Demo: @Autowired rules not supported with Spring's JUnit rules

This commit introduces a test that demonstrates that custom JUnit 4
rules can be @Autowired into a test instance but that they will not be
applied by JUnit since JUnit only ever sees such fields as null and
therefore ignores them.

Issue: SPR-15927

spring-test/src/test/java/org/springframework/test/context/junit4/rules/AutowiredRuleTests.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2002-2017 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.test.context.junit4.rules;
+
+import org.junit.ClassRule;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TestRule;
+import org.junit.runner.Description;
+import org.junit.runners.model.Statement;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import static org.junit.Assert.*;
+
+/**
+ * Integration tests for an issue raised in https://jira.spring.io/browse/SPR-15927.
+ *
+ * @author Sam Brannen
+ * @since 5.0
+ */
+public class AutowiredRuleTests {
+
+	@ClassRule
+	public static final SpringClassRule springClassRule = new SpringClassRule();
+
+	@Rule
+	public final SpringMethodRule springMethodRule = new SpringMethodRule();
+
+	@Autowired
+	@Rule
+	public AutowiredTestRule autowiredTestRule;
+
+	@Test
+	public void test() {
+		assertNotNull("TestRule should have been @Autowired", autowiredTestRule);
+
+		// Rationale for the following assertion:
+		//
+		// The field value for the custom rule is null when JUnit sees it. JUnit then
+		// ignores the null value, and at a later point in time Spring injects the rule
+		// from the ApplicationContext and overrides the null field value. But that's too
+		// late: JUnit never sees the rule supplied by Spring via dependency injection.
+		assertFalse("@Autowired TestRule should NOT have been applied", autowiredTestRule.applied);
+	}
+
+	@Configuration
+	static class Config {
+
+		@Bean
+		AutowiredTestRule autowiredTestRule() {
+			return new AutowiredTestRule();
+		}
+	}
+
+	static class AutowiredTestRule implements TestRule {
+
+		private boolean applied = false;
+
+		@Override
+		public Statement apply(Statement base, Description description) {
+			this.applied = true;
+			return base;
+		}
+	}
+
+}