Improve configuration properties back-compatibility

Refine `SystemEnvironmentPropertyMapper` to support environment
variables that would have worked in Spring Boot 1.5. Specifically,
camelCase property bindings now support an additional underscore. The
recommended way to map `fooBar` is still `PREFIX_FOOBAR`, however,
`PREFIX_FOO_BAR` will now also work.

Fixes gh-10873

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

@@ -18,7 +18,9 @@ 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.function.Function;
 import java.util.stream.IntStream;
 
@@ -49,6 +51,37 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 	private SystemEnvironmentPropertyMapper() {
 	}
 
+	@Override
+	public List<PropertyMapping> map(PropertySource<?> propertySource,
+			ConfigurationPropertyName configurationPropertyName) {
+		Set<String> names = new LinkedHashSet<>();
+		names.add(convertName(configurationPropertyName));
+		names.add(convertLegacyName(configurationPropertyName));
+		List<PropertyMapping> result = new ArrayList<>();
+		names.forEach((name) -> result
+				.add(new PropertyMapping(name, configurationPropertyName)));
+		if (isListShortcutPossible(configurationPropertyName)) {
+			result.addAll(mapListShortcut(propertySource, configurationPropertyName));
+		}
+		return result;
+	}
+
+	private boolean isListShortcutPossible(ConfigurationPropertyName name) {
+		return (name.isLastElementIndexed() && isNumber(name.getLastElement(Form.UNIFORM))
+				&& name.getNumberOfElements() >= 1);
+	}
+
+	private List<PropertyMapping> mapListShortcut(PropertySource<?> propertySource,
+			ConfigurationPropertyName name) {
+		String result = convertName(name, name.getNumberOfElements() - 1) + "__";
+		if (propertySource.containsProperty(result)) {
+			int index = Integer.parseInt(name.getLastElement(Form.UNIFORM));
+			return Collections.singletonList(
+					new PropertyMapping(result, name, new ElementExtractor(index)));
+		}
+		return Collections.emptyList();
+	}
+
 	@Override
 	public List<PropertyMapping> map(PropertySource<?> propertySource,
 			String propertySourceName) {
@@ -89,19 +122,6 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 		return mappings;
 	}
 
-	@Override
-	public List<PropertyMapping> map(PropertySource<?> propertySource,
-			ConfigurationPropertyName configurationPropertyName) {
-		String name = convertName(configurationPropertyName);
-		List<PropertyMapping> result = Collections
-				.singletonList(new PropertyMapping(name, configurationPropertyName));
-		if (isListShortcutPossible(configurationPropertyName)) {
-			result = new ArrayList<>(result);
-			result.addAll(mapListShortcut(propertySource, configurationPropertyName));
-		}
-		return result;
-	}
-
 	private String convertName(ConfigurationPropertyName name) {
 		return convertName(name, name.getNumberOfElements());
 	}
@@ -115,20 +135,17 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 		return result.toString();
 	}
 
-	private boolean isListShortcutPossible(ConfigurationPropertyName name) {
-		return (name.isLastElementIndexed() && isNumber(name.getLastElement(Form.UNIFORM))
-				&& name.getNumberOfElements() >= 1);
+	private String convertLegacyName(ConfigurationPropertyName name) {
+		StringBuilder result = new StringBuilder();
+		for (int i = 0; i < name.getNumberOfElements(); i++) {
+			result.append(result.length() == 0 ? "" : "_");
+			result.append(convertLegacyNameElement(name.getElement(i, Form.ORIGINAL)));
+		}
+		return result.toString();
 	}
 
-	private List<PropertyMapping> mapListShortcut(PropertySource<?> propertySource,
-			ConfigurationPropertyName name) {
-		String result = convertName(name, name.getNumberOfElements() - 1) + "__";
-		if (propertySource.containsProperty(result)) {
-			int index = Integer.parseInt(name.getLastElement(Form.UNIFORM));
-			return Collections.singletonList(
-					new PropertyMapping(result, name, new ElementExtractor(index)));
-		}
-		return Collections.emptyList();
+	private Object convertLegacyNameElement(String element) {
+		return element.replace("-", "_").toUpperCase();
 	}
 
 	private CharSequence processElementValue(CharSequence value) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/BackCompatibiltyBinderIntegrationTests.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2012-2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.context.properties.bind;
+
+import java.util.Collections;
+
+import org.junit.Test;
+
+import org.springframework.core.env.StandardEnvironment;
+import org.springframework.core.env.SystemEnvironmentPropertySource;
+import org.springframework.mock.env.MockEnvironment;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests to ensure that the {@link Binder} offers at least some support for
+ * Boot 1.5 style binding.
+ *
+ * @author Phillip Webb
+ */
+public class BackCompatibiltyBinderIntegrationTests {
+
+	// gh-10873
+
+	@Test
+	public void bindWhenBindingCamelCaseToEnvironmentWithExtractUnderscore()
+			throws Exception {
+		MockEnvironment environment = new MockEnvironment();
+		SystemEnvironmentPropertySource propertySource = new SystemEnvironmentPropertySource(
+				StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
+				Collections.singletonMap("FOO_ZK_NODES", "foo"));
+		environment.getPropertySources().addFirst(propertySource);
+		ExampleCamelCaseBean result = Binder.get(environment)
+				.bind("foo", Bindable.of(ExampleCamelCaseBean.class)).get();
+		assertThat(result.getZkNodes()).isEqualTo("foo");
+	}
+
+	public static class ExampleCamelCaseBean {
+
+		private String zkNodes;
+
+		public String getZkNodes() {
+			return this.zkNodes;
+		}
+
+		public void setZkNodes(String zkNodes) {
+			this.zkNodes = zkNodes;
+		}
+
+	}
+
+}

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

@@ -65,7 +65,8 @@ public class SystemEnvironmentPropertyMapperTests extends AbstractPropertyMapper
 		assertThat(namesFromConfiguration("host[0].name")).containsExactly("HOST_0_NAME");
 		assertThat(namesFromConfiguration("host.f00.name"))
 				.containsExactly("HOST_F00_NAME");
-		assertThat(namesFromConfiguration("foo.the-bar")).containsExactly("FOO_THEBAR");
+		assertThat(namesFromConfiguration("foo.the-bar")).containsExactly("FOO_THEBAR",
+				"FOO_THE_BAR");
 	}
 
 	@Test