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/IllegalComponentStateException.java

/*
 * Copyright (c) 1996, 1997, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.awt;

/**
 * Signals that an AWT component is not in an appropriate state for
 * the requested operation.
 *
 * @author      Jonni Kanerva
 */
public class IllegalComponentStateException extends IllegalStateException {
    /*
     * JDK 1.1 serialVersionUID
     */
     private static final long serialVersionUID = -1889339587208144238L;

    /**
     * Constructs an IllegalComponentStateException with no detail message.
     * A detail message is a String that describes this particular exception.
     */
    public IllegalComponentStateException() {
        super();
    }

    /**
     * Constructs an IllegalComponentStateException with the specified detail
     * message.  A detail message is a String that describes this particular
     * exception.
     * @param s the String that contains a detailed message
     */
    public IllegalComponentStateException(String s) {
        super(s);
    }
}

⏎ java/awt/IllegalComponentStateException.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)