Reduce ConfigurationPropertyName GC pressure

Rewrite `ConfigurationPropertyName` in an attempt to consume less memory
and to reduce GC pressure from `toString()`.

Prior to this commit the `toString()` method would always construct a
new value from the name elements. This is sub-optimal since on on many
occasions the `ConfigurationPropertyName` is created from an already
well-formed String. The updated code now attempts to directly use the
original value for both `toString` and `equals` whenever possible.

Further refinements have also been made to the way that elements are
stored. Rather than a list or objects, we now use arrays that contains
the split points and types. This helps to reduce the amount of memory
required to store the name.

Closes gh-13414

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

@@ -52,14 +52,12 @@ class ConfigurationPropertySourcesPropertySource
 
 	private ConfigurationProperty findConfigurationProperty(String name) {
 		try {
-			if (ConfigurationPropertyName.isValid(name)) {
-				return findConfigurationProperty(ConfigurationPropertyName.of(name));
-			}
+			return findConfigurationProperty(ConfigurationPropertyName.of(name, true));
 		}
 		catch (Exception ex) {
-		}
 			return null;
 		}
+	}
 
 	private ConfigurationProperty findConfigurationProperty(
 			ConfigurationPropertyName name) {

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

@@ -234,10 +234,10 @@ public class ConfigurationPropertyNameTests {
 	}
 
 	@Test
-	public void adaptWhenElementValueProcessorIsNullShouldThrowException() {
-		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown.expectMessage("ElementValueProcessor must not be null");
-		ConfigurationPropertyName.adapt("foo", '.', null);
+	public void adaptWhenElementValueProcessorIsNullShouldAdapt() {
+		ConfigurationPropertyName name = ConfigurationPropertyName.adapt("foo", '.',
+				null);
+		assertThat(name.toString()).isEqualTo("foo");
 	}
 
 	@Test
@@ -304,6 +304,12 @@ public class ConfigurationPropertyNameTests {
 		assertThat(name.getNumberOfElements()).isEqualTo(3);
 	}
 
+	@Test
+	public void adaptUnderscoreShouldReturnEmpty() {
+		assertThat(ConfigurationPropertyName.adapt("_", '_').isEmpty()).isTrue();
+		assertThat(ConfigurationPropertyName.adapt("_", '.').isEmpty()).isTrue();
+	}
+
 	@Test
 	public void isEmptyWhenEmptyShouldReturnTrue() {
 		assertThat(ConfigurationPropertyName.of("").isEmpty()).isTrue();
@@ -538,6 +544,15 @@ public class ConfigurationPropertyNameTests {
 						"foo.bar", "foo.bard", "foo.baz");
 	}
 
+	@Test
+	public void compareDifferentLengthsShouldSortNames() {
+		ConfigurationPropertyName name = ConfigurationPropertyName
+				.of("spring.resources.chain.strategy.content");
+		ConfigurationPropertyName other = ConfigurationPropertyName
+				.of("spring.resources.chain.strategy.content.enabled");
+		assertThat(name.compareTo(other)).isLessThan(0);
+	}
+
 	@Test
 	public void toStringShouldBeLowerCaseDashed() {
 		ConfigurationPropertyName name = ConfigurationPropertyName.adapt("fOO.b_-a-r",
@@ -545,6 +560,12 @@ public class ConfigurationPropertyNameTests {
 		assertThat(name.toString()).isEqualTo("foo.b-a-r");
 	}
 
+	@Test
+	public void toStringFromOfShouldBeLowerCaseDashed() {
+		ConfigurationPropertyName name = ConfigurationPropertyName.of("foo.bar-baz");
+		assertThat(name.toString()).isEqualTo("foo.bar-baz");
+	}
+
 	@Test
 	public void equalsAndHashCode() {
 		ConfigurationPropertyName n01 = ConfigurationPropertyName.of("foo[bar]");

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

@@ -104,6 +104,19 @@ public class SpringConfigurationPropertySourcesTests {
 		assertThat(iterator.hasNext()).isFalse();
 	}
 
+	@Test
+	public void shouldAdaptSystemEnvironmentPropertySourceWithUnderscoreValue() {
+		MutablePropertySources sources = new MutablePropertySources();
+		sources.addLast(new SystemEnvironmentPropertySource(
+				StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
+				Collections.singletonMap("_", "1234")));
+		Iterator<ConfigurationPropertySource> iterator = new SpringConfigurationPropertySources(
+				sources).iterator();
+		ConfigurationPropertyName name = ConfigurationPropertyName.of("bar");
+		assertThat(iterator.next().getConfigurationProperty(name)).isNull();
+		assertThat(iterator.hasNext()).isFalse();
+	}
+
 	@Test
 	public void shouldAdaptMultiplePropertySources() {
 		MutablePropertySources sources = new MutablePropertySources();