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/swing/DebugGraphicsInfo.java

/*
 * Copyright (c) 1997, 2008, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package javax.swing;

import java.awt.*;
import java.util.*;

/** Class used by DebugGraphics for maintaining information about how
  * to render graphics calls.
  *
  * @author Dave Karlton
  */
class DebugGraphicsInfo {
    Color                flashColor = Color.red;
    int                  flashTime = 100;
    int                  flashCount = 2;
    Hashtable<JComponent, Integer> componentToDebug;
    JFrame               debugFrame = null;
    java.io.PrintStream  stream = System.out;

    void setDebugOptions(JComponent component, int debug) {
        if (debug == 0) {
            return;
        }
        if (componentToDebug == null) {
            componentToDebug = new Hashtable<JComponent, Integer>();
        }
        if (debug > 0) {
            componentToDebug.put(component, Integer.valueOf(debug));
        } else {
            componentToDebug.remove(component);
        }
    }

    int getDebugOptions(JComponent component) {
        if (componentToDebug == null) {
            return 0;
        } else {
            Integer integer = componentToDebug.get(component);

            return integer == null ? 0 : integer.intValue();
        }
    }

    void log(String string) {
        stream.println(string);
    }
}

⏎ javax/swing/DebugGraphicsInfo.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)