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JMockit is a Java toolkit for automated developer testing. It contains mocking/faking APIs and a code coverage tool, supporting both JUnit and TestNG. The mocking APIs allow all kinds of Java code, without testability restrictions, to be tested in isolation from selected dependencies.

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/*
 * Copyright (c) 2006 JMockit developers
 * This file is subject to the terms of the MIT license (see LICENSE.txt).
 */
package mockit.internal.classGeneration;

import java.lang.reflect.Type;
import javax.annotation.*;

import mockit.asm.*;
import mockit.internal.*;
import mockit.internal.util.*;

/**
 * Allows the creation of new implementation classes for interfaces and abstract classes.
 */
public abstract class ImplementationClass
{
   @Nonnull protected final Class sourceClass;
   @Nonnull protected String generatedClassName;

   protected ImplementationClass(@Nonnull Type mockedType) { this(Utilities.getClassType(mockedType)); }

   protected ImplementationClass(@Nonnull Class mockedClass) {
      this(mockedClass, GeneratedClasses.getNameForGeneratedClass(mockedClass, null));
   }

   protected ImplementationClass(@Nonnull Class sourceClass, @Nonnull String desiredClassName) {
      this.sourceClass = sourceClass;
      generatedClassName = desiredClassName;
   }

   @Nonnull
   public final Class generateClass() {
      ClassReader classReader = ClassFile.createReaderOrGetFromCache(sourceClass);

      ClassVisitor modifier = createMethodBodyGenerator(classReader.getBytecode());
      classReader.accept(modifier);

      return defineNewClass(modifier);
   }

   @Nonnull
   protected abstract ClassVisitor createMethodBodyGenerator(@Nonnull byte[] classfile);

   @Nonnull
   private Class defineNewClass(@Nonnull ClassVisitor modifier) {
      final ClassLoader parentLoader = ClassLoad.getClassLoaderWithAccess(sourceClass);
      final byte[] modifiedClassfile = modifier.toByteArray();

      try {
         @SuppressWarnings("unchecked")
         Class generatedClass = (Class) new ClassLoader(parentLoader) {
            @Override
            protected Class findClass(String name) throws ClassNotFoundException {
               if (!name.equals(generatedClassName)) {
                  return parentLoader.loadClass(name);
               }

               return defineClass(name, modifiedClassfile, 0, modifiedClassfile.length);
            }
         }.findClass(generatedClassName);

         return generatedClass;
      }
      catch (ClassNotFoundException e) {
         throw new RuntimeException("Unable to define class: " + generatedClassName, e);
      }
   }
}