Demo how to use SpringClassRule in nested test classes

Due to restrictions imposed by JUnit 4, the SpringClassRule must be
declared as a public static field, which makes it impossible to be
declared directly within a nested (i.e., inner) test class.

This commit demonstrates a work-around that makes it possible to use
the SpringClassRule and SpringMethodRule in a nested (i.e., inner) test
class when using a custom JUnit 4 runner such as the
HierarchicalContextRunner from Stefan Bechtold.

The trick is to have inner test classes extend a class that properly
declares the SpringClassRule and SpringMethodRule. The
SpringRuleConfigurer in this commit serves as an example.

Note, however, that each such nested test class must declare its own
@ContextConfiguration. Furthermore, TestExecutionListeners in the
Spring TestContext Framework are not applied to the enclosing instance
of such an inner test class, meaning that @Autowired fields in the
enclosing instance will not be injected, etc.

Issue: SPR-14150

spring-test/src/test/java/org/springframework/test/context/junit4/nested/NestedTestsWithSpringRulesTests.java

@@ -0,0 +1,97 @@
+/*
+ * Copyright 2002-2016 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.test.context.junit4.nested;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.context.ContextConfiguration;
+import org.springframework.test.context.junit4.nested.NestedTestsWithSpringRulesTests.TopLevelConfig;
+import org.springframework.test.context.junit4.rules.SpringClassRule;
+import org.springframework.test.context.junit4.rules.SpringMethodRule;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+import de.bechte.junit.runners.context.HierarchicalContextRunner;
+
+/**
+ * JUnit 4 based integration tests for <em>nested</em> test classes that are
+ * executed via a custom JUnit 4 {@link HierarchicalContextRunner} and Spring's
+ * {@link SpringClassRule} and {@link SpringMethodRule} support.
+ *
+ * @author Sam Brannen
+ * @since 5.0
+ */
+@RunWith(HierarchicalContextRunner.class)
+@ContextConfiguration(classes = TopLevelConfig.class)
+public class NestedTestsWithSpringRulesTests extends SpringRuleConfigurer {
+
+	@Autowired
+	String foo;
+
+
+	@Test
+	public void topLevelTest() {
+		assertEquals("foo", foo);
+	}
+
+
+	@ContextConfiguration(classes = NestedConfig.class)
+	public class NestedTestCase extends SpringRuleConfigurer {
+
+		@Autowired
+		String bar;
+
+
+		@Test
+		public void nestedTest() throws Exception {
+			// Note: the following would fail since TestExecutionListeners in
+			// the Spring TestContext Framework are not applied to the enclosing
+			// instance of an inner test class.
+			//
+			// assertEquals("foo", foo);
+
+			assertNull("@Autowired field in enclosing instance should be null.", foo);
+			assertEquals("bar", bar);
+		}
+	}
+
+	// -------------------------------------------------------------------------
+
+	@Configuration
+	public static class TopLevelConfig {
+
+		@Bean
+		String foo() {
+			return "foo";
+		}
+	}
+
+	@Configuration
+	public static class NestedConfig {
+
+		@Bean
+		String bar() {
+			return "bar";
+		}
+	}
+
+}

spring-test/src/test/java/org/springframework/test/context/junit4/nested/SpringRuleConfigurer.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2002-2016 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.test.context.junit4.nested;
+
+import org.junit.ClassRule;
+import org.junit.Rule;
+
+import org.springframework.test.context.junit4.rules.SpringClassRule;
+import org.springframework.test.context.junit4.rules.SpringMethodRule;
+
+/**
+ * Abstract base test class that preconfigures the {@link SpringClassRule} and
+ * {@link SpringMethodRule}.
+ *
+ * @author Sam Brannen
+ * @since 5.0
+ */
+public abstract class SpringRuleConfigurer {
+
+	@ClassRule
+	public static final SpringClassRule SPRING_CLASS_RULE = new SpringClassRule();
+
+	@Rule
+	public final SpringMethodRule springMethodRule = new SpringMethodRule();
+
+}