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PowerMock Java agent support.
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/*
 *   Copyright 2016 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.powermock.modules.agent;

import javassist.ClassPool;
import javassist.CtClass;
import org.powermock.core.agent.JavaAgentClassRegister;
import org.powermock.core.transformers.ClassWrapper;
import org.powermock.core.transformers.ClassWrapperFactory;
import org.powermock.core.transformers.MockTransformerChain;
import org.powermock.core.transformers.TransformStrategy;
import org.powermock.core.transformers.javassist.JavassistMockTransformerChainFactory;
import org.powermock.core.transformers.javassist.support.JavaAssistClassWrapperFactory;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import java.util.Collection;
import java.util.HashSet;
import java.util.Set;

class PowerMockClassTransformer extends AbstractClassTransformer implements ClassFileTransformer {
    
    private static final MockTransformerChain MOCK_TRANSFORMER_CHAIN = new JavassistMockTransformerChainFactory()
                                                                           .createDefaultChain(TransformStrategy.INST_REDEFINE);
    private final ClassWrapperFactory wrapperFactory;
    private volatile Set classesToTransform;
    private volatile JavaAgentClassRegister javaAgentClassRegister;
    
    PowerMockClassTransformer() {
        super();
        wrapperFactory = new JavaAssistClassWrapperFactory();
    }
    
    public void setClassesToTransform(Collection classesToTransform) {
        this.classesToTransform = new HashSet(classesToTransform);
    }
    
    public void setJavaAgentClassRegister(JavaAgentClassRegister javaAgentClassRegister) {
        this.javaAgentClassRegister = javaAgentClassRegister;
    }
    
    public byte[] transform(
                               ClassLoader loader, String className,
                               Class classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer
    ) throws IllegalClassFormatException {
        
        if (loader == null || shouldIgnore(className)) {
            return null;
        }
        try {
            String normalizedClassName = className.replace("/", ".");
            if (classesToTransform != null && classesToTransform.contains(normalizedClassName)) {
                CtClass ctClass = getCtClass(classfileBuffer);
                
                ctClass = transform(ctClass);

                /*
                 * ClassPool may cause huge memory consumption if the number of CtClass
                 * objects becomes amazingly large (this rarely happens since Javassist
                 * tries to reduce memory consumption in various ways). To avoid this
                 * problem, you can explicitly remove an unnecessary CtClass object from
                 * the ClassPool. If you call detach() on a CtClass object, then that
                 * CtClass object is removed from the ClassPool.
                 */
                ctClass.detach();
                
                javaAgentClassRegister.registerClass(loader, normalizedClassName);
                
                return ctClass.toBytecode();
            }
            
            return null;
        } catch (Exception e) {
            throw new RuntimeException("Failed to redefine class " + className, e);
        }
    }
    
    private CtClass getCtClass(final byte[] classfileBuffer) throws IOException {
        final CtClass ctClass;
        
        ByteArrayInputStream is = new ByteArrayInputStream(classfileBuffer);
        try {
            ctClass = ClassPool.getDefault().makeClass(is);
        } finally {
            is.close();
        }
        return ctClass;
    }
    
    private CtClass transform(CtClass ctClass) throws Exception {
        ClassWrapper wrapped = wrapperFactory.wrap(ctClass);
        wrapped = MOCK_TRANSFORMER_CHAIN.transform(wrapped);
        return wrapped.unwrap();
    }
}