Restore RestartClassLoader's Java 6 compatibility

Closes gh-12826

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/RestartClassLoader.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.devtools.restart.classloader;
 
 import java.io.IOException;
+import java.lang.reflect.Method;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLClassLoader;
@@ -30,6 +31,7 @@ import org.apache.commons.logging.LogFactory;
 import org.springframework.boot.devtools.restart.classloader.ClassLoaderFile.Kind;
 import org.springframework.core.SmartClassLoader;
 import org.springframework.util.Assert;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * Disposable {@link ClassLoader} used to support application restarting. Provides parent
@@ -41,6 +43,8 @@ import org.springframework.util.Assert;
  */
 public class RestartClassLoader extends URLClassLoader implements SmartClassLoader {
 
+	private final ClassLoadingLockSupplier classLoadingLockSupplier;
+
 	private final Log logger;
 
 	private final ClassLoaderFileRepository updatedFiles;
@@ -85,6 +89,11 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 		if (logger.isDebugEnabled()) {
 			logger.debug("Created RestartClassLoader " + toString());
 		}
+		Method classLoadingLockMethod = ReflectionUtils.findMethod(ClassLoader.class,
+				"getClassLoadingLock", String.class);
+		this.classLoadingLockSupplier = classLoadingLockMethod != null
+						? new StandardClassLoadingLockSupplier()
+						: new Java6ClassLoadingLockSupplier();
 	}
 
 	@Override
@@ -142,7 +151,7 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 		if (file != null && file.getKind() == Kind.DELETED) {
 			throw new ClassNotFoundException(name);
 		}
-		synchronized (getClassLoadingLock(name)) {
+		synchronized (this.classLoadingLockSupplier.getClassLoadingLock(this, name)) {
 			Class<?> loadedClass = findLoadedClass(name);
 			if (loadedClass == null) {
 				try {
@@ -232,4 +241,32 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 
 	}
 
+	private interface ClassLoadingLockSupplier {
+
+		Object getClassLoadingLock(RestartClassLoader classLoader, String className);
+
+	}
+
+	private static final class Java6ClassLoadingLockSupplier
+			implements ClassLoadingLockSupplier {
+
+		@Override
+		public Object getClassLoadingLock(RestartClassLoader classLoader,
+				String className) {
+			return classLoader;
+		}
+
+	}
+
+	private static final class StandardClassLoadingLockSupplier
+			implements ClassLoadingLockSupplier {
+
+		@Override
+		public Object getClassLoadingLock(RestartClassLoader classLoader,
+				String className) {
+			return classLoader.getClassLoadingLock(className);
+		}
+
+	}
+
 }