https://tomcat.apache.org/: Tomcat Servlet Engine Core Classes and Standard implementations 
public class org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl implements javax.security.auth.callback.CallbackHandler
  minor version: 0
  major version: 59
  flags: flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl
  super_class: java.lang.Object
{
  private static final org.apache.tomcat.util.res.StringManager sm;
    descriptor: Lorg/apache/tomcat/util/res/StringManager;
    flags: (0x001a) ACC_PRIVATE, ACC_STATIC, ACC_FINAL

  private static javax.security.auth.callback.CallbackHandler instance;
    descriptor: Ljavax/security/auth/callback/CallbackHandler;
    flags: (0x000a) ACC_PRIVATE, ACC_STATIC

  static void <clinit>();
    descriptor: ()V
    flags: (0x0008) ACC_STATIC
    Code:
      stack=2, locals=0, args_size=0
         0: .line 42
            ldc Lorg/apache/catalina/authenticator/jaspic/CallbackHandlerImpl;
            invokestatic org.apache.tomcat.util.res.StringManager.getManager:(Ljava/lang/Class;)Lorg/apache/tomcat/util/res/StringManager;
            putstatic org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl.sm:Lorg/apache/tomcat/util/res/StringManager;
         1: .line 48
            new org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl
            dup
            invokespecial org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl.<init>:()V
            putstatic org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl.instance:Ljavax/security/auth/callback/CallbackHandler;
         2: .line 49
            return
      LocalVariableTable:
        Start  End  Slot  Name  Signature

  public static javax.security.auth.callback.CallbackHandler getInstance();
    descriptor: ()Ljavax/security/auth/callback/CallbackHandler;
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: .line 53
            getstatic org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl.instance:Ljavax/security/auth/callback/CallbackHandler;
            areturn
      LocalVariableTable:
        Start  End  Slot  Name  Signature

  private void <init>();
    descriptor: ()V
    flags: (0x0002) ACC_PRIVATE
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl this
         0: .line 57
            aload 0 /* this */
            invokespecial java.lang.Object.<init>:()V
         1: .line 59
            return
        end local 0 // org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    2     0  this  Lorg/apache/catalina/authenticator/jaspic/CallbackHandlerImpl;

  public void handle(javax.security.auth.callback.Callback[]);
    descriptor: ([Ljavax/security/auth/callback/Callback;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=7, locals=11, args_size=2
        start local 0 // org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl this
        start local 1 // javax.security.auth.callback.Callback[] callbacks
         0: .line 65
            aconst_null
            astore 2 /* name */
        start local 2 // java.lang.String name
         1: .line 66
            aconst_null
            astore 3 /* principal */
        start local 3 // java.security.Principal principal
         2: .line 67
            aconst_null
            astore 4 /* subject */
        start local 4 // javax.security.auth.Subject subject
         3: .line 68
            aconst_null
            astore 5 /* groups */
        start local 5 // java.lang.String[] groups
         4: .line 70
            aload 1 /* callbacks */
            ifnull 26
         5: .line 74
            aload 1 /* callbacks */
            dup
            astore 9
            arraylength
            istore 8
            iconst_0
            istore 7
            goto 22
      StackMap locals: org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl javax.security.auth.callback.Callback[] java.lang.String java.security.Principal javax.security.auth.Subject java.lang.String[] top int int javax.security.auth.callback.Callback[]
      StackMap stack:
         6: aload 9
            iload 7
            aaload
            astore 6 /* callback */
        start local 6 // javax.security.auth.callback.Callback callback
         7: .line 75
            aload 6 /* callback */
            instanceof javax.security.auth.message.callback.CallerPrincipalCallback
            ifeq 13
         8: .line 76
            aload 6 /* callback */
            checkcast javax.security.auth.message.callback.CallerPrincipalCallback
            astore 10 /* cpc */
        start local 10 // javax.security.auth.message.callback.CallerPrincipalCallback cpc
         9: .line 77
            aload 10 /* cpc */
            invokevirtual javax.security.auth.message.callback.CallerPrincipalCallback.getName:()Ljava/lang/String;
            astore 2 /* name */
        10: .line 78
            aload 10 /* cpc */
            invokevirtual javax.security.auth.message.callback.CallerPrincipalCallback.getPrincipal:()Ljava/security/Principal;
            astore 3 /* principal */
        11: .line 79
            aload 10 /* cpc */
            invokevirtual javax.security.auth.message.callback.CallerPrincipalCallback.getSubject:()Ljavax/security/auth/Subject;
            astore 4 /* subject */
        end local 10 // javax.security.auth.message.callback.CallerPrincipalCallback cpc
        12: .line 80
            goto 21
      StackMap locals: org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl javax.security.auth.callback.Callback[] java.lang.String java.security.Principal javax.security.auth.Subject java.lang.String[] javax.security.auth.callback.Callback int int javax.security.auth.callback.Callback[]
      StackMap stack:
        13: aload 6 /* callback */
            instanceof javax.security.auth.message.callback.GroupPrincipalCallback
            ifeq 17
        14: .line 81
            aload 6 /* callback */
            checkcast javax.security.auth.message.callback.GroupPrincipalCallback
            astore 10 /* gpc */
        start local 10 // javax.security.auth.message.callback.GroupPrincipalCallback gpc
        15: .line 82
            aload 10 /* gpc */
            invokevirtual javax.security.auth.message.callback.GroupPrincipalCallback.getGroups:()[Ljava/lang/String;
            astore 5 /* groups */
        end local 10 // javax.security.auth.message.callback.GroupPrincipalCallback gpc
        16: .line 83
            goto 21
        17: .line 86
      StackMap locals:
      StackMap stack:
            ldc Lorg/apache/catalina/authenticator/jaspic/CallbackHandlerImpl;
            invokestatic org.apache.juli.logging.LogFactory.getLog:(Ljava/lang/Class;)Lorg/apache/juli/logging/Log;
            astore 10 /* log */
        start local 10 // org.apache.juli.logging.Log log
        18: .line 87
            aload 10 /* log */
            getstatic org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl.sm:Lorg/apache/tomcat/util/res/StringManager;
            ldc "callbackHandlerImpl.jaspicCallbackMissing"
            iconst_1
            anewarray java.lang.Object
            dup
            iconst_0
        19: .line 88
            aload 6 /* callback */
            invokevirtual java.lang.Object.getClass:()Ljava/lang/Class;
            invokevirtual java.lang.Class.getName:()Ljava/lang/String;
            aastore
        20: .line 87
            invokevirtual org.apache.tomcat.util.res.StringManager.getString:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/String;
            invokeinterface org.apache.juli.logging.Log.error:(Ljava/lang/Object;)V
        end local 10 // org.apache.juli.logging.Log log
        end local 6 // javax.security.auth.callback.Callback callback
        21: .line 74
      StackMap locals: org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl javax.security.auth.callback.Callback[] java.lang.String java.security.Principal javax.security.auth.Subject java.lang.String[] top int int javax.security.auth.callback.Callback[]
      StackMap stack:
            iinc 7 1
      StackMap locals:
      StackMap stack:
        22: iload 7
            iload 8
            if_icmplt 6
        23: .line 93
            aload 0 /* this */
            aload 3 /* principal */
            aload 2 /* name */
            aload 5 /* groups */
            invokevirtual org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl.getPrincipal:(Ljava/security/Principal;Ljava/lang/String;[Ljava/lang/String;)Ljava/security/Principal;
            astore 6 /* gp */
        start local 6 // java.security.Principal gp
        24: .line 94
            aload 4 /* subject */
            ifnull 26
            aload 6 /* gp */
            ifnull 26
        25: .line 95
            aload 4 /* subject */
            invokevirtual javax.security.auth.Subject.getPrivateCredentials:()Ljava/util/Set;
            aload 6 /* gp */
            invokeinterface java.util.Set.add:(Ljava/lang/Object;)Z
            pop
        end local 6 // java.security.Principal gp
        26: .line 98
      StackMap locals: org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl javax.security.auth.callback.Callback[] java.lang.String java.security.Principal javax.security.auth.Subject java.lang.String[]
      StackMap stack:
            return
        end local 5 // java.lang.String[] groups
        end local 4 // javax.security.auth.Subject subject
        end local 3 // java.security.Principal principal
        end local 2 // java.lang.String name
        end local 1 // javax.security.auth.callback.Callback[] callbacks
        end local 0 // org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl this
      LocalVariableTable:
        Start  End  Slot       Name  Signature
            0   27     0       this  Lorg/apache/catalina/authenticator/jaspic/CallbackHandlerImpl;
            0   27     1  callbacks  [Ljavax/security/auth/callback/Callback;
            1   27     2       name  Ljava/lang/String;
            2   27     3  principal  Ljava/security/Principal;
            3   27     4    subject  Ljavax/security/auth/Subject;
            4   27     5     groups  [Ljava/lang/String;
            7   21     6   callback  Ljavax/security/auth/callback/Callback;
            9   12    10        cpc  Ljavax/security/auth/message/callback/CallerPrincipalCallback;
           15   16    10        gpc  Ljavax/security/auth/message/callback/GroupPrincipalCallback;
           18   21    10        log  Lorg/apache/juli/logging/Log;
           24   26     6         gp  Ljava/security/Principal;
    Exceptions:
      throws java.io.IOException, javax.security.auth.callback.UnsupportedCallbackException
    MethodParameters:
           Name  Flags
      callbacks  

  private java.security.Principal getPrincipal(java.security.Principal, java.lang.String, java.lang.String[]);
    descriptor: (Ljava/security/Principal;Ljava/lang/String;[Ljava/lang/String;)Ljava/security/Principal;
    flags: (0x0002) ACC_PRIVATE
    Code:
      stack=6, locals=5, args_size=4
        start local 0 // org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl this
        start local 1 // java.security.Principal principal
        start local 2 // java.lang.String name
        start local 3 // java.lang.String[] groups
         0: .line 103
            aload 1 /* principal */
            instanceof org.apache.catalina.realm.GenericPrincipal
            ifeq 2
         1: .line 104
            aload 1 /* principal */
            areturn
         2: .line 106
      StackMap locals:
      StackMap stack:
            aload 2 /* name */
            ifnonnull 4
            aload 1 /* principal */
            ifnull 4
         3: .line 107
            aload 1 /* principal */
            invokeinterface java.security.Principal.getName:()Ljava/lang/String;
            astore 2 /* name */
         4: .line 109
      StackMap locals:
      StackMap stack:
            aload 2 /* name */
            ifnonnull 6
         5: .line 110
            aconst_null
            areturn
         6: .line 113
      StackMap locals:
      StackMap stack:
            aload 3 /* groups */
            ifnull 7
            aload 3 /* groups */
            arraylength
            ifne 9
         7: .line 114
      StackMap locals:
      StackMap stack:
            invokestatic java.util.Collections.emptyList:()Ljava/util/List;
            astore 4 /* roles */
        start local 4 // java.util.List roles
         8: .line 115
            goto 10
        end local 4 // java.util.List roles
         9: .line 116
      StackMap locals:
      StackMap stack:
            aload 3 /* groups */
            invokestatic java.util.Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
            astore 4 /* roles */
        start local 4 // java.util.List roles
        10: .line 119
      StackMap locals: java.util.List
      StackMap stack:
            new org.apache.catalina.realm.GenericPrincipal
            dup
            aload 2 /* name */
            aconst_null
            aload 4 /* roles */
            aload 1 /* principal */
            invokespecial org.apache.catalina.realm.GenericPrincipal.<init>:(Ljava/lang/String;Ljava/lang/String;Ljava/util/List;Ljava/security/Principal;)V
            areturn
        end local 4 // java.util.List roles
        end local 3 // java.lang.String[] groups
        end local 2 // java.lang.String name
        end local 1 // java.security.Principal principal
        end local 0 // org.apache.catalina.authenticator.jaspic.CallbackHandlerImpl this
      LocalVariableTable:
        Start  End  Slot       Name  Signature
            0   11     0       this  Lorg/apache/catalina/authenticator/jaspic/CallbackHandlerImpl;
            0   11     1  principal  Ljava/security/Principal;
            0   11     2       name  Ljava/lang/String;
            0   11     3     groups  [Ljava/lang/String;
            8    9     4      roles  Ljava/util/List<Ljava/lang/String;>;
           10   11     4      roles  Ljava/util/List<Ljava/lang/String;>;
    MethodParameters:
           Name  Flags
      principal  
      name       
      groups     
}
SourceFile: "CallbackHandlerImpl.java"