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/sound/midi/spi/MidiDeviceProvider.java

/*
 * Copyright (c) 1999, 2014, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package javax.sound.midi.spi;

import javax.sound.midi.MidiDevice;

/**
 * A {@code MidiDeviceProvider} is a factory or provider for a particular type
 * of MIDI device. This mechanism allows the implementation to determine how
 * resources are managed in the creation and management of a device.
 *
 * @author Kara Kytle
 */
public abstract class MidiDeviceProvider {

    /**
     * Indicates whether the device provider supports the device represented by
     * the specified device info object.
     *
     * @param  info an info object that describes the device for which support
     *         is queried
     * @return {@code true} if the specified device is supported, otherwise
     *         {@code false}
     */
    public boolean isDeviceSupported(MidiDevice.Info info) {

        MidiDevice.Info infos[] = getDeviceInfo();

        for(int i=0; i<infos.length; i++) {
            if( info.equals( infos[i] ) ) {
                return true;
            }
        }
        return false;
    }

    /**
     * Obtains the set of info objects representing the device or devices
     * provided by this {@code MidiDeviceProvider}.
     *
     * @return set of device info objects
     */
    public abstract MidiDevice.Info[] getDeviceInfo();

    /**
     * Obtains an instance of the device represented by the info object.
     *
     * @param  info an info object that describes the desired device
     * @return device instance
     * @throws IllegalArgumentException if the info object specified does not
     *         match the info object for a device supported by this
     *         {@code MidiDeviceProvider}
     */
    public abstract MidiDevice getDevice(MidiDevice.Info info);
}

⏎ javax/sound/midi/spi/MidiDeviceProvider.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

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