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/GraphicsCallback.java

/*
 * Copyright (c) 1999, 2000, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.awt;

import java.awt.peer.LightweightPeer;
import sun.awt.SunGraphicsCallback;


abstract class GraphicsCallback extends SunGraphicsCallback {

    static final class PaintCallback extends GraphicsCallback {
        private static PaintCallback instance = new PaintCallback();

        private PaintCallback() {}
        public void run(Component comp, Graphics cg) {
            comp.paint(cg);
        }
        static PaintCallback getInstance() {
            return instance;
        }
    }
    static final class PrintCallback extends GraphicsCallback {
        private static PrintCallback instance = new PrintCallback();

        private PrintCallback() {}
        public void run(Component comp, Graphics cg) {
            comp.print(cg);
        }
        static PrintCallback getInstance() {
            return instance;
        }
    }
    static final class PaintAllCallback extends GraphicsCallback {
        private static PaintAllCallback instance = new PaintAllCallback();

        private PaintAllCallback() {}
        public void run(Component comp, Graphics cg) {
            comp.paintAll(cg);
        }
        static PaintAllCallback getInstance() {
            return instance;
        }
    }
    static final class PrintAllCallback extends GraphicsCallback {
        private static PrintAllCallback instance = new PrintAllCallback();

        private PrintAllCallback() {}
        public void run(Component comp, Graphics cg) {
            comp.printAll(cg);
        }
        static PrintAllCallback getInstance() {
            return instance;
        }
    }
    static final class PeerPaintCallback extends GraphicsCallback {
        private static PeerPaintCallback instance = new PeerPaintCallback();

        private PeerPaintCallback() {}
        public void run(Component comp, Graphics cg) {
            comp.validate();
            if (comp.peer instanceof LightweightPeer) {
                comp.lightweightPaint(cg);
            } else {
                comp.peer.paint(cg);
            }
        }
        static PeerPaintCallback getInstance() {
            return instance;
        }
    }
    static final class PeerPrintCallback extends GraphicsCallback {
        private static PeerPrintCallback instance = new PeerPrintCallback();

        private PeerPrintCallback() {}
        public void run(Component comp, Graphics cg) {
            comp.validate();
            if (comp.peer instanceof LightweightPeer) {
                comp.lightweightPrint(cg);
            } else {
                comp.peer.print(cg);
            }
        }
        static PeerPrintCallback getInstance() {
            return instance;
        }
    }
    static final class PaintHeavyweightComponentsCallback
        extends GraphicsCallback
    {
        private static PaintHeavyweightComponentsCallback instance =
            new PaintHeavyweightComponentsCallback();

        private PaintHeavyweightComponentsCallback() {}
        public void run(Component comp, Graphics cg) {
            if (comp.peer instanceof LightweightPeer) {
                comp.paintHeavyweightComponents(cg);
            } else {
                comp.paintAll(cg);
            }
        }
        static PaintHeavyweightComponentsCallback getInstance() {
            return instance;
        }
    }
    static final class PrintHeavyweightComponentsCallback
        extends GraphicsCallback
    {
        private static PrintHeavyweightComponentsCallback instance =
            new PrintHeavyweightComponentsCallback();

        private PrintHeavyweightComponentsCallback() {}
        public void run(Component comp, Graphics cg) {
            if (comp.peer instanceof LightweightPeer) {
                comp.printHeavyweightComponents(cg);
            } else {
                comp.printAll(cg);
            }
        }
        static PrintHeavyweightComponentsCallback getInstance() {
            return instance;
        }
    }
}

⏎ java/awt/GraphicsCallback.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)