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/management/openmbean/InvalidKeyException.java

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

package javax.management.openmbean;

/**
 * This runtime exception is thrown to indicate that a method parameter which was expected to be
 * an item name of a <i>composite data</i> or a row index of a <i>tabular data</i> is not valid.
 *
 *
 * @since 1.5
 */
public class InvalidKeyException extends IllegalArgumentException {

    private static final long serialVersionUID = 4224269443946322062L;

    /**
     * An InvalidKeyException with no detail message.
     */
    public InvalidKeyException() {
        super();
    }

    /**
     * An InvalidKeyException with a detail message.
     *
     * @param msg the detail message.
     */
    public InvalidKeyException(String msg) {
        super(msg);
    }

}

⏎ javax/management/openmbean/InvalidKeyException.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)