Add a parent last classloader for restart use

Add a parent last classloader for use with application restarts. The
classloader provides a layer on top of the regular classloader to
contain the classes that might change when an application is restarted.

See gh-3084

spring-boot-developer-tools/src/main/java/org/springframework/boot/developertools/restart/classloader/RestartClassLoader.java

@@ -0,0 +1,111 @@
+/*
+ * 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.developertools.restart.classloader;
+
+import java.io.IOException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.Enumeration;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import org.springframework.core.SmartClassLoader;
+import org.springframework.util.Assert;
+
+/**
+ * Disposable {@link ClassLoader} used to support application restarting. Provides parent
+ * last loading for the specified URLs.
+ *
+ * @author Andy Clement
+ * @author Phillip Webb
+ * @since 1.3.0
+ */
+public class RestartClassLoader extends URLClassLoader implements SmartClassLoader {
+
+	private final Log logger;
+
+	/**
+	 * Create a new {@link RestartClassLoader} instance.
+	 * @param parent the parent classloader URLs were created.
+	 * @param urls the urls managed by the classloader
+	 */
+	public RestartClassLoader(ClassLoader parent, URL[] urls) {
+		this(parent, urls, LogFactory.getLog(RestartClassLoader.class));
+	}
+
+	/**
+	 * Create a new {@link RestartClassLoader} instance.
+	 * @param parent the parent classloader URLs were created.
+	 * @param urls the urls managed by the classloader
+	 * @param logger the logger used for messages
+	 */
+	public RestartClassLoader(ClassLoader parent, URL[] urls, Log logger) {
+		super(urls, parent);
+		Assert.notNull(parent, "Parent must not be null");
+		Assert.notNull(logger, "Logger must not be null");
+		this.logger = logger;
+		if (logger.isDebugEnabled()) {
+			logger.debug("Created RestartClassLoader " + toString());
+		}
+	}
+
+	@Override
+	public Enumeration<URL> getResources(String name) throws IOException {
+		// Use the parent since we're shadowing resource and we don't want duplicates
+		return getParent().getResources(name);
+	}
+
+	@Override
+	public URL getResource(String name) {
+		URL resource = findResource(name);
+		if (resource != null) {
+			return resource;
+		}
+		return getParent().getResource(name);
+	}
+
+	@Override
+	public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
+		Class<?> loadedClass = findLoadedClass(name);
+		if (loadedClass == null) {
+			try {
+				loadedClass = findClass(name);
+			}
+			catch (ClassNotFoundException ex) {
+				loadedClass = getParent().loadClass(name);
+			}
+		}
+		if (resolve) {
+			resolveClass(loadedClass);
+		}
+		return loadedClass;
+	}
+
+	@Override
+	protected void finalize() throws Throwable {
+		if (this.logger.isDebugEnabled()) {
+			this.logger.debug("Finalized classloader " + toString());
+		}
+		super.finalize();
+	}
+
+	@Override
+	public boolean isClassReloadable(Class<?> classType) {
+		return (classType.getClassLoader() instanceof RestartClassLoader);
+	}
+
+}

spring-boot-developer-tools/src/main/java/org/springframework/boot/developertools/restart/classloader/package-info.java

@@ -0,0 +1,21 @@
+/*
+ * 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.
+ */
+
+/**
+ * Classloaders used for reload support
+ */
+package org.springframework.boot.developertools.restart.classloader;
+