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/nio/channels/IllegalSelectorException.java

/*
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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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// -- This file was mechanically generated: Do not edit! -- //

package java.nio.channels;


/**
 * Unchecked exception thrown when an attempt is made to register a channel
 * with a selector that was not created by the provider that created the
 * channel.
 *
 * @since 1.4
 */

public class IllegalSelectorException
    extends IllegalArgumentException
{

    private static final long serialVersionUID = -8406323347253320987L;

    /**
     * Constructs an instance of this class.
     */
    public IllegalSelectorException() { }

}

⏎ java/nio/channels/IllegalSelectorException.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

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