
- * "{@code my.list[1]=b}", "{@code my.list[2]=c}".
  *
  * @author Phillip Webb
  * @author Madhura Bhave
@@ -45,28 +36,26 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 
 	public static final PropertyMapper INSTANCE = new SystemEnvironmentPropertyMapper();
 
-	private SystemEnvironmentPropertyMapper() {
-	}
-
 	@Override
-	public List<PropertyMapping> map(
+	public PropertyMapping[] map(ConfigurationPropertyName configurationPropertyName) {
-			ConfigurationPropertyName configurationPropertyName) {
+		String name = convertName(configurationPropertyName);
-		Set<String> names = new LinkedHashSet<>();
+		String legacyName = convertLegacyName(configurationPropertyName);
-		names.add(convertName(configurationPropertyName));
+		if (name.equals(legacyName)) {
-		names.add(convertLegacyName(configurationPropertyName));
+			return new PropertyMapping[] {
-		List<PropertyMapping> result = new ArrayList<>();
+					new PropertyMapping(name, configurationPropertyName) };
-		names.forEach((name) -> result
+		}
-				.add(new PropertyMapping(name, configurationPropertyName)));
+		return new PropertyMapping[] {
-		return result;
+				new PropertyMapping(name, configurationPropertyName),
+				new PropertyMapping(legacyName, configurationPropertyName), };
 	}
 
 	@Override
-	public List<PropertyMapping> map(String propertySourceName) {
+	public PropertyMapping[] map(String propertySourceName) {
 		ConfigurationPropertyName name = convertName(propertySourceName);
 		if (name == null || name.isEmpty()) {
-			return Collections.emptyList();
+			return NO_MAPPINGS;
 		}
-		return Collections.singletonList(new PropertyMapping(propertySourceName, name));
+		return new PropertyMapping[] { new PropertyMapping(propertySourceName, name) };
 	}
 
 	private ConfigurationPropertyName convertName(String propertySourceName) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/AbstractPropertyMapperTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.context.properties.source;
 
+import java.util.Arrays;
 import java.util.Iterator;
 
 /**
@@ -34,7 +35,7 @@ public abstract class AbstractPropertyMapperTests {
 	}
 
 	protected final Iterator<String> namesFromString(String name, Object value) {
-		return getMapper().map(name).stream()
+		return Arrays.stream(getMapper().map(name))
 				.map((mapping) -> mapping.getConfigurationPropertyName().toString())
 				.iterator();
 	}
@@ -44,7 +45,7 @@ public abstract class AbstractPropertyMapperTests {
 	}
 
 	protected final Iterator<String> namesFromConfiguration(String name, String value) {
-		return getMapper().map(ConfigurationPropertyName.of(name)).stream()
+		return Arrays.stream(getMapper().map(ConfigurationPropertyName.of(name)))
 				.map(PropertyMapping::getPropertySourceName).iterator();
 	}
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/SystemEnvironmentPropertyMapperTests.java

@@ -16,8 +16,6 @@
 
 package org.springframework.boot.context.properties.source;
 
-import java.util.List;
-
 import org.junit.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -61,7 +59,7 @@ public class SystemEnvironmentPropertyMapperTests extends AbstractPropertyMapper
 
 	@Test
 	public void underscoreShouldNotMapToEmptyString() {
-		List<PropertyMapping> mappings = getMapper().map("_");
+		PropertyMapping[] mappings = getMapper().map("_");
 		boolean applicable = false;
 		for (PropertyMapping mapping : mappings) {
 			applicable = mapping.isApplicable(ConfigurationPropertyName.of(""));
@@ -71,7 +69,7 @@ public class SystemEnvironmentPropertyMapperTests extends AbstractPropertyMapper
 
 	@Test
 	public void underscoreWithWhitespaceShouldNotMapToEmptyString() {
-		List<PropertyMapping> mappings = getMapper().map("  _");
+		PropertyMapping[] mappings = getMapper().map("  _");
 		boolean applicable = false;
 		for (PropertyMapping mapping : mappings) {
 			applicable = mapping.isApplicable(ConfigurationPropertyName.of(""));

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/TestPropertyMapper.java

@@ -17,8 +17,6 @@
 package org.springframework.boot.context.properties.source;
 
 import java.util.Collections;
-import java.util.List;
-import java.util.function.Function;
 
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
@@ -46,21 +44,17 @@ class TestPropertyMapper implements PropertyMapper {
 		}
 	}
 
-	public void addFromConfigurationProperty(ConfigurationPropertyName from, String to,
-			Function<Object, Object> extractor) {
-		this.fromConfig.add(from, new PropertyMapping(to, from, extractor));
-	}
-
 	@Override
-	public List<PropertyMapping> map(String propertySourceName) {
+	public PropertyMapping[] map(String propertySourceName) {
-		return this.fromSource.getOrDefault(propertySourceName, Collections.emptyList());
+		return this.fromSource.getOrDefault(propertySourceName, Collections.emptyList())
+				.toArray(new PropertyMapping[0]);
 	}
 
 	@Override
-	public List<PropertyMapping> map(
+	public PropertyMapping[] map(ConfigurationPropertyName configurationPropertyName) {
-			ConfigurationPropertyName configurationPropertyName) {
+		return this.fromConfig
-		return this.fromConfig.getOrDefault(configurationPropertyName,
+				.getOrDefault(configurationPropertyName, Collections.emptyList())
-				Collections.emptyList());
+				.toArray(new PropertyMapping[0]);
 	}
 
 }