Introduce SystemEnvironmentPropertySource

Properties such as 'spring.profiles.active' cannot be specified at the
command line under Bash and other shells due to variable naming
constraints. This change allows for exchanging underscores for periods
as well as capitalizing property names for more idiomatic naming when
dealing with environment variables.

For example, Spring will respect equally either of the following:

    spring.profiles.active=p1 java -classpath ... MyApp

    SPRING_PROFILES_ACTIVE=p1 java -classpath ... MyApp

The former is not possible under Bash, while the latter is. No code or
configuration changes are required; SystemEnvironmentPropertySource
adapts for these varations automatically.

SystemEnvironmentPropertySource is added by default as
"systemEnvironment" to StandardEnvironment and all subtypes, taking the
place of the plain MapPropertySource that was in use before this change.

Issue: SPR-8869

org.springframework.core/src/test/java/org/springframework/core/env/StandardEnvironmentTests.java

@@ -71,6 +71,13 @@ public class StandardEnvironmentTests {
 		assertThat(sources.size(), is(2));
 	}
 
+	@Test
+	public void propertySourceTypes() {
+		ConfigurableEnvironment env = new StandardEnvironment();
+		MutablePropertySources sources = env.getPropertySources();
+		assertThat(sources.get(StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME), instanceOf(SystemEnvironmentPropertySource.class));
+	}
+
 	@Test
 	public void activeProfilesIsEmptyByDefault() {
 		assertThat(environment.getActiveProfiles().length, is(0));

org.springframework.core/src/test/java/org/springframework/core/env/SystemEnvironmentPropertySourceTests.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright 2002-2011 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.core.env;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.junit.Assert.assertThat;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.junit.Before;
+import org.junit.Test;
+
+/**
+ * Unit tests for {@link SystemEnvironmentPropertySource}.
+ *
+ * @author Chris Beams
+ * @since 3.1
+ */
+public class SystemEnvironmentPropertySourceTests {
+
+	private Map<String, Object> envMap;
+	private PropertySource<?> ps;
+
+	@Before
+	public void setUp() {
+		envMap = new HashMap<String, Object>();
+		ps = new SystemEnvironmentPropertySource("sysEnv", envMap);
+	}
+
+	@Test
+	public void none() {
+		//envMap.put("a.key", "a_value");
+
+		assertThat(ps.containsProperty("a.key"), equalTo(false));
+		assertThat(ps.getProperty("a.key"), equalTo(null));
+	}
+
+	@Test
+	public void normalWithoutPeriod() {
+		envMap.put("akey", "avalue");
+
+		assertThat(ps.containsProperty("akey"), equalTo(true));
+		assertThat(ps.getProperty("akey"), equalTo((Object)"avalue"));
+	}
+
+	@Test
+	public void normalWithPeriod() {
+		envMap.put("a.key", "a.value");
+
+		assertThat(ps.containsProperty("a.key"), equalTo(true));
+		assertThat(ps.getProperty("a.key"), equalTo((Object)"a.value"));
+	}
+
+	@Test
+	public void withUnderscore() {
+		envMap.put("a_key", "a_value");
+
+		assertThat(ps.containsProperty("a_key"), equalTo(true));
+		assertThat(ps.containsProperty("a.key"), equalTo(true));
+
+		assertThat(ps.getProperty("a_key"), equalTo((Object)"a_value"));
+		assertThat( ps.getProperty("a.key"), equalTo((Object)"a_value"));
+	}
+
+	@Test
+	public void withBothPeriodAndUnderscore() {
+		envMap.put("a_key", "a_value");
+		envMap.put("a.key", "a.value");
+
+		assertThat(ps.getProperty("a_key"), equalTo((Object)"a_value"));
+		assertThat( ps.getProperty("a.key"), equalTo((Object)"a.value"));
+	}
+
+	@Test
+	public void withUppercase() {
+		envMap.put("A_KEY", "a_value");
+
+		assertThat(ps.containsProperty("A_KEY"), equalTo(true));
+		assertThat(ps.containsProperty("A.KEY"), equalTo(true));
+		assertThat(ps.containsProperty("a_key"), equalTo(true));
+		assertThat(ps.containsProperty("a.key"), equalTo(true));
+
+		assertThat(ps.getProperty("A_KEY"), equalTo((Object)"a_value"));
+		assertThat(ps.getProperty("A.KEY"), equalTo((Object)"a_value"));
+		assertThat(ps.getProperty("a_key"), equalTo((Object)"a_value"));
+		assertThat(ps.getProperty("a.key"), equalTo((Object)"a_value"));
+	}
+
+}