/*
 * Copyright 2002-2019 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.instrument.IllegalClassFormatException;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;
import java.util.ArrayList;
import java.util.List;

import org.springframework.instrument.InstrumentationSavingAgent;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;

/**
 * {@link LoadTimeWeaver} relying on VM {@link Instrumentation}.
 *
 * <p>Start the JVM specifying the Java agent to be used &mdash; for example, as
 * follows where <code>spring-instrument-{version}.jar</code> is a JAR file
 * containing the {@link InstrumentationSavingAgent} class shipped with Spring
 * and where <code>{version}</code> is the release version of the Spring
 * Framework (e.g., {@code 5.1.5.RELEASE}).
 *
 * <p><code>-javaagent:path/to/spring-instrument-{version}.jar</code>
 *
 * <p>In Eclipse, for example, add something similar to the following to the
 * JVM arguments for the Eclipse "Run configuration":
 *
 * <p><code>-javaagent:${project_loc}/lib/spring-instrument-{version}.jar</code>
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 2.0
 * @see InstrumentationSavingAgent
 */
public class InstrumentationLoadTimeWeaver implements LoadTimeWeaver {

        private static final boolean AGENT_CLASS_PRESENT = ClassUtils.isPresent(
                        "org.springframework.instrument.InstrumentationSavingAgent",
                        InstrumentationLoadTimeWeaver.class.getClassLoader());


        @Nullable
        private final ClassLoader classLoader;

        @Nullable
        private final Instrumentation instrumentation;

        private final List<ClassFileTransformer> transformers = new ArrayList<>(4);


        /**
         * Create a new InstrumentationLoadTimeWeaver for the default ClassLoader.
         */
        public InstrumentationLoadTimeWeaver() {
                this(ClassUtils.getDefaultClassLoader());
        }

        /**
         * Create a new InstrumentationLoadTimeWeaver for the given ClassLoader.
         * @param classLoader the ClassLoader that registered transformers are supposed to apply to
         */
        public InstrumentationLoadTimeWeaver(@Nullable ClassLoader classLoader) {
                this.classLoader = classLoader;
                this.instrumentation = getInstrumentation();
        }


        @Override
        public void addTransformer(ClassFileTransformer transformer) {
                Assert.notNull(transformer, "Transformer must not be null");
                FilteringClassFileTransformer actualTransformer =
                                new FilteringClassFileTransformer(transformer, this.classLoader);
                synchronized (this.transformers) {
                        Assert.state(this.instrumentation != null,
                                        "Must start with Java agent to use InstrumentationLoadTimeWeaver. See Spring documentation.");
                        this.instrumentation.addTransformer(actualTransformer);
                        this.transformers.add(actualTransformer);
                }
        }

        /**
         * We have the ability to weave the current class loader when starting the
         * JVM in this way, so the instrumentable class loader will always be the
         * current loader.
         */
        @Override
        public ClassLoader getInstrumentableClassLoader() {
                Assert.state(this.classLoader != null, "No ClassLoader available");
                return this.classLoader;
        }

        /**
         * This implementation always returns a {@link SimpleThrowawayClassLoader}.
         */
        @Override
        public ClassLoader getThrowawayClassLoader() {
                return new SimpleThrowawayClassLoader(getInstrumentableClassLoader());
        }

        /**
         * Remove all registered transformers, in inverse order of registration.
         */
        public void removeTransformers() {
                synchronized (this.transformers) {
                        if (this.instrumentation != null && !this.transformers.isEmpty()) {
                                for (int i = this.transformers.size() - 1; i >= 0; i--) {
                                        this.instrumentation.removeTransformer(this.transformers.get(i));
                                }
                                this.transformers.clear();
                        }
                }
        }


        /**
         * Check whether an Instrumentation instance is available for the current VM.
         * @see #getInstrumentation()
         */
        public static boolean isInstrumentationAvailable() {
                return (getInstrumentation() != null);
        }

        /**
         * Obtain the Instrumentation instance for the current VM, if available.
         * @return the Instrumentation instance, or {@code null} if none found
         * @see #isInstrumentationAvailable()
         */
        @Nullable
        private static Instrumentation getInstrumentation() {
                if (AGENT_CLASS_PRESENT) {
                        return InstrumentationAccessor.getInstrumentation();
                }
                else {
                        return null;
                }
        }


        /**
         * Inner class to avoid InstrumentationSavingAgent dependency.
         */
        private static class InstrumentationAccessor {

                public static Instrumentation getInstrumentation() {
                        return InstrumentationSavingAgent.getInstrumentation();
                }
        }


        /**
         * Decorator that only applies the given target transformer to a specific ClassLoader.
         */
        private static class FilteringClassFileTransformer implements ClassFileTransformer {

                private final ClassFileTransformer targetTransformer;

                @Nullable
                private final ClassLoader targetClassLoader;

                public FilteringClassFileTransformer(
                                ClassFileTransformer targetTransformer, @Nullable ClassLoader targetClassLoader) {

                        this.targetTransformer = targetTransformer;
                        this.targetClassLoader = targetClassLoader;
                }

                @Override
                @Nullable
                public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
                                ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {

                        if (this.targetClassLoader != loader) {
                                return null;
                        }
                        return this.targetTransformer.transform(
                                        loader, className, classBeingRedefined, protectionDomain, classfileBuffer);
                }

                @Override
                public String toString() {
                        return "FilteringClassFileTransformer for: " + this.targetTransformer.toString();
                }
        }

}