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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 Rogério Liesenfeld
 * This file is subject to the terms of the MIT license (see LICENSE.txt).
 */
package mockit.internal.startup;

import java.lang.instrument.*;
import java.security.*;
import javax.annotation.*;
import static java.lang.reflect.Modifier.*;

import mockit.external.asm.*;
import mockit.internal.*;
import mockit.internal.expectations.mocking.*;
import mockit.internal.mockups.*;
import static mockit.external.asm.ClassReader.*;
import static mockit.external.asm.Opcodes.*;

final class MockingBridgeFields
{
   private MockingBridgeFields() {}

   static void createSyntheticFieldsInJREClassToHoldMockingBridges(@Nonnull Instrumentation instrumentation)
   {
      instrumentation.addTransformer(new FieldAdditionTransformer(instrumentation));

      // Loads some JRE classes expected to not be loaded yet.
      NegativeArraySizeException.class.getName();
      javax.print.PrintException.class.getName();
   }

   private static final class FieldAdditionTransformer implements ClassFileTransformer
   {
      private static final int FIELD_ACCESS = ACC_PUBLIC + ACC_STATIC + ACC_SYNTHETIC;
      @Nonnull private final Instrumentation instrumentation;
      private boolean fieldsAdded;

      FieldAdditionTransformer(@Nonnull Instrumentation instrumentation) { this.instrumentation = instrumentation; }

      @Nullable @Override
      public byte[] transform(
         @Nullable ClassLoader loader, @Nonnull String className, @Nullable Class classBeingRedefined,
         @Nullable ProtectionDomain protectionDomain, @Nonnull byte[] classfileBuffer)
      {
         if (!fieldsAdded && loader == null) { // adds the fields to the first public JRE class to be loaded
            ClassReader cr = new ClassReader(classfileBuffer);

            if (isPublic(cr.getAccess())) {
               fieldsAdded = true;
               instrumentation.removeTransformer(this);
               MockingBridge.setHostClassName(className);
               return getModifiedJREClassWithAddedFields(cr);
            }
         }

         return null;
      }

      @Nonnull
      private byte[] getModifiedJREClassWithAddedFields(@Nonnull ClassReader classReader)
      {
         final ClassWriter cw = new ClassWriter(classReader);

         ClassVisitor cv = new ClassVisitor(cw) {
            @Override
            public void visitEnd()
            {
               addField(MockedBridge.MB);
               addField(MockupBridge.MB);
               addField(MockMethodBridge.MB);
            }

            private void addField(@Nonnull MockingBridge mb)
            {
               cw.visitField(FIELD_ACCESS, mb.id, "Ljava/lang/reflect/InvocationHandler;", null, null);
            }
         };

         classReader.accept(cv, SKIP_FRAMES);
         return cw.toByteArray();
      }
   }
}