Introduce ImportSelector interface

Allows @Enable* a layer of indirection for deciding which @Configuration
class(es) to @Import.

The @Import annotation may now accept @Configuration class literals
and/or ImportSelector class literals.

org.springframework.context/src/main/java/org/springframework/context/annotation/ConfigurationClassParser.java

@@ -27,6 +27,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.Stack;
 
+import org.springframework.beans.BeanUtils;
 import org.springframework.beans.factory.parsing.Location;
 import org.springframework.beans.factory.parsing.Problem;
 import org.springframework.beans.factory.parsing.ProblemReporter;
@@ -37,6 +38,7 @@ import org.springframework.core.type.MethodMetadata;
 import org.springframework.core.type.StandardAnnotationMetadata;
 import org.springframework.core.type.classreading.MetadataReader;
 import org.springframework.core.type.classreading.MetadataReaderFactory;
+import org.springframework.core.type.filter.AssignableTypeFilter;
 import org.springframework.util.StringUtils;
 
 /**
@@ -142,7 +144,7 @@ class ConfigurationClassParser {
 		List<Map<String, Object>> allImportAttribs =
 			AnnotationUtils.findAllAnnotationAttributes(Import.class, metadata.getClassName(), true);
 		for (Map<String, Object> importAttribs : allImportAttribs) {
-			processImport(configClass, (String[]) importAttribs.get("value"));
+			processImport(configClass, (String[]) importAttribs.get("value"), true);
 		}
 
 		if (metadata.isAnnotated(ImportResource.class.getName())) {
@@ -162,16 +164,29 @@ class ConfigurationClassParser {
 		}
 	}
 
-	private void processImport(ConfigurationClass configClass, String[] classesToImport) throws IOException {
+	private void processImport(ConfigurationClass configClass, String[] classesToImport, boolean checkForCircularImports) throws IOException {
-		if (this.importStack.contains(configClass)) {
+		if (checkForCircularImports && this.importStack.contains(configClass)) {
 			this.problemReporter.error(new CircularImportProblem(configClass, this.importStack, configClass.getMetadata()));
 		}
 		else {
 			this.importStack.push(configClass);
-			for (String classToImport : classesToImport) {
+			AnnotationMetadata importingClassMetadata = configClass.getMetadata();
-				this.importStack.registerImport(configClass.getMetadata().getClassName(), classToImport);
+			for (String candidate : classesToImport) {
-				MetadataReader reader = this.metadataReaderFactory.getMetadataReader(classToImport);
+				MetadataReader reader = this.metadataReaderFactory.getMetadataReader(candidate);
-				processConfigurationClass(new ConfigurationClass(reader, null));
+				if (new AssignableTypeFilter(ImportSelector.class).match(reader, metadataReaderFactory)) {
+					// the candidate class is an ImportSelector -> delegate to it to determine imports
+					try {
+						ImportSelector selector = BeanUtils.instantiateClass(Class.forName(candidate), ImportSelector.class);
+						processImport(configClass, selector.selectImports(importingClassMetadata), false);
+					} catch (ClassNotFoundException ex) {
+						throw new IllegalStateException(ex);
+					}
+				}
+				else {
+					// the candidate class not an ImportSelector -> process it as a @Configuration class
+					this.importStack.registerImport(importingClassMetadata.getClassName(), candidate);
+					processConfigurationClass(new ConfigurationClass(reader, null));
+				}
 			}
 			this.importStack.pop();
 		}

org.springframework.context/src/main/java/org/springframework/context/annotation/ImportSelector.java

@@ -0,0 +1,40 @@
+/*
+ * 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.context.annotation;
+
+import org.springframework.core.type.AnnotationMetadata;
+
+
+/**
+ * Interface to be implemented by types that determine
+ * which @{@link Configuration} class(es) should be imported based on
+ * a given selection criteria, usually an annotation attribute.
+ *
+ * @author Chris Beams
+ * @since 3.1
+ * @see Import
+ */
+public interface ImportSelector {
+
+	/**
+	 * Select and return the names of which class(es) should be imported.
+	 * @param importingClassMetadata the AnnotationMetodata of the
+	 * importing @{@link Configuration} class.
+	 */
+	String[] selectImports(AnnotationMetadata importingClassMetadata);
+
+}