Allow default CacheAwareContextLoaderDelegate configuration via system property

Prior to this commit, the default CacheAwareContextLoaderDelegate could
be configured by extending AbstractTestContextBootstrapper and
overriding getCacheAwareContextLoaderDelegate(); however, this required
that the user configure the custom TestContextBootstrapper via
@BootstrapWith.

This commit introduces a new
"spring.test.context.default.CacheAwareContextLoaderDelegate" property
that can be configured via a JVM system property or via the
SpringProperties mechanism. BootstrapUtils uses this new property to
load the default CacheAwareContextLoaderDelegate. If the property is
not defined, BootstrapUtils will fall back to creating a
DefaultCacheAwareContextLoaderDelegate as it did previously.

This allows third parties to configure the default
CacheAwareContextLoaderDelegate transparently for the user -- for
example, to intercept context loading in order to load the context in a
different manner -- for example, to make use of ahead of time (AOT)
techniques for implementing a different type of ApplicationContext at
build time.

Closes gh-27540

spring-test/src/test/java/org/springframework/test/context/support/CustomDefaultCacheAwareContextLoaderDelegateTests.java

@@ -0,0 +1,95 @@
+/*
+ * Copyright 2002-2021 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
+ *
+ *      https://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.support;
+
+import org.junit.jupiter.api.Test;
+import org.junit.platform.testkit.engine.EngineTestKit;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.support.GenericApplicationContext;
+import org.springframework.core.SpringProperties;
+import org.springframework.test.context.CacheAwareContextLoaderDelegate;
+import org.springframework.test.context.MergedContextConfiguration;
+import org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.platform.engine.discovery.DiscoverySelectors.selectClass;
+
+/**
+ * Integration tests for configuring a custom default {@link CacheAwareContextLoaderDelegate}
+ * via {@link SpringProperties}.
+ *
+ * @author sbrannen
+ * @since 5.3.11
+ */
+class CustomDefaultCacheAwareContextLoaderDelegateTests {
+
+	@Test
+	void customDefaultCacheAwareContextLoaderDelegateConfiguredViaSpringProperties() {
+		String key = CacheAwareContextLoaderDelegate.DEFAULT_CACHE_AWARE_CONTEXT_LOADER_DELEGATE_PROPERTY_NAME;
+
+		try {
+			SpringProperties.setProperty(key, AotCacheAwareContextLoaderDelegate.class.getName());
+
+			EngineTestKit.engine("junit-jupiter")//
+					.selectors(selectClass(TestCase.class))//
+					.execute()//
+					.testEvents()//
+					.assertStatistics(stats -> stats.started(1).succeeded(1).failed(0));
+		}
+		finally {
+			SpringProperties.setProperty(key, null);
+		}
+	}
+
+
+	@SpringJUnitConfig
+	static class TestCase {
+
+		@Test
+		void test(@Autowired String foo) {
+			// foo will be "bar" unless the AotCacheAwareContextLoaderDelegate is registered.
+			assertThat(foo).isEqualTo("AOT");
+		}
+
+
+		@Configuration
+		static class Config {
+
+			@Bean
+			String foo() {
+				return "bar";
+			}
+		}
+	}
+
+	static class AotCacheAwareContextLoaderDelegate extends DefaultCacheAwareContextLoaderDelegate {
+
+		@Override
+		protected ApplicationContext loadContextInternal(MergedContextConfiguration mergedContextConfiguration) {
+			GenericApplicationContext applicationContext = new GenericApplicationContext();
+			applicationContext.registerBean("foo", String.class, () -> "AOT");
+			applicationContext.refresh();
+			return applicationContext;
+		}
+	}
+
+}