JRE 8 rt.jar is the JAR file for JRE 8 RT (Runtime) libraries. JRE (Java Runtime) 8 is the runtime environment included in JDK 8. JRE 8 rt.jar libraries are divided into 6 packages:

com.* - Internal Oracle and Sun Microsystems libraries
java.* - Standard Java API libraries.
javax.* - Extended Java API libraries.
jdk.* -  JDK supporting libraries.
org.* - Third party libraries.
sun.* - Old libraries developed by Sun Microsystems.

JAR File Information:

Directory of C:\fyicenter\jdk-1.8.0_191\jre\lib
      63,596,151 rt.jar

Here is the list of Java classes of the javax.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ javax/security/auth/x500/package-info.java

/*
 * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/**
 * This package contains the classes that should be used to store
 * X500 Principal and X500 Private Credentials in a
 * <i>Subject</i>.
 *
 * <h2>Package Specification</h2>
 *
 * <ul>
 *   <li><a href="http://www.ietf.org/rfc/rfc1779.txt">
 *     RFC 1779: A String Representation of Distinguished Names</a></li>
 *   <li><a href="http://www.ietf.org/rfc/rfc2253.txt">
 *     RFC 2253: Lightweight Directory Access Protocol (v3):
 *     UTF-8 String Representation of Distinguished Names</a></li>
 *   <li><a href="http://www.ietf.org/rfc/rfc3280.txt">
 *     RFC 3280: Internet X.509 Public Key Infrastructure
 *     Certificate and Certificate Revocation List (CRL) Profile</a></li>
 *   <li><a href="http://www.ietf.org/rfc/rfc4512.txt">
 *     RFC 4512: Lightweight Directory Access Protocol (LDAP):
 *     Directory Information Models</a></li>
 * </ul>
 *
 * @since JDK1.4
 */
package javax.security.auth.x500;

⏎ javax/security/auth/x500/package-info.java

Or download all of them as a single archive file:

File name: jre-rt-javax-1.8.0_191-src.zip
File size: 5381005 bytes
Release date: 2018-10-28
Download 

⇒ JRE 8 rt.jar - org.* Package Source Code

⇐ JRE 8 rt.jar - java.* Package Source Code

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)