Add prefix support for environment variables

Creates an environmentPrefix property that is used to filter the
SystemEnvironmentPropertySource's map of environment
variables and create a new one only with the prefixed properties.
This new map is used on a new SystemEnvironmentPropertySource
that replaces the former one.

Fixes spring-projects#3450

Author: lucassaldanha

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

@@ -177,6 +177,8 @@ public class SpringApplication {
 
 	private ConfigurableEnvironment environment;
 
+	private String environmentPrefix = "";
+
 	private Class<? extends ConfigurableApplicationContext> applicationContextClass;
 
 	private boolean webEnvironment;
@@ -428,6 +430,11 @@ protected void configureEnvironment(ConfigurableEnvironment environment, String[
 	protected void configurePropertySources(ConfigurableEnvironment environment,
 			String[] args) {
 		MutablePropertySources sources = environment.getPropertySources();
+
+		if(this.environmentPrefix != null && !this.environmentPrefix.equals("")) {
+			filterSystemEnvironmentPropertySourceByPrefix(environment);
+		}
+
 		if (this.defaultProperties != null && !this.defaultProperties.isEmpty()) {
 			sources.addLast(new MapPropertySource("defaultProperties",
 					this.defaultProperties));
@@ -448,6 +455,26 @@ protected void configurePropertySources(ConfigurableEnvironment environment,
 		}
 	}
 
+	private void filterSystemEnvironmentPropertySourceByPrefix(
+			ConfigurableEnvironment environment) {
+		final Map<String, Object> systemEnvironment = environment.getSystemEnvironment();
+		final Map<String, Object> prefixedSystemEnvironment = new HashMap<String, Object>(
+				systemEnvironment.size());
+
+		for (String key : systemEnvironment.keySet()) {
+			if (key.startsWith(environmentPrefix)) {
+				prefixedSystemEnvironment.put(key.substring(environmentPrefix.length()),
+						systemEnvironment.get(key));
+			}
+		}
+
+		environment.getPropertySources()
+				.replace(StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
+						new org.springframework.core.env.SystemEnvironmentPropertySource(
+								StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
+								prefixedSystemEnvironment));
+	}
+
 	/**
 	 * Configure which profiles are active (or active by default) for this application
 	 * environment. Consider overriding this method to programmatically enforce profile
@@ -842,6 +869,15 @@ public void setEnvironment(ConfigurableEnvironment environment) {
 		this.environment = environment;
 	}
 
+	/**
+	 * Sets the environment variables prefix that will be used to filter those variables
+	 * for this application.
+	 * @param environmentPrefix the prefix
+	 */
+	public void setEnvironmentPrefix(String environmentPrefix) {
+		this.environmentPrefix = environmentPrefix.toUpperCase();
+	}
+
 	/**
 	 * Returns a mutable set of the sources that will be added to an ApplicationContext
 	 * when {@link #run(String...)} is called.
@@ -1051,5 +1087,4 @@ private static <E> Set<E> asUnmodifiableOrderedSet(Collection<E> elements) {
 		Collections.sort(list, AnnotationAwareOrderComparator.INSTANCE);
 		return new LinkedHashSet<E>(list);
 	}
-
 }