Register nested @Configuration classes automatically

The following is now possible:

@Configuration
public class AppConfig {
    @Inject DataSource dataSource;

    @Bean
    public MyBean myBean() {
        return new MyBean(dataSource);
    }

    @Configuration
    static class DatabaseConfig {
        @Bean
        DataSource dataSource() {
            return new EmbeddedDatabaseBuilder().build();
        }
    }
}

public static void main(String... args) {
    AnnotationConfigApplicationContext ctx =
        new AnnotationConfigApplicationContext(AppConfig.class);
    ctx.getBean(MyBean.class);     // works
    ctx.getBean(DataSource.class); // works
}

Notice that the @Import annotation was not used and that only AppConfig
was registered against the context. By virtue of the fact that
DatabaseConfig is a member class of AppConfig, it is automatically
registered when AppConfig is registered. This avoids an awkward and
redundant @Import annotation when the relationship is already implicitly
clear.

See @Configuration Javadoc for details.

Issue: SPR-8186

org.springframework.context/src/test/java/org/springframework/context/annotation/NestedConfigurationClassTests.java

@@ -0,0 +1,141 @@
+/*
+ * 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 static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThat;
+
+import org.junit.Test;
+
+import test.beans.TestBean;
+
+/**
+ * Tests ensuring that nested static @Configuration classes are automatically detected
+ * and registered without the need for explicit registration or @Import. See SPR-8186.
+ *
+ * @author Chris Beams
+ * @since 3.1
+ */
+public class NestedConfigurationClassTests {
+
+	@Test
+	public void oneLevelDeep() {
+		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+		ctx.register(L0Config.L1Config.class);
+		ctx.refresh();
+
+		assertFalse(ctx.containsBean("l0Bean"));
+
+		ctx.getBean(L0Config.L1Config.class);
+		ctx.getBean("l1Bean");
+
+		ctx.getBean(L0Config.L1Config.L2Config.class);
+		ctx.getBean("l2Bean");
+
+		// ensure that override order is correct
+		assertThat(ctx.getBean("overrideBean", TestBean.class).getName(), is("override-l1"));
+	}
+
+	@Test
+	public void twoLevelsDeep() {
+		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+		ctx.register(L0Config.class);
+		ctx.refresh();
+
+		ctx.getBean(L0Config.class);
+		ctx.getBean("l0Bean");
+
+		ctx.getBean(L0Config.L1Config.class);
+		ctx.getBean("l1Bean");
+
+		ctx.getBean(L0Config.L1Config.L2Config.class);
+		ctx.getBean("l2Bean");
+
+		// ensure that override order is correct
+		assertThat(ctx.getBean("overrideBean", TestBean.class).getName(), is("override-l0"));
+	}
+
+	@Test
+	public void twoLevelsDeepWithInheritance() {
+		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+		ctx.register(S1Config.class);
+		ctx.refresh();
+
+		ctx.getBean(S1Config.class);
+		ctx.getBean("l0Bean");
+
+		ctx.getBean(L0Config.L1Config.class);
+		ctx.getBean("l1Bean");
+
+		ctx.getBean(L0Config.L1Config.L2Config.class);
+		ctx.getBean("l2Bean");
+
+		// ensure that override order is correct
+		assertThat(ctx.getBean("overrideBean", TestBean.class).getName(), is("override-s1"));
+	}
+
+
+	@Configuration
+	static class L0Config {
+		@Bean
+		public TestBean l0Bean() {
+			return new TestBean("l0");
+		}
+
+		@Bean
+		public TestBean overrideBean() {
+			return new TestBean("override-l0");
+		}
+
+		@Configuration
+		static class L1Config {
+			@Bean
+			public TestBean l1Bean() {
+				return new TestBean("l1");
+			}
+
+			@Bean
+			public TestBean overrideBean() {
+				return new TestBean("override-l1");
+			}
+
+			@Configuration
+			protected static class L2Config {
+				@Bean
+				public TestBean l2Bean() {
+					return new TestBean("l2");
+				}
+
+				@Bean
+				public TestBean overrideBean() {
+					return new TestBean("override-l2");
+				}
+			}
+		}
+	}
+
+
+	@Configuration
+	static class S1Config extends L0Config {
+		@Bean
+		public TestBean overrideBean() {
+			return new TestBean("override-s1");
+		}
+	}
+
+}