Introduce ConfigurableEnvironment#merge

Prior to this change, AbstractApplicationContext#setParent replaced the
child context's Environment with the parent's Environment if available.
This has the negative effect of potentially changing the type of the
child context's Environment, and in any case causes property sources
added directly against the child environment to be ignored. This
situation could easily occur if a WebApplicationContext child had a
non-web ApplicationContext set as its parent. In this case the parent
Environment type would (likely) be StandardEnvironment, while the child
Environment type would (likely) be StandardServletEnvironment. By
directly inheriting the parent environment, critical property sources
such as ServletContextPropertySource are lost entirely.

This commit introduces the concept of merging an environment through
the new ConfigurableEnvironment#merge method. Instead of replacing the
child's environment with the parent's,
AbstractApplicationContext#setParent now merges property sources as
well as active and default profile names from the parent into the
child. In this way, distinct environment objects are maintained with
specific types and property sources preserved. See #merge Javadoc for
additional details.

Issue: SPR-9444, SPR-9439

spring-core/src/main/java/org/springframework/core/env/AbstractEnvironment.java

@@ -387,6 +387,20 @@ public abstract class AbstractEnvironment implements ConfigurableEnvironment {
 		return systemProperties;
 	}
 
+	public void merge(ConfigurableEnvironment parent) {
+		for (PropertySource<?> ps : parent.getPropertySources()) {
+			if (!this.propertySources.contains(ps.getName())) {
+				this.propertySources.addLast(ps);
+			}
+		}
+		for (String profile : parent.getActiveProfiles()) {
+			this.activeProfiles.add(profile);
+		}
+		for (String profile : parent.getDefaultProfiles()) {
+			this.defaultProfiles.add(profile);
+		}
+	}
+
 
 	//---------------------------------------------------------------------
 	// Implementation of ConfigurablePropertyResolver interface

spring-core/src/main/java/org/springframework/core/env/ConfigurableEnvironment.java

@@ -149,4 +149,24 @@ public interface ConfigurableEnvironment extends Environment, ConfigurableProper
 	 */
 	Map<String, Object> getSystemProperties();
 
+	/**
+	 * Append the given parent environment's active profiles, default profiles and
+	 * property sources to this (child) environment's respective collections of each.
+	 * <p>For any identically-named {@code PropertySource} instance existing in both
+	 * parent and child, the child instance is to be preserved and the parent instance
+	 * discarded. This has the effect of allowing overriding of property sources by the
+	 * child as well as avoiding redundant searches through common property source types,
+	 * e.g. system environment and system properties.
+	 * <p>Active and default profile names are also filtered for duplicates, to avoid
+	 * confusion and redundant storage.
+	 * <p>The parent environment remains unmodified in any case. Note that any changes to
+	 * the parent environment occurring after the call to {@code merge} will not be
+	 * reflected in the child. Therefore, care should be taken to configure parent
+	 * property sources and profile information prior to calling {@code merge}.
+	 * @param parent the environment to merge with
+	 * @since 3.2
+	 * @see org.springframework.context.support.AbstractApplicationContext#setParent
+	 */
+	void merge(ConfigurableEnvironment parent);
+
 }