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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;

import java.util.*;

import javax.annotation.*;

import mockit.internal.util.*;

/**
 * A {@link ClassVisitor} that generates classes in bytecode form. More precisely this visitor generates 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("ParameterHidesMemberVariable")
public final class ClassWriter extends ClassVisitor
{
   /**
    * The class bytecode from which this class writer will generate a new/modified class.
    */
   @Nonnull final byte[] code;

   /**
    * true if the stack map frames must be recomputed from scratch.
    * 
    * If this flag is set, then the stack map frames are recomputed from the methods bytecode. The arguments of the
    * {@link MethodVisitor#visitMaxStack} method are also ignored and recomputed from the bytecode. In other words,
    * computeFrames implies computeMaxs.
    */
   private final boolean computeFrames;

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

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

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

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

   @Nullable private Interfaces interfaces;
   @Nullable private SignatureWriter signatureWriter;
   @Nonnull private final SourceInfoWriter sourceInfo;
   @Nullable private OuterClassWriter outerClassWriter;
   @Nullable private InnerClassesWriter innerClassesWriter;
   @Nullable final BootstrapMethods bootstrapMethods;

   /**
    * The fields of this class.
    */
   @Nonnull private final List fields;

   /**
    * The methods of this class.
    */
   @Nonnull private final List methods;

   /**
    * Constructs a new ClassWriter object and enables optimizations for "mostly add" bytecode transformations.
    * These optimizations are the following:
    * 

    * 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 {@link 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;
      version = classReader.getVersion();
      computeFrames = version >= ClassVersion.V1_7;

      cp = new ConstantPoolGeneration();
      sourceInfo = new SourceInfoWriter(cp);

      bootstrapMethods = classReader.positionAtBootstrapMethodsAttribute() ? new BootstrapMethods(cp, classReader) : null;

      new ConstantPoolCopying(classReader, this).copyPool(bootstrapMethods);

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

   /**
    * Constructs a new ClassWriter object meant for the generation of new bytecode only.
    *
    * @param code the bytecode of an existing class/interface whose methods will have brand new implementations in the new class created
    *             from this writer
    */
   public ClassWriter(@Nonnull byte[] code) {
      this.code = code;
      this.version = ClassMetadataReader.readVersion(code);
      computeFrames = version >= ClassVersion.V1_7;

      cp = new ConstantPoolGeneration();
      sourceInfo = new SourceInfoWriter(cp);

      bootstrapMethods = null;
      fields = new ArrayList();
      methods = new ArrayList();
   }

   @Override
   public void visit(
      int version, int access, @Nonnull String name, @Nullable String signature, @Nullable String superName, @Nullable String[] interfaces
   ) {
      this.version = version;
      this.access = access;
      this.name = cp.newClass(name);
      thisName = name;

      createMarkerAttributes(version);

      if (signature != null) {
         signatureWriter = new SignatureWriter(cp, signature);
      }

      this.superName = superName == null ? 0 : cp.newClass(superName);

      if (interfaces != null && interfaces.length > 0) {
         this.interfaces = new Interfaces(cp, interfaces);
      }

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

   @Override
   public void visitSource(@Nullable String file) {
      sourceInfo.setSourceFileName(file);
   }

   @Override
   public void visitOuterClass(@Nonnull String owner, @Nullable String name, @Nullable String desc) {
      outerClassWriter = new OuterClassWriter(cp, owner, name, desc);
   }

   @Nonnull @Override
   public AnnotationVisitor visitAnnotation(@Nonnull String desc) {
      return addAnnotation(desc);
   }

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

      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
   ) {
      FieldWriter field = new FieldWriter(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
   ) {
      MethodWriter method = new MethodWriter(this, access, name, desc, signature, exceptions, computeFrames);
      methods.add(method);
      return method;
   }

   /**
    * Returns the bytecode of the class that was build 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.data;
   }

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

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

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

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

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

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

   @Nonnegative
   private int getInterfacesSize() {
      return interfaces == null ? 0 : 2 * interfaces.getCount();
   }

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

      for (FieldWriter fb : fields) {
         size += fb.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(version);
      cp.put(out);

      putAccess(out, 0);
      out.putShort(name);
      out.putShort(superName);

      int interfaceCount = interfaces == null ? 0 : interfaces.getCount();
      out.putShort(interfaceCount);

      if (interfaceCount > 0) {
         interfaces.put(out);
      }

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

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

      if (bootstrapMethods != null) {
         bootstrapMethods.put(out);
      }

      if (signatureWriter != null) {
         signatureWriter.put(out);
      }

      sourceInfo.put(out);

      if (outerClassWriter != null) {
         outerClassWriter.put(out);
      }

      putMarkerAttributes(out);

      if (innerClassesWriter != null) {
         innerClassesWriter.put(out);
      }
   }

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

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

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

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

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

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

      return attributeCount;
   }

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