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/lang/model/element/PackageElement.java

/*
 * Copyright (c) 2005, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package javax.lang.model.element;

import java.util.List;

/**
 * Represents a package program element.  Provides access to information
 * about the package and its members.
 *
 * @author Joseph D. Darcy
 * @author Scott Seligman
 * @author Peter von der Ahé
 * @see javax.lang.model.util.Elements#getPackageOf
 * @since 1.6
 */
public interface PackageElement extends Element, QualifiedNameable {

    /**
     * Returns the fully qualified name of this package.
     * This is also known as the package's <i>canonical</i> name.
     *
     * @return the fully qualified name of this package, or an
     * empty name if this is an unnamed package
     * @jls 6.7 Fully Qualified Names and Canonical Names
     */
    Name getQualifiedName();

    /**
     * Returns the simple name of this package.  For an unnamed
     * package, an empty name is returned.
     *
     * @return the simple name of this package or an empty name if
     * this is an unnamed package
     */
    @Override
    Name getSimpleName();

    /**
     * Returns the {@linkplain NestingKind#TOP_LEVEL top-level}
     * classes and interfaces within this package.  Note that
     * subpackages are <em>not</em> considered to be enclosed by a
     * package.
     *
     * @return the top-level classes and interfaces within this
     * package
     */
    @Override
    List<? extends Element> getEnclosedElements();

    /**
     * Returns {@code true} is this is an unnamed package and {@code
     * false} otherwise.
     *
     * @return {@code true} is this is an unnamed package and {@code
     * false} otherwise
     * @jls 7.4.2 Unnamed Packages
     */
    boolean isUnnamed();

    /**
     * Returns {@code null} since a package is not enclosed by another
     * element.
     *
     * @return {@code null}
     */
    @Override
    Element getEnclosingElement();
}

⏎ javax/lang/model/element/PackageElement.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

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