https://openjdk.java.net/ 
interface sun.security.ssl.SSLExtension$SSLExtensionSpec
  minor version: 0
  major version: 59
  flags: flags: (0x0600) ACC_INTERFACE, ACC_ABSTRACT
  this_class: sun.security.ssl.SSLExtension$SSLExtensionSpec
  super_class: java.lang.Object
{
}
SourceFile: "SSLExtension.java"
NestHost: sun.security.ssl.SSLExtension
InnerClasses:
  abstract SSLExtensionSpec = sun.security.ssl.SSLExtension$SSLExtensionSpec of sun.security.ssl.SSLExtension