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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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package mockit.asm.classes;

import java.util.*;

import javax.annotation.*;

import mockit.asm.*;
import mockit.asm.constantPool.*;
import mockit.asm.fields.*;
import mockit.asm.jvmConstants.*;
import mockit.asm.methods.*;
import mockit.asm.util.*;
import mockit.internal.util.*;

/**
 * A {@link ClassVisitor} that generates classes in bytecode form, that is, a byte array conforming to the
 * Java class file format.
 * 
 * It can be used alone, to generate a Java class "from scratch", or with one or more {@link ClassReader} and adapter class visitor to
 * generate a modified class from one or more existing Java classes.
 */
@SuppressWarnings({"OverlyCoupledClass", "ClassWithTooManyFields"})
public final class ClassWriter extends ClassVisitor
{
   /**
    * The class bytecode from which this class writer will generate a new/modified class.
    */
   @Nonnull public final byte[] code;

   /**
    * Minor and major version numbers of the class to be generated.
    */
   private int classVersion;

   /**
    * The constant pool item that contains the internal name of this class.
    */
   @Nonnegative private int nameItemIndex;

   /**
    * The internal name of this class.
    */
   private String thisName;

   /**
    * The constant pool item that contains the internal name of the super class of this class.
    */
   @Nonnegative private int superNameItemIndex;

   @Nonnull private final List attributeWriters;
   @Nullable final BootstrapMethodsWriter bootstrapMethodsWriter;
   @Nullable private InterfaceWriter interfaceWriter;
   @Nullable private InnerClassesWriter innerClassesWriter;
   @Nonnull private final List fields;
   @Nonnull private final List methods;

   /**
    * Initializes a new class writer, applying the following two optimizations that are useful for "mostly add" bytecode transformations:
    * 

    * The constant pool from the original class is copied as is in the new class, which saves time.
    * New constant pool entries will be added at the end if necessary, but unused constant pool entries won't be removed.
    * Methods that are not transformed are copied as is in the new class, directly from the original class bytecode (i.e. without
    * emitting visit events for all the method instructions), which saves a lot of time. Untransformed methods are detected by the
    * fact that the {@link ClassReader} receives MethodVisitor objects that come from a ClassWriter (and not from any
    * other {@link ClassVisitor} instance).
    * 
    *
    * @param classReader the {@link ClassReader} used to read the original class; it will be used to copy the entire constant pool from the
    *                    original class and also to copy other fragments of original bytecode where applicable
    */
   public ClassWriter(@Nonnull ClassReader classReader) {
      code = classReader.code;
      classVersion = classReader.getVersion();

      cp = new ConstantPoolGeneration();

      bootstrapMethodsWriter = classReader.positionAtBootstrapMethodsAttribute() ? new BootstrapMethodsWriter(cp, classReader) : null;
      new ConstantPoolCopying(classReader, this).copyPool(bootstrapMethodsWriter);

      attributeWriters = new ArrayList<>(5);

      if (bootstrapMethodsWriter != null) {
         attributeWriters.add(bootstrapMethodsWriter);
      }

      fields = new ArrayList<>();
      methods = new ArrayList<>();
   }

   public int getClassVersion() { return classVersion; }
   public String getInternalClassName() { return thisName; }

   @Override
   public void visit(int version, int access, @Nonnull String name, @Nonnull ClassInfo additionalInfo) {
      classVersion = version;
      classOrMemberAccess = access;
      nameItemIndex = cp.newClass(name);
      thisName = name;

      createMarkerAttributes(version);

      String superName = additionalInfo.superName;
      superNameItemIndex = superName == null ? 0 : cp.newClass(superName);

      createInterfaceWriterIfApplicable(additionalInfo.interfaces);
      createSignatureWriterIfApplicable(additionalInfo.signature);
      createSourceFileWriterIfApplicable(additionalInfo.sourceFileName);
      createNestWritersIfApplicable(additionalInfo.hostClassName, additionalInfo.nestMembers);

      if (superName != null) {
         ClassLoad.addSuperClass(name, superName);
      }
   }

   private void createInterfaceWriterIfApplicable(@Nonnull String[] interfaces) {
      if (interfaces.length > 0) {
         interfaceWriter = new InterfaceWriter(cp, interfaces);
      }
   }

   private void createSignatureWriterIfApplicable(@Nullable String signature) {
      if (signature != null) {
         attributeWriters.add(new SignatureWriter(cp, signature));
      }
   }

   private void createSourceFileWriterIfApplicable(@Nullable String sourceFileName) {
      if (sourceFileName != null) {
         attributeWriters.add(new SourceFileWriter(cp, sourceFileName));
      }
   }

   private void createNestWritersIfApplicable(@Nullable String hostClassName, @Nullable String[] memberClassNames) {
      if (hostClassName != null) {
         attributeWriters.add(new NestHostWriter(cp, hostClassName));
      }
      else if (memberClassNames != null) {
         attributeWriters.add(new NestMembersWriter(cp, memberClassNames));
      }
   }

   @Override
   public void visitInnerClass(@Nonnull String name, @Nullable String outerName, @Nullable String innerName, int access) {
      if (innerClassesWriter == null) {
         innerClassesWriter = new InnerClassesWriter(cp);
         attributeWriters.add(innerClassesWriter);
      }

      innerClassesWriter.add(name, outerName, innerName, access);
   }

   @Nonnull @Override
   public FieldVisitor visitField(
      int access, @Nonnull String name, @Nonnull String desc, @Nullable String signature, @Nullable Object value
   ) {
      FieldVisitor field = new FieldVisitor(this, access, name, desc, signature, value);
      fields.add(field);
      return field;
   }

   @Nonnull @Override
   public MethodWriter visitMethod(
      int access, @Nonnull String name, @Nonnull String desc, @Nullable String signature, @Nullable String[] exceptions
   ) {
      boolean computeFrames = classVersion >= ClassVersion.V1_7;
      MethodWriter method = new MethodWriter(this, access, name, desc, signature, exceptions, computeFrames);
      methods.add(method);
      return method;
   }

   /**
    * Returns the bytecode of the class that was built with this class writer.
    */
   @Nonnull @Override
   public byte[] toByteArray() {
      cp.checkConstantPoolMaxSize();

      int size = getBytecodeSize(); // the real size of the bytecode of this class

      // Allocates a byte vector of this size, in order to avoid unnecessary arraycopy operations in the ByteVector.enlarge() method.
      ByteVector out = new ByteVector(size);

      putClassAttributes(out);
      putAnnotations(out);
      return out.getData();
   }

   @Nonnegative
   private int getBytecodeSize() {
      int size = 24 + getMarkerAttributesSize() + getFieldsSize() + getMethodsSize();

      if (interfaceWriter != null) {
         size += interfaceWriter.getSize();
      }

      for (AttributeWriter attributeWriter : attributeWriters) {
         size += attributeWriter.getSize();
      }

      return size + getAnnotationsSize() + cp.getSize();
   }

   @Nonnegative
   private int getFieldsSize() {
      int size = 0;

      for (FieldVisitor fv : fields) {
         size += fv.getSize();
      }

      return size;
   }

   @Nonnegative
   private int getMethodsSize() {
      int size = 0;

      for (MethodWriter mb : methods) {
         size += mb.getSize();
      }

      return size;
   }

   private void putClassAttributes(@Nonnull ByteVector out) {
      out.putInt(0xCAFEBABE).putInt(classVersion);
      cp.put(out);

      putAccess(out, 0);
      out.putShort(nameItemIndex).putShort(superNameItemIndex);

      if (interfaceWriter == null) {
         out.putShort(0);
      }
      else {
         interfaceWriter.put(out);
      }

      BaseWriter.put(out, fields);
      BaseWriter.put(out, methods);

      int attributeCount = getAttributeCount();
      out.putShort(attributeCount);

      for (AttributeWriter attributeWriter : attributeWriters) {
         attributeWriter.put(out);
      }

      putMarkerAttributes(out);
   }

   @Nonnegative
   private int getAttributeCount() {
      int attributeCount = getMarkerAttributeCount() + attributeWriters.size();

      if (annotations != null) {
         attributeCount++;
      }

      return attributeCount;
   }

   @Nonnull
   public DynamicItem addInvokeDynamicReference(
      @Nonnull String name, @Nonnull String desc, @Nonnull MethodHandle bsm, @Nonnull Object... bsmArgs
   ) {
      assert bootstrapMethodsWriter != null;
      return bootstrapMethodsWriter.addInvokeDynamicReference(name, desc, bsm, bsmArgs);
   }

   public boolean isJava6OrNewer() { return classVersion >= ClassVersion.V1_6; }
}