Add ConfigurationPropertySource.from method

Add a factory method to `ConfigurationPropertySource` that allows a
single Spring `PropertySource` instance to be adapted.

Closes gh-22494

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -19,6 +19,7 @@ package org.springframework.boot.context.properties.source;
 import java.util.function.Predicate;
 
 import org.springframework.boot.origin.OriginTrackedValue;
+import org.springframework.core.env.PropertySource;
 
 /**
  * A source of {@link ConfigurationProperty ConfigurationProperties}.
@@ -81,4 +82,18 @@ public interface ConfigurationPropertySource {
 		return null;
 	}
 
+	/**
+	 * Return a single new {@link ConfigurationPropertySource} adapted from the given
+	 * Spring {@link PropertySource} or {@code null} if the source cannot be adapted.
+	 * @param source the Spring property source to adapt
+	 * @return an adapted source or {@code null} {@link SpringConfigurationPropertySource}
+	 * @since 2.4.0
+	 */
+	static ConfigurationPropertySource from(PropertySource<?> source) {
+		if (source instanceof ConfigurationPropertySourcesPropertySource) {
+			return null;
+		}
+		return SpringConfigurationPropertySource.from(source);
+	}
+
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -110,7 +110,7 @@ public final class ConfigurationPropertySources {
 	 * {@link SpringConfigurationPropertySource}
 	 */
 	public static Iterable<ConfigurationPropertySource> from(PropertySource<?> source) {
-		return Collections.singleton(SpringConfigurationPropertySource.from(source));
+		return Collections.singleton(ConfigurationPropertySource.from(source));
 	}
 
 	/**

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

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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.boot.context.properties.source;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.mock.env.MockPropertySource;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link ConfigurationPropertySource}.
+ *
+ * @author Phillip Webb
+ */
+class ConfigurationPropertySourceTests {
+
+	@Test
+	void fromCreatesConfigurationPropertySourcesPropertySource() {
+		MockPropertySource source = new MockPropertySource();
+		source.setProperty("spring", "boot");
+		ConfigurationPropertySource adapted = ConfigurationPropertySource.from(source);
+		assertThat(adapted.getConfigurationProperty(ConfigurationPropertyName.of("spring")).getValue())
+				.isEqualTo("boot");
+	}
+
+	@Test
+	void fromWhenSourceIsAlreadyConfigurationPropertySourcesPropertySourceReturnsNull() {
+		ConfigurationPropertySourcesPropertySource source = mock(ConfigurationPropertySourcesPropertySource.class);
+		assertThat(ConfigurationPropertySource.from(source)).isNull();
+	}
+
+}