Avoid ClassCastExceptions in DevTools when using Spring HATEOAS

Spring HATEOAS’s DummyInvocationUtils, that is used by
ControllerLinkBuilder, uses Objenesis. By default, Objenesis caches
the instances that it creates and uses the class name as the cache key.
This is problematic when DevTools’ restart support is enabled as the
class loader changes with each restart and ClassCastExceptions occur
due to Objenesis returning a cached instance that was created by an
old restart class loader.

This commit works around the problem described above by auto-configuring
a bean that uses reflection to replace DummyInvocationUtils’ default
Objenesis instance with one that has caching disabled.

Closes gh-3784

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/HateoasObjenesisCacheDisabler.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2012-2015 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.devtools.autoconfigure;
+
+import java.lang.reflect.Field;
+
+import javax.annotation.PostConstruct;
+
+import org.springframework.objenesis.ObjenesisStd;
+import org.springframework.util.ClassUtils;
+import org.springframework.util.ReflectionUtils;
+
+/**
+ * Replaces the Objenesis instance in Spring HATEOAS's {@code DummyInvocationUtils} with
+ * one that does not perform any caching. The cache is problematic as it's keyed on class
+ * name which leads to {@code ClassCastExceptions} as the class loader changes across
+ * restarts.
+ *
+ * @author Andy Wilkinson
+ * @since 1.3.0
+ */
+class HateoasObjenesisCacheDisabler {
+
+	private static boolean cacheDisabled;
+
+	@PostConstruct
+	void disableCaching() {
+		if (!cacheDisabled) {
+			cacheDisabled = true;
+			doDisableCaching();
+		}
+	}
+
+	private void doDisableCaching() {
+		try {
+			Class<?> type = ClassUtils.forName(
+					"org.springframework.hateoas.core.DummyInvocationUtils",
+					getClass().getClassLoader());
+			Field objenesis = ReflectionUtils.findField(type, "OBJENESIS");
+			if (objenesis != null) {
+				ReflectionUtils.makeAccessible(objenesis);
+				ReflectionUtils.setField(objenesis, null, new ObjenesisStd(false));
+			}
+		}
+		catch (Exception ex) {
+			// Assume that Spring HATEOAS is not on the classpath and continue
+		}
+	}
+
+}

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/LocalDevToolsAutoConfiguration.java

@@ -126,6 +126,11 @@ public class LocalDevToolsAutoConfiguration {
 					this.properties.getRestart().getAllExclude());
 		}
 
+		@Bean
+		public HateoasObjenesisCacheDisabler hateoasObjenesisCacheDisabler() {
+			return new HateoasObjenesisCacheDisabler();
+		}
+
 		@Bean
 		public FileSystemWatcherFactory fileSystemWatcherFactory() {
 			return new FileSystemWatcherFactory() {