Support 'required properties' precondition

Users may now call #setRequiredProperties(String...) against the
Environment (via its ConfigurablePropertyResolver interface) in order
to indicate which properties must be present.

Environment#validateRequiredProperties() is invoked by
AbstractApplicationContext during the refresh() lifecycle to perform
the actual check and a MissingRequiredPropertiesException is thrown
if the precondition is not satisfied.

Issue: SPR-8323

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

@@ -318,6 +318,40 @@ public class PropertySourcesPropertyResolverTests {
 		resolver.getPropertyAsClass("some.class", SomeType.class);
 	}
 
+	@Test
+	public void setRequiredProperties_andValidateRequiredProperties() {
+		// no properties have been marked as required -> validation should pass
+		propertyResolver.validateRequiredProperties();
+
+		// mark which properties are required
+		propertyResolver.setRequiredProperties("foo", "bar");
+
+		// neither foo nor bar properties are present -> validating should throw
+		try {
+			propertyResolver.validateRequiredProperties();
+			fail("expected validation exception");
+		} catch (MissingRequiredPropertiesException ex) {
+			assertThat(ex.getMessage(), equalTo(
+					"The following properties were declared as required " +
+					"but could not be resolved: [foo, bar]"));
+		}
+
+		// add foo property -> validation should fail only on missing 'bar' property
+		testProperties.put("foo", "fooValue");
+		try {
+			propertyResolver.validateRequiredProperties();
+			fail("expected validation exception");
+		} catch (MissingRequiredPropertiesException ex) {
+			assertThat(ex.getMessage(), equalTo(
+					"The following properties were declared as required " +
+					"but could not be resolved: [bar]"));
+		}
+
+		// add bar property -> validation should pass, even with an empty string value
+		testProperties.put("bar", "");
+		propertyResolver.validateRequiredProperties();
+	}
+
 	static interface SomeType { }
 	static class SpecificType implements SomeType { }
 }