https://developers.google.com/protocol-buffers/protobuf-java/: Core Protocol Buffers library. Protocol Buffers are a way of encoding structured data in an efficient yet extensible format. 
interface com.google.protobuf.GeneratedMessageV3$ExtensionDescriptorRetriever
  minor version: 0
  major version: 59
  flags: flags: (0x0600) ACC_INTERFACE, ACC_ABSTRACT
  this_class: com.google.protobuf.GeneratedMessageV3$ExtensionDescriptorRetriever
  super_class: java.lang.Object
{
  public abstract com.google.protobuf.Descriptors$FieldDescriptor getDescriptor();
    descriptor: ()Lcom/google/protobuf/Descriptors$FieldDescriptor;
    flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT
}
SourceFile: "GeneratedMessageV3.java"
NestHost: com.google.protobuf.GeneratedMessageV3
InnerClasses:
  public final FieldDescriptor = com.google.protobuf.Descriptors$FieldDescriptor of com.google.protobuf.Descriptors
  abstract ExtensionDescriptorRetriever = com.google.protobuf.GeneratedMessageV3$ExtensionDescriptorRetriever of com.google.protobuf.GeneratedMessageV3