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/ClosedSelectorException.java

/*
 * Copyright (c) 2000, 2007, Oracle and/or its affiliates. All rights reserved.
 *
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

// -- This file was mechanically generated: Do not edit! -- //

package java.nio.channels;


/**
 * Unchecked exception thrown when an attempt is made to invoke an I/O
 * operation upon a closed selector.
 *
 * @since 1.4
 */

public class ClosedSelectorException
    extends IllegalStateException
{

    private static final long serialVersionUID = 6466297122317847835L;

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

}

⏎ java/nio/channels/ClosedSelectorException.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)