Add Configurations class

Add a general purpose `Configurations` class that encapsulates the
sorting and merging rules that are usually apply. The class is
particularly useful in tests where configuration classes often need
to be specified, but an `@Import` or `ImportSelector` cannot be easily
used.

Two `Configurations` subclasses have been initially added. The
`UserConfigurations` class can be used to represent user defined
configuration and the `AutoConfigurations` class can be used to
represent a subset of auto-configurations. Auto configurations are
sorted using the same `@AutoConfiguraionBefore`/`@AutoConfiguraionAfter`
logic as the `@EnableAutoConfiguration` annotation.

Fixes gh-9795

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/AutoConfigurationSorter.java

@@ -168,7 +168,7 @@ class AutoConfigurationSorter {
 		}
 
 		private int getOrder() {
-			if (this.autoConfigurationMetadata.wasProcessed(this.className)) {
+			if (wasProcessed()) {
 				return this.autoConfigurationMetadata.getInteger(this.className,
 						"AutoConfigureOrder", Ordered.LOWEST_PRECEDENCE);
 			}
@@ -179,7 +179,7 @@ class AutoConfigurationSorter {
 		}
 
 		private Set<String> readBefore() {
-			if (this.autoConfigurationMetadata.wasProcessed(this.className)) {
+			if (wasProcessed()) {
 				return this.autoConfigurationMetadata.getSet(this.className,
 						"AutoConfigureBefore", Collections.<String>emptySet());
 			}
@@ -187,13 +187,18 @@ class AutoConfigurationSorter {
 		}
 
 		private Set<String> readAfter() {
-			if (this.autoConfigurationMetadata.wasProcessed(this.className)) {
+			if (wasProcessed()) {
 				return this.autoConfigurationMetadata.getSet(this.className,
 						"AutoConfigureAfter", Collections.<String>emptySet());
 			}
 			return getAnnotationValue(AutoConfigureAfter.class);
 		}
 
+		private boolean wasProcessed() {
+			return (this.autoConfigurationMetadata != null
+					&& this.autoConfigurationMetadata.wasProcessed(this.className));
+		}
+
 		private Set<String> getAnnotationValue(Class<?> annotation) {
 			Map<String, Object> attributes = getAnnotationMetadata()
 					.getAnnotationAttributes(annotation.getName(), true);

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/AutoConfigurations.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2012-2017 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.boot.autoconfigure;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import org.springframework.boot.context.annotation.Configurations;
+import org.springframework.core.Ordered;
+import org.springframework.core.type.classreading.SimpleMetadataReaderFactory;
+import org.springframework.util.ClassUtils;
+
+/**
+ * {@link Configurations} representing auto-configuration {@code @Configuration} classes.
+ *
+ * @author Philip Webb
+ * @since 2.0.0
+ */
+public class AutoConfigurations extends Configurations implements Ordered {
+
+	private static AutoConfigurationSorter SORTER = new AutoConfigurationSorter(
+			new SimpleMetadataReaderFactory(), null);
+
+	private static final Ordered ORDER = new AutoConfigurationImportSelector();
+
+	protected AutoConfigurations(Collection<Class<?>> classes) {
+		super(classes);
+	}
+
+	@Override
+	protected Collection<Class<?>> sort(Collection<Class<?>> classes) {
+		List<String> names = classes.stream().map(Class::getName)
+				.collect(Collectors.toCollection(ArrayList::new));
+		List<String> sorted = SORTER.getInPriorityOrder(names);
+		return sorted.stream()
+				.map(className -> ClassUtils.resolveClassName(className, null))
+				.collect(Collectors.toCollection(ArrayList::new));
+	}
+
+	@Override
+	public int getOrder() {
+		return ORDER.getOrder();
+	}
+
+	@Override
+	protected AutoConfigurations merge(Set<Class<?>> mergedClasses) {
+		return new AutoConfigurations(mergedClasses);
+	}
+
+	public static AutoConfigurations of(Class<?>... classes) {
+		return new AutoConfigurations(Arrays.asList(classes));
+	}
+
+}

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/AutoConfigurationsTests.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2012-2017 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.boot.autoconfigure;
+
+import org.junit.Test;
+
+import org.springframework.boot.context.annotation.Configurations;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link AutoConfigurations}.
+ *
+ * @author Phillip Webb
+ */
+public class AutoConfigurationsTests {
+
+	@Test
+	public void ofShouldCreateOrderedConfigurations() throws Exception {
+		Configurations configurations = AutoConfigurations.of(AutoConfigureA.class,
+				AutoConfigureB.class);
+		assertThat(Configurations.getClasses(configurations))
+				.containsExactly(AutoConfigureB.class, AutoConfigureA.class);
+	}
+
+	@AutoConfigureAfter(AutoConfigureB.class)
+	public static class AutoConfigureA {
+
+	}
+
+	public static class AutoConfigureB {
+
+	}
+
+}