Add prefix support for property source

We configure the `SystemEnvironmentPropertySource` as a `Prefixed` property
source. When adapting this to a `ConfigurationPropertySource, a
`PrefixedConfigurationPropertySource` will be created for it.
A `PrefixedConfigurationPropertySource` will resolve property
such as `foo.bar` to `my.foo.bar` for a prefix of `my`.

Closes gh-3450

Author: mbhave

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -996,6 +996,14 @@ The `+random.int*+` syntax is `OPEN value (,max) CLOSE` where the `OPEN,CLOSE` a
 If `max` is provided, then `value` is the minimum value and `max` is the maximum value (exclusive).
 
 
+[[boot-features-external-config-system-environment]]
+=== Configuring System Environment Properties
+Spring Boot supports setting a prefix for environment properties.
+This is useful if the system environment is shared by multiple Spring Boot applications with different configuration requirements.
+The prefix for system environment properties can be set directly on `SpringApplication`.
+
+For example, if you set the prefix to `input`, a property such as `foo.bar` will also be resolved as `input.foo.bar` in the system environment.
+
 
 [[boot-features-external-config-typesafe-configuration-properties]]
 === Type-safe Configuration Properties

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -246,6 +246,8 @@ public class SpringApplication {
 
 	private boolean lazyInitialization = false;
 
+	private String environmentPrefix;
+
 	private ApplicationContextFactory applicationContextFactory = ApplicationContextFactory.DEFAULT;
 
 	private ApplicationStartup applicationStartup = ApplicationStartup.DEFAULT;
@@ -362,6 +364,9 @@ private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners
 		listeners.environmentPrepared(bootstrapContext, environment);
 		DefaultPropertiesPropertySource.moveToEnd(environment);
 		configureAdditionalProfiles(environment);
+		if (environment.getProperty("spring.main.environment-prefix") != null) {
+			throw new IllegalStateException("Environment prefix cannot be set via properties.");
+		}
 		bindToSpringApplication(environment);
 		if (!this.isCustomEnvironment) {
 			environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
@@ -1174,6 +1179,14 @@ public void setResourceLoader(ResourceLoader resourceLoader) {
 		this.resourceLoader = resourceLoader;
 	}
 
+	public String getEnvironmentPrefix() {
+		return this.environmentPrefix;
+	}
+
+	public void setEnvironmentPrefix(String environmentPrefix) {
+		this.environmentPrefix = environmentPrefix;
+	}
+
 	/**
 	 * Sets the type of Spring {@link ApplicationContext} that will be created. If not
 	 * specified defaults to {@link #DEFAULT_SERVLET_WEB_CONTEXT_CLASS} for web based

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

@@ -74,6 +74,15 @@ default ConfigurationPropertySource withAliases(ConfigurationPropertyNameAliases
 		return new AliasedConfigurationPropertySource(this, aliases);
 	}
 
+	/**
+	 * Return a variant of this source that supports a prefix.
+	 * @param prefix the prefix for properties in the source
+	 * @return a {@link ConfigurationPropertySource} instance supporting a prefix
+	 */
+	default ConfigurationPropertySource withPrefix(String prefix) {
+		return new PrefixedConfigurationPropertySource(this, prefix);
+	}
+
 	/**
 	 * Return the underlying source that is actually providing the properties.
 	 * @return the underlying property source or {@code null}.

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

@@ -73,4 +73,9 @@ default IterableConfigurationPropertySource withAliases(ConfigurationPropertyNam
 		return new AliasedIterableConfigurationPropertySource(this, aliases);
 	}
 
+	@Override
+	default IterableConfigurationPropertySource withPrefix(String prefix) {
+		return new PrefixedIterableConfigurationPropertySource(this, prefix);
+	}
+
 }

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

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2012-2021 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
+ *
+ *      https://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.source;
+
+import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
+
+/**
+ * A {@link ConfigurationPropertySource} supporting a prefix.
+ *
+ * @author Madhura Bhave
+ */
+class PrefixedConfigurationPropertySource implements ConfigurationPropertySource {
+
+	private final ConfigurationPropertySource source;
+
+	private final String prefix;
+
+	PrefixedConfigurationPropertySource(ConfigurationPropertySource source, String prefix) {
+		Assert.notNull(source, "Source must not be null");
+		Assert.notNull(prefix, "Prefix must not be null");
+		this.source = source;
+		this.prefix = prefix;
+	}
+
+	@Override
+	public ConfigurationProperty getConfigurationProperty(ConfigurationPropertyName name) {
+		ConfigurationProperty configurationProperty = this.source.getConfigurationProperty(getPrefixedName(name));
+		if (configurationProperty == null) {
+			return null;
+		}
+		return ConfigurationProperty.of(name, configurationProperty.getValue(), configurationProperty.getOrigin());
+	}
+
+	private ConfigurationPropertyName getPrefixedName(ConfigurationPropertyName name) {
+		String prefix = (StringUtils.hasText(this.prefix)) ? this.prefix + "." : "";
+		return ConfigurationPropertyName.of(prefix + name);
+	}
+
+	@Override
+	public ConfigurationPropertyState containsDescendantOf(ConfigurationPropertyName name) {
+		return this.source.containsDescendantOf(getPrefixedName(name));
+	}
+
+	@Override
+	public Object getUnderlyingSource() {
+		return this.source.getUnderlyingSource();
+	}
+
+	protected ConfigurationPropertySource getSource() {
+		return this.source;
+	}
+
+	protected String getPrefix() {
+		return this.prefix;
+	}
+
+}

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

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2012-2021 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
+ *
+ *      https://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.source;
+
+import java.util.stream.Stream;
+
+/**
+ * An iterable {@link PrefixedConfigurationPropertySource}.
+ *
+ * @author Madhura Bhave
+ */
+class PrefixedIterableConfigurationPropertySource extends PrefixedConfigurationPropertySource
+		implements IterableConfigurationPropertySource {
+
+	PrefixedIterableConfigurationPropertySource(IterableConfigurationPropertySource source, String prefix) {
+		super(source, prefix);
+	}
+
+	@Override
+	public Stream<ConfigurationPropertyName> stream() {
+		ConfigurationPropertyName prefix = ConfigurationPropertyName.of(getPrefix());
+		return getSource().stream().map((propertyName) -> {
+			if (prefix.isAncestorOf(propertyName)) {
+				String name = propertyName.toString();
+				return ConfigurationPropertyName.of(name.substring(getPrefix().length() + 1));
+			}
+			return propertyName;
+		});
+	}
+
+	@Override
+	protected IterableConfigurationPropertySource getSource() {
+		return (IterableConfigurationPropertySource) super.getSource();
+	}
+
+}

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

@@ -24,6 +24,7 @@
 import java.util.Random;
 import java.util.function.Function;
 
+import org.springframework.boot.env.Prefixed;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.env.MutablePropertySources;
 import org.springframework.core.env.PropertySource;
@@ -63,6 +64,9 @@ private ConfigurationPropertySource adapt(PropertySource<?> source) {
 			return result;
 		}
 		result = SpringConfigurationPropertySource.from(source);
+		if (source instanceof Prefixed) {
+			result = result.withPrefix(((Prefixed) source).getPrefix());
+		}
 		this.cache.put(source, result);
 		return result;
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/Prefixed.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2012-2021 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
+ *
+ *      https://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.env;
+
+/**
+ * Interface that can be implemented by a
+ * {@link org.springframework.core.env.PropertySource} that can be used with a prefix.
+ *
+ * @author Madhura Bhave
+ * @since 2.5.0
+ */
+@FunctionalInterface
+public interface Prefixed {
+
+	String getPrefix();
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/SystemEnvironmentPropertySourceEnvironmentPostProcessor.java

@@ -27,11 +27,12 @@
 import org.springframework.core.env.PropertySource;
 import org.springframework.core.env.StandardEnvironment;
 import org.springframework.core.env.SystemEnvironmentPropertySource;
+import org.springframework.util.StringUtils;
 
 /**
  * An {@link EnvironmentPostProcessor} that replaces the systemEnvironment
  * {@link SystemEnvironmentPropertySource} with an
- * {@link OriginAwareSystemEnvironmentPropertySource} that can track the
+ * {@link OriginAndPrefixAwareSystemEnvironmentPropertySource} that can track the
  * {@link SystemEnvironmentOrigin} for every system environment property.
  *
  * @author Madhura Bhave
@@ -51,16 +52,16 @@ public void postProcessEnvironment(ConfigurableEnvironment environment, SpringAp
 		String sourceName = StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME;
 		PropertySource<?> propertySource = environment.getPropertySources().get(sourceName);
 		if (propertySource != null) {
-			replacePropertySource(environment, sourceName, propertySource);
+			replacePropertySource(environment, sourceName, propertySource, application.getEnvironmentPrefix());
 		}
 	}
 
 	@SuppressWarnings("unchecked")
 	private void replacePropertySource(ConfigurableEnvironment environment, String sourceName,
-			PropertySource<?> propertySource) {
+			PropertySource<?> propertySource, String environmentPrefix) {
 		Map<String, Object> originalSource = (Map<String, Object>) propertySource.getSource();
-		SystemEnvironmentPropertySource source = new OriginAwareSystemEnvironmentPropertySource(sourceName,
-				originalSource);
+		SystemEnvironmentPropertySource source = new OriginAndPrefixAwareSystemEnvironmentPropertySource(sourceName,
+				originalSource, environmentPrefix);
 		environment.getPropertySources().replace(sourceName, source);
 	}
 
@@ -76,11 +77,36 @@ public void setOrder(int order) {
 	/**
 	 * {@link SystemEnvironmentPropertySource} that also tracks {@link Origin}.
 	 */
-	protected static class OriginAwareSystemEnvironmentPropertySource extends SystemEnvironmentPropertySource
-			implements OriginLookup<String> {
+	protected static class OriginAndPrefixAwareSystemEnvironmentPropertySource extends SystemEnvironmentPropertySource
+			implements OriginLookup<String>, Prefixed {
 
-		OriginAwareSystemEnvironmentPropertySource(String name, Map<String, Object> source) {
+		private final String environmentPrefix;
+
+		OriginAndPrefixAwareSystemEnvironmentPropertySource(String name, Map<String, Object> source,
+				String environmentPrefix) {
 			super(name, source);
+			this.environmentPrefix = getEnvironmentPrefix(environmentPrefix);
+		}
+
+		private String getEnvironmentPrefix(String environmentPrefix) {
+			String prefix = environmentPrefix;
+			if (!StringUtils.hasText(environmentPrefix)) {
+				return "";
+			}
+			if (environmentPrefix.endsWith(".") || environmentPrefix.endsWith("_") || environmentPrefix.endsWith("-")) {
+				prefix = environmentPrefix.substring(0, environmentPrefix.length() - 1);
+			}
+			return prefix;
+		}
+
+		@Override
+		public boolean containsProperty(String name) {
+			return super.containsProperty(name);
+		}
+
+		@Override
+		public Object getProperty(String name) {
+			return super.getProperty(name);
 		}
 
 		@Override
@@ -92,6 +118,11 @@ public Origin getOrigin(String key) {
 			return null;
 		}
 
+		@Override
+		public String getPrefix() {
+			return this.environmentPrefix;
+		}
+
 	}
 
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -1233,6 +1233,21 @@ void addBootstrapperCanRegisterBeans() {
 		assertThat(applicationContext.getBean("test")).isEqualTo("boot");
 	}
 
+	@Test
+	void settingEnvironmentPrefixViaPropertiesThrowsException() {
+		assertThatIllegalStateException()
+				.isThrownBy(() -> new SpringApplication().run("--spring.main.environment-prefix=my"));
+	}
+
+	@Test
+	void bindsEnvironmentPrefixToSpringApplication() {
+		SpringApplication application = new SpringApplication(ExampleConfig.class);
+		application.setEnvironmentPrefix("my");
+		application.setWebApplicationType(WebApplicationType.NONE);
+		this.context = application.run();
+		assertThat(application.getEnvironmentPrefix()).isEqualTo("my");
+	}
+
 	private <S extends AvailabilityState> ArgumentMatcher<ApplicationEvent> isAvailabilityChangeEventWithState(
 			S state) {
 		return (argument) -> (argument instanceof AvailabilityChangeEvent<?>)

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

@@ -53,6 +53,7 @@
 import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.GenericBeanDefinition;
 import org.springframework.beans.factory.support.RootBeanDefinition;
+import org.springframework.boot.SpringApplication;
 import org.springframework.boot.context.properties.bind.BindException;
 import org.springframework.boot.context.properties.bind.DefaultValue;
 import org.springframework.boot.context.properties.bind.validation.BindValidationException;
@@ -473,6 +474,22 @@ void loadWhenDotsInSystemEnvironmentPropertiesShouldBind() {
 		assertThat(bean.getBar()).isEqualTo("baz");
 	}
 
+	@Test
+	@SuppressWarnings("unchecked")
+	void loadWhenEnvironmentPrefixSetShouldBind() {
+		MutablePropertySources sources = this.context.getEnvironment().getPropertySources();
+		sources.replace(StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
+				new SystemEnvironmentPropertySource(StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
+						Collections.singletonMap("MY_SPRING_FOO_NAME", "Jane")));
+		SpringApplication application = new SpringApplication(PrefixConfiguration.class);
+		application.setApplicationContextFactory((webApplicationType) -> ConfigurationPropertiesTests.this.context);
+		application.setEnvironmentPrefix("my");
+		application.setEnvironment(this.context.getEnvironment());
+		application.run();
+		BasicProperties bean = this.context.getBean(BasicProperties.class);
+		assertThat(bean.name).isEqualTo("Jane");
+	}
+
 	@Test
 	void loadWhenOverridingPropertiesShouldBind() {
 		MutablePropertySources sources = this.context.getEnvironment().getPropertySources();