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 java.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ java/lang/reflect/package-info.java

/*
 * Copyright (c) 1998, 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/**
 * Provides classes and interfaces for obtaining reflective
 * information about classes and objects.  Reflection allows
 * programmatic access to information about the fields, methods and
 * constructors of loaded classes, and the use of reflected fields,
 * methods, and constructors to operate on their underlying
 * counterparts, within security restrictions.
 *
 * <p>{@code AccessibleObject} allows suppression of access checks if
 * the necessary {@code ReflectPermission} is available.
 *
 * <p>{@code Array} provides static methods to dynamically create and
 * access arrays.
 *
 * <p>Classes in this package, along with {@code java.lang.Class}
 * accommodate applications such as debuggers, interpreters, object
 * inspectors, class browsers, and services such as Object
 * Serialization and JavaBeans that need access to either the public
 * members of a target object (based on its runtime class) or the
 * members declared by a given class.
 *
 * @since JDK1.1
 */
package java.lang.reflect;

⏎ java/lang/reflect/package-info.java

Or download all of them as a single archive file:

File name: jre-rt-java-1.8.0_191-src.zip
File size: 6664831 bytes
Release date: 2018-10-28
Download 

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

⇐ JRE 8 plugin.jar - Java Deploy Control Panel Plugin

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)