/*
 * Copyright 2002-2018 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.instrument.classloading;

import java.lang.instrument.ClassFileTransformer;
import java.lang.reflect.Method;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.core.DecoratingClassLoader;
import org.springframework.core.OverridingClassLoader;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ReflectionUtils;

LoadTimeWeaver which uses reflection to delegate to an underlying ClassLoader with well-known transformation hooks. The underlying ClassLoader is expected to support the following weaving methods (as defined in the LoadTimeWeaver interface): 

public void addTransformer(java.lang.instrument.ClassFileTransformer): for registering the given ClassFileTransformer on this ClassLoader 
public ClassLoader getThrowawayClassLoader(): for obtaining a throwaway class loader for this ClassLoader (optional; ReflectiveLoadTimeWeaver will fall back to a SimpleThrowawayClassLoader if that method isn't available) 
Please note that the above methods must reside in a class that is
publicly accessible, although the class itself does not have to be visible
to the application's class loader.
The reflective nature of this LoadTimeWeaver is particularly useful when the
underlying ClassLoader implementation is loaded in a different class loader itself
(such as the application server's class loader which is not visible to the
web application). There is no direct API dependency between this LoadTimeWeaver
adapter and the underlying ClassLoader, just a 'loose' method contract.
This is the LoadTimeWeaver to use e.g. with the Resin application server
version 3.1+.
Author:
Costin Leau, Juergen Hoeller
See Also:
addTransformer(ClassFileTransformer)
getThrowawayClassLoader()
SimpleThrowawayClassLoader
Since:
2.0/**
 * {@link LoadTimeWeaver} which uses reflection to delegate to an underlying ClassLoader
 * with well-known transformation hooks. The underlying ClassLoader is expected to
 * support the following weaving methods (as defined in the {@link LoadTimeWeaver}
 * interface):
 * <ul>
 * <li>{@code public void addTransformer(java.lang.instrument.ClassFileTransformer)}:
 * for registering the given ClassFileTransformer on this ClassLoader
 * <li>{@code public ClassLoader getThrowawayClassLoader()}:
 * for obtaining a throwaway class loader for this ClassLoader (optional;
 * ReflectiveLoadTimeWeaver will fall back to a SimpleThrowawayClassLoader if
 * that method isn't available)
 * </ul>
 *
 * <p>Please note that the above methods <i>must</i> reside in a class that is
 * publicly accessible, although the class itself does not have to be visible
 * to the application's class loader.
 *
 * <p>The reflective nature of this LoadTimeWeaver is particularly useful when the
 * underlying ClassLoader implementation is loaded in a different class loader itself
 * (such as the application server's class loader which is not visible to the
 * web application). There is no direct API dependency between this LoadTimeWeaver
 * adapter and the underlying ClassLoader, just a 'loose' method contract.
 *
 * <p>This is the LoadTimeWeaver to use e.g. with the Resin application server
 * version 3.1+.
 *
 * @author Costin Leau
 * @author Juergen Hoeller
 * @since 2.0
 * @see #addTransformer(java.lang.instrument.ClassFileTransformer)
 * @see #getThrowawayClassLoader()
 * @see SimpleThrowawayClassLoader
 */
public class ReflectiveLoadTimeWeaver implements LoadTimeWeaver {

	private static final String ADD_TRANSFORMER_METHOD_NAME = "addTransformer";

	private static final String GET_THROWAWAY_CLASS_LOADER_METHOD_NAME = "getThrowawayClassLoader";

	private static final Log logger = LogFactory.getLog(ReflectiveLoadTimeWeaver.class);


	private final ClassLoader classLoader;

	private final Method addTransformerMethod;

	@Nullable
	private final Method getThrowawayClassLoaderMethod;


	
Create a new ReflectiveLoadTimeWeaver for the current context class
loader, which needs to support the required weaving methods.
/**
	 * Create a new ReflectiveLoadTimeWeaver for the current context class
	 * loader, <i>which needs to support the required weaving methods</i>.
	 */
	public ReflectiveLoadTimeWeaver() {
		this(ClassUtils.getDefaultClassLoader());
	}

	
Create a new SimpleLoadTimeWeaver for the given class loader.
Params:
classLoader – the ClassLoader to delegate to for weaving (must support the required weaving methods).
Throws:
IllegalStateException – if the supplied ClassLoader does not support the required weaving methods/**
	 * Create a new SimpleLoadTimeWeaver for the given class loader.
	 * @param classLoader the {@code ClassLoader} to delegate to for
	 * weaving (<i>must</i> support the required weaving methods).
	 * @throws IllegalStateException if the supplied {@code ClassLoader}
	 * does not support the required weaving methods
	 */
	public ReflectiveLoadTimeWeaver(@Nullable ClassLoader classLoader) {
		Assert.notNull(classLoader, "ClassLoader must not be null");
		this.classLoader = classLoader;

		Method addTransformerMethod = ClassUtils.getMethodIfAvailable(
				this.classLoader.getClass(), ADD_TRANSFORMER_METHOD_NAME, ClassFileTransformer.class);
		if (addTransformerMethod == null) {
			throw new IllegalStateException(
					"ClassLoader [" + classLoader.getClass().getName() + "] does NOT provide an " +
					"'addTransformer(ClassFileTransformer)' method.");
		}
		this.addTransformerMethod = addTransformerMethod;

		Method getThrowawayClassLoaderMethod = ClassUtils.getMethodIfAvailable(
				this.classLoader.getClass(), GET_THROWAWAY_CLASS_LOADER_METHOD_NAME);
		// getThrowawayClassLoader method is optional
		if (getThrowawayClassLoaderMethod == null) {
			if (logger.isDebugEnabled()) {
				logger.debug("The ClassLoader [" + classLoader.getClass().getName() + "] does NOT provide a " +
						"'getThrowawayClassLoader()' method; SimpleThrowawayClassLoader will be used instead.");
			}
		}
		this.getThrowawayClassLoaderMethod = getThrowawayClassLoaderMethod;
	}


	@Override
	public void addTransformer(ClassFileTransformer transformer) {
		Assert.notNull(transformer, "Transformer must not be null");
		ReflectionUtils.invokeMethod(this.addTransformerMethod, this.classLoader, transformer);
	}

	@Override
	public ClassLoader getInstrumentableClassLoader() {
		return this.classLoader;
	}

	@Override
	public ClassLoader getThrowawayClassLoader() {
		if (this.getThrowawayClassLoaderMethod != null) {
			ClassLoader target = (ClassLoader)
					ReflectionUtils.invokeMethod(this.getThrowawayClassLoaderMethod, this.classLoader);
			return (target instanceof DecoratingClassLoader ? target :
					new OverridingClassLoader(this.classLoader, target));
		}
		else {
			return new SimpleThrowawayClassLoader(this.classLoader);
		}
	}

}