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/awt/datatransfer/ClipboardOwner.java

/*
 * Copyright (c) 1996, 2002, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.awt.datatransfer;

/**
 * Defines the interface for classes that will provide data to
 * a clipboard. An instance of this interface becomes the owner
 * of the contents of a clipboard (clipboard owner) if it is
 * passed as an argument to
 * {@link java.awt.datatransfer.Clipboard#setContents} method of
 * the clipboard and this method returns successfully.
 * The instance remains the clipboard owner until another application
 * or another object within this application asserts ownership
 * of this clipboard.
 *
 * @see java.awt.datatransfer.Clipboard
 *
 * @author      Amy Fowler
 */

public interface ClipboardOwner {

    /**
     * Notifies this object that it is no longer the clipboard owner.
     * This method will be called when another application or another
     * object within this application asserts ownership of the clipboard.
     *
     * @param clipboard the clipboard that is no longer owned
     * @param contents the contents which this owner had placed on the clipboard
     */
    public void lostOwnership(Clipboard clipboard, Transferable contents);

}

⏎ java/awt/datatransfer/ClipboardOwner.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)