https://tomcat.apache.org/: Tomcat Servlet Engine Core Classes and Standard implementations 
public class org.apache.catalina.realm.X509SubjectDnRetriever implements org.apache.catalina.realm.X509UsernameRetriever
  minor version: 0
  major version: 59
  flags: flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: org.apache.catalina.realm.X509SubjectDnRetriever
  super_class: java.lang.Object
{
  public void <init>();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // org.apache.catalina.realm.X509SubjectDnRetriever this
         0: .line 25
            aload 0 /* this */
            invokespecial java.lang.Object.<init>:()V
            return
        end local 0 // org.apache.catalina.realm.X509SubjectDnRetriever this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lorg/apache/catalina/realm/X509SubjectDnRetriever;

  public java.lang.String getUsername(java.security.cert.X509Certificate);
    descriptor: (Ljava/security/cert/X509Certificate;)Ljava/lang/String;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=2
        start local 0 // org.apache.catalina.realm.X509SubjectDnRetriever this
        start local 1 // java.security.cert.X509Certificate clientCert
         0: .line 29
            aload 1 /* clientCert */
            invokevirtual java.security.cert.X509Certificate.getSubjectDN:()Ljava/security/Principal;
            invokeinterface java.security.Principal.getName:()Ljava/lang/String;
            areturn
        end local 1 // java.security.cert.X509Certificate clientCert
        end local 0 // org.apache.catalina.realm.X509SubjectDnRetriever this
      LocalVariableTable:
        Start  End  Slot        Name  Signature
            0    1     0        this  Lorg/apache/catalina/realm/X509SubjectDnRetriever;
            0    1     1  clientCert  Ljava/security/cert/X509Certificate;
    MethodParameters:
            Name  Flags
      clientCert  
}
SourceFile: "X509SubjectDnRetriever.java"