Add detection of default config classes to SpringApplicationConfiguration

I guess this didn't get done before because of the legacy structure of the
class hierarchy of TestContextLoaders. It makes sense that we should just prefer
netsted @Configuration though, so that's what this change does.

Fixes gh-271

spring-boot/src/main/java/org/springframework/boot/test/SpringApplicationContextLoader.java

@@ -33,6 +33,7 @@ import org.springframework.mock.web.MockServletContext;
 import org.springframework.test.context.ContextLoader;
 import org.springframework.test.context.MergedContextConfiguration;
 import org.springframework.test.context.support.AbstractContextLoader;
+import org.springframework.test.context.support.AnnotationConfigContextLoaderUtils;
 import org.springframework.test.context.web.WebAppConfiguration;
 import org.springframework.test.context.web.WebMergedContextConfiguration;
 import org.springframework.util.ObjectUtils;
@@ -82,9 +83,31 @@ public class SpringApplicationContextLoader extends AbstractContextLoader {
 		Set<Object> sources = new LinkedHashSet<Object>();
 		sources.addAll(Arrays.asList(mergedConfig.getClasses()));
 		sources.addAll(Arrays.asList(mergedConfig.getLocations()));
+		if (sources.isEmpty()) {
+			Class<?>[] defaultConfigClasses = detectDefaultConfigurationClasses(mergedConfig
+					.getTestClass());
+			sources.addAll(Arrays.asList(defaultConfigClasses));
+		}
 		return sources;
 	}
 
+	/**
+	 * Detect the default configuration classes for the supplied test class.
+	 * 
+	 * <p>
+	 * The default implementation simply delegates to
+	 * {@link AnnotationConfigContextLoaderUtils#detectDefaultConfigurationClasses(Class)}.
+	 * 
+	 * @param declaringClass the test class that declared {@code @ContextConfiguration}
+	 * @return an array of default configuration classes, potentially empty but never
+	 * {@code null}
+	 * @see AnnotationConfigContextLoaderUtils
+	 */
+	protected Class<?>[] detectDefaultConfigurationClasses(Class<?> declaringClass) {
+		return AnnotationConfigContextLoaderUtils
+				.detectDefaultConfigurationClasses(declaringClass);
+	}
+
 	private Map<String, Object> getArgs(MergedContextConfiguration mergedConfig) {
 		Map<String, Object> args = new LinkedHashMap<String, Object>();
 		// Not running an embedded server, just setting up web context

spring-boot/src/test/java/org/springframework/boot/test/SpringApplicationConfigurationDefaultConfigurationTests.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2012-2013 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.test;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+import static org.junit.Assert.assertNotNull;
+
+/**
+ * @author Dave Syer
+ */
+@RunWith(SpringJUnit4ClassRunner.class)
+@SpringApplicationConfiguration
+public class SpringApplicationConfigurationDefaultConfigurationTests {
+
+	@Autowired
+	private Config config;
+
+	@Test
+	public void nestedConfigClasses() {
+		assertNotNull(this.config);
+	}
+
+	@Configuration
+	protected static class Config {
+
+	}
+
+}