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/naming/OperationNotSupportedException.java

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

/**
  * This exception is thrown when a context implementation does not support
  * the operation being invoked.
  * For example, if a server does not support the Context.bind() method
  * it would throw OperationNotSupportedException when the bind() method
  * is invoked on it.
  * <p>
  * Synchronization and serialization issues that apply to NamingException
  * apply directly here.
  *
  * @author Rosanna Lee
  * @author Scott Seligman
  * @since 1.3
  */

public class OperationNotSupportedException extends NamingException {
    /**
      * Constructs a new instance of OperationNotSupportedException.
      * All fields default to null.
      */
    public OperationNotSupportedException() {
        super();
    }

    /**
      * Constructs a new instance of OperationNotSupportedException using an
      * explanation. All other fields default to null.
      *
      * @param  explanation     Possibly null additional detail about this exception
      * @see java.lang.Throwable#getMessage
      */
    public OperationNotSupportedException(String explanation) {
        super(explanation);
    }

    /**
     * Use serialVersionUID from JNDI 1.1.1 for interoperability
     */
    private static final long serialVersionUID = 5493232822427682064L;
}

⏎ javax/naming/OperationNotSupportedException.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)