/**
   * 跳过 Code 属性的标志。 如果设置了此标志，则不会解析也不访问Code属性。
   */
  public static final int SKIP_CODE = 1;

  /**
   * 使给定的访问者访问传递给此 ClassReader 构造方法的JVMS Class 文件结构
   *
   * @param classVisitor 必须访问此类的访问者。
   * @param parsingOptions 用于解析此类的选项。 #SKIP_CODE #SKIP_DEBUG #SKIP_FRAMES或#EXPAND_FRAMES中的一个或多个
   */
  public void accept(final ClassVisitor classVisitor, final int parsingOptions) {
    accept(classVisitor, new Attribute[0], parsingOptions);
  }

  /**
   * @param attributePrototypes 在类访问期间必须解析的属性的原型.任何类型不等于原型类型的属性都不会被解析:其字节数组值也将一成不变地传递给 ClassWriter
   * 如果该值包含对常量池的引用，或者与由 reader 和 writer 之间的类适配器转换的类元素具有句法或语义链接，则可能会破坏它
   */
  public void accept(
      final ClassVisitor classVisitor,
      final Attribute[] attributePrototypes,
      final int parsingOptions) {
    Context context = new Context();
    context.attributePrototypes = attributePrototypes;
    context.parsingOptions = parsingOptions;
    context.charBuffer = new char[maxStringLength];
    // Read the access_flags, this_class, super_class, interface_count and interfaces fields.
    char[] charBuffer = context.charBuffer;
    int currentOffset = header;
    int accessFlags = readUnsignedShort(currentOffset);
    // 本类名
    String thisClass = readClass(currentOffset + 2, charBuffer);
    // 父类名
    String superClass = readClass(currentOffset + 4, charBuffer);
    String[] interfaces = new String[readUnsignedShort(currentOffset + 6)];
    currentOffset += 8;
    for (int i = 0; i < interfaces.length; ++i) {
      interfaces[i] = readClass(currentOffset, charBuffer);
      currentOffset += 2;
    }
    // 读取 class 属性（变量按照JVMS的4.7节的顺序排列）。
    // 属性偏移量不包括attribute_name_index和attribute_length字段。
    // InnerClasses 属性的偏移量，或0。
    int innerClassesOffset = 0;
    // EnclosingMethod 属性的偏移量, or 0.
    int enclosingMethodOffset = 0;
    // - The string corresponding to the Signature attribute, or null.
    String signature = null;
    // - The string corresponding to the SourceFile attribute, or null.
    String sourceFile = null;
    // - The string corresponding to the SourceDebugExtension attribute, or null.
    String sourceDebugExtension = null;
    // - The offset of the RuntimeVisibleAnnotations attribute, or 0.
    int runtimeVisibleAnnotationsOffset = 0;
    // - The offset of the RuntimeInvisibleAnnotations attribute, or 0.
    int runtimeInvisibleAnnotationsOffset = 0;
    // - The offset of the RuntimeVisibleTypeAnnotations attribute, or 0.
    int runtimeVisibleTypeAnnotationsOffset = 0;
    // - The offset of the RuntimeInvisibleTypeAnnotations attribute, or 0.
    int runtimeInvisibleTypeAnnotationsOffset = 0;
    // - The offset of the Module attribute, or 0.
    int moduleOffset = 0;
    // - The offset of the ModulePackages attribute, or 0.
    int modulePackagesOffset = 0;
    // - The string corresponding to the ModuleMainClass attribute, or null.
    String moduleMainClass = null;
    // - The string corresponding to the NestHost attribute, or null.
    String nestHostClass = null;
    // - The offset of the NestMembers attribute, or 0.
    int nestMembersOffset = 0;
    // - The non standard attributes (linked with their {@link Attribute#nextAttribute} field).
    //   This list in the <i>reverse order</i> or their order in the ClassFile structure.
    Attribute attributes = null;
    int currentAttributeOffset = getFirstAttributeOffset();
    for (int i = readUnsignedShort(currentAttributeOffset - 2); i > 0; --i) {
      // Read the attribute_info's attribute_name and attribute_length fields.
      String attributeName = readUTF8(currentAttributeOffset, charBuffer);
      int attributeLength = readInt(currentAttributeOffset + 2);
      currentAttributeOffset += 6;
      // The tests are sorted in decreasing frequency order (based on frequencies observed on
      // typical classes).
      if (Constants.SOURCE_FILE.equals(attributeName)) {
        sourceFile = readUTF8(currentAttributeOffset, charBuffer);
      } else if (Constants.INNER_CLASSES.equals(attributeName)) {
        innerClassesOffset = currentAttributeOffset;
      } else if (Constants.ENCLOSING_METHOD.equals(attributeName)) {
        enclosingMethodOffset = currentAttributeOffset;
      } else if (Constants.NEST_HOST.equals(attributeName)) {
        nestHostClass = readClass(currentAttributeOffset, charBuffer);
      } else if (Constants.NEST_MEMBERS.equals(attributeName)) {
        nestMembersOffset = currentAttributeOffset;
      } else if (Constants.SIGNATURE.equals(attributeName)) {
        signature = readUTF8(currentAttributeOffset, charBuffer);
      } else if (Constants.RUNTIME_VISIBLE_ANNOTATIONS.equals(attributeName)) {
        runtimeVisibleAnnotationsOffset = currentAttributeOffset;
      } else if (Constants.RUNTIME_VISIBLE_TYPE_ANNOTATIONS.equals(attributeName)) {
        runtimeVisibleTypeAnnotationsOffset = currentAttributeOffset;
      } else if (Constants.DEPRECATED.equals(attributeName)) {
        accessFlags |= Opcodes.ACC_DEPRECATED;
      } else if (Constants.SYNTHETIC.equals(attributeName)) {
        accessFlags |= Opcodes.ACC_SYNTHETIC;
      } else if (Constants.SOURCE_DEBUG_EXTENSION.equals(attributeName)) {
        sourceDebugExtension =
            readUtf(currentAttributeOffset, attributeLength, new char[attributeLength]);
      } else if (Constants.RUNTIME_INVISIBLE_ANNOTATIONS.equals(attributeName)) {
        runtimeInvisibleAnnotationsOffset = currentAttributeOffset;
      } else if (Constants.RUNTIME_INVISIBLE_TYPE_ANNOTATIONS.equals(attributeName)) {
        runtimeInvisibleTypeAnnotationsOffset = currentAttributeOffset;
      } else if (Constants.MODULE.equals(attributeName)) {
        moduleOffset = currentAttributeOffset;
      } else if (Constants.MODULE_MAIN_CLASS.equals(attributeName)) {
        moduleMainClass = readClass(currentAttributeOffset, charBuffer);
      } else if (Constants.MODULE_PACKAGES.equals(attributeName)) {
        modulePackagesOffset = currentAttributeOffset;
      } else if (!Constants.BOOTSTRAP_METHODS.equals(attributeName)) {
        // The BootstrapMethods attribute is read in the constructor.
        Attribute attribute =
            readAttribute(
                attributePrototypes,
                attributeName,
                currentAttributeOffset,
                attributeLength,
                charBuffer,
                -1,
                null);
        attribute.nextAttribute = attributes;
        attributes = attribute;
      }
      currentAttributeOffset += attributeLength;
    }
    // Visit class declaration. The minor_version and major_version fields start 6 bytes before
    // the first constant pool entry, which itself starts at cpInfoOffsets[1] - 1 (by definition).
    classVisitor.visit(
        readInt(cpInfoOffsets[1] - 7), accessFlags, thisClass, signature, superClass, interfaces);
    // Visit the SourceFile and SourceDebugExtenstion attributes.
    if ((parsingOptions & SKIP_DEBUG) == 0
        && (sourceFile != null || sourceDebugExtension != null)) {
      classVisitor.visitSource(sourceFile, sourceDebugExtension);
    }
    // Visit the Module, ModulePackages and ModuleMainClass attributes.
    if (moduleOffset != 0) {
      readModuleAttributes(
          classVisitor, context, moduleOffset, modulePackagesOffset, moduleMainClass);
    }
    // Visit the NestHost attribute.
    if (nestHostClass != null) {
      classVisitor.visitNestHost(nestHostClass);
    }
    // Visit the EnclosingMethod attribute.
    if (enclosingMethodOffset != 0) {
      String className = readClass(enclosingMethodOffset, charBuffer);
      int methodIndex = readUnsignedShort(enclosingMethodOffset + 2);
      String name = methodIndex == 0 ? null : readUTF8(cpInfoOffsets[methodIndex], charBuffer);
      String type = methodIndex == 0 ? null : readUTF8(cpInfoOffsets[methodIndex] + 2, charBuffer);
      classVisitor.visitOuterClass(className, name, type);
    }
    // Visit the RuntimeVisibleAnnotations attribute.
    if (runtimeVisibleAnnotationsOffset != 0) {
      int numAnnotations = readUnsignedShort(runtimeVisibleAnnotationsOffset);
      int currentAnnotationOffset = runtimeVisibleAnnotationsOffset + 2;
      while (numAnnotations-- > 0) {
        // Parse the type_index field.
        String annotationDescriptor = readUTF8(currentAnnotationOffset, charBuffer);
        currentAnnotationOffset += 2;
        // Parse num_element_value_pairs and element_value_pairs and visit these values.
        currentAnnotationOffset =
            readElementValues(
                classVisitor.visitAnnotation(annotationDescriptor, /* visible = */ true),
                currentAnnotationOffset,
                /* named = */ true,
                charBuffer);
      }
    }
    // Visit the RuntimeInvisibleAnnotations attribute.
    if (runtimeInvisibleAnnotationsOffset != 0) {
      int numAnnotations = readUnsignedShort(runtimeInvisibleAnnotationsOffset);
      int currentAnnotationOffset = runtimeInvisibleAnnotationsOffset + 2;
      while (numAnnotations-- > 0) {
        // Parse the type_index field.
        String annotationDescriptor = readUTF8(currentAnnotationOffset, charBuffer);
        currentAnnotationOffset += 2;
        // Parse num_element_value_pairs and element_value_pairs and visit these values.
        currentAnnotationOffset =
            readElementValues(
                classVisitor.visitAnnotation(annotationDescriptor, /* visible = */ false),
                currentAnnotationOffset,
                /* named = */ true,
                charBuffer);
      }
    }
    // Visit the RuntimeVisibleTypeAnnotations attribute.
    if (runtimeVisibleTypeAnnotationsOffset != 0) {
      int numAnnotations = readUnsignedShort(runtimeVisibleTypeAnnotationsOffset);
      int currentAnnotationOffset = runtimeVisibleTypeAnnotationsOffset + 2;
      while (numAnnotations-- > 0) {
        // Parse the target_type, target_info and target_path fields.
        currentAnnotationOffset = readTypeAnnotationTarget(context, currentAnnotationOffset);
        // Parse the type_index field.
        String annotationDescriptor = readUTF8(currentAnnotationOffset, charBuffer);
        currentAnnotationOffset += 2;
        // Parse num_element_value_pairs and element_value_pairs and visit these values.
        currentAnnotationOffset =
            readElementValues(
                classVisitor.visitTypeAnnotation(
                    context.currentTypeAnnotationTarget,
                    context.currentTypeAnnotationTargetPath,
                    annotationDescriptor,
                    /* visible = */ true),
                currentAnnotationOffset,
                /* named = */ true,
                charBuffer);
      }
    }
    // Visit the RuntimeInvisibleTypeAnnotations attribute.
    if (runtimeInvisibleTypeAnnotationsOffset != 0) {
      int numAnnotations = readUnsignedShort(runtimeInvisibleTypeAnnotationsOffset);
      int currentAnnotationOffset = runtimeInvisibleTypeAnnotationsOffset + 2;
      while (numAnnotations-- > 0) {
        // Parse the target_type, target_info and target_path fields.
        currentAnnotationOffset = readTypeAnnotationTarget(context, currentAnnotationOffset);
        // Parse the type_index field.
        String annotationDescriptor = readUTF8(currentAnnotationOffset, charBuffer);
        currentAnnotationOffset += 2;
        // Parse num_element_value_pairs and element_value_pairs and visit these values.
        currentAnnotationOffset =
            readElementValues(
                classVisitor.visitTypeAnnotation(
                    context.currentTypeAnnotationTarget,
                    context.currentTypeAnnotationTargetPath,
                    annotationDescriptor,
                    /* visible = */ false),
                currentAnnotationOffset,
                /* named = */ true,
                charBuffer);
      }
    }
    // Visit the non standard attributes.
    while (attributes != null) {
      // Copy and reset the nextAttribute field so that it can also be used in ClassWriter.
      Attribute nextAttribute = attributes.nextAttribute;
      attributes.nextAttribute = null;
      classVisitor.visitAttribute(attributes);
      attributes = nextAttribute;
    }
    // Visit the NestedMembers attribute.
    if (nestMembersOffset != 0) {
      int numberOfNestMembers = readUnsignedShort(nestMembersOffset);
      int currentNestMemberOffset = nestMembersOffset + 2;
      while (numberOfNestMembers-- > 0) {
        classVisitor.visitNestMember(readClass(currentNestMemberOffset, charBuffer));
        currentNestMemberOffset += 2;
      }
    }
    // Visit the InnerClasses attribute.
    if (innerClassesOffset != 0) {
      int numberOfClasses = readUnsignedShort(innerClassesOffset);
      int currentClassesOffset = innerClassesOffset + 2;
      while (numberOfClasses-- > 0) {
        classVisitor.visitInnerClass(
            readClass(currentClassesOffset, charBuffer),
            readClass(currentClassesOffset + 2, charBuffer),
            readUTF8(currentClassesOffset + 4, charBuffer),
            readUnsignedShort(currentClassesOffset + 6));
        currentClassesOffset += 8;
      }
    }
    // 访问字段和方法
    int fieldsCount = readUnsignedShort(currentOffset);
    currentOffset += 2;
    while (fieldsCount-- > 0) {
      currentOffset = readField(classVisitor, context, currentOffset);
    }
    int methodsCount = readUnsignedShort(currentOffset);
    currentOffset += 2;
    while (methodsCount-- > 0) {
      // 读取方法
      currentOffset = readMethod(classVisitor, context, currentOffset);
    }
    // Visit the end of the class.
    classVisitor.visitEnd();
  }