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/lang/model/type/MirroredTypeException.java

/*
 * Copyright (c) 2005, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package javax.lang.model.type;

import java.io.ObjectInputStream;
import java.io.IOException;
import javax.lang.model.element.Element;


/**
 * Thrown when an application attempts to access the {@link Class} object
 * corresponding to a {@link TypeMirror}.
 *
 * @author Joseph D. Darcy
 * @author Scott Seligman
 * @author Peter von der Ahé
 * @see MirroredTypesException
 * @see Element#getAnnotation(Class)
 * @since 1.6
 */
public class MirroredTypeException extends MirroredTypesException {

    private static final long serialVersionUID = 269;

    private transient TypeMirror type;          // cannot be serialized

    /**
     * Constructs a new MirroredTypeException for the specified type.
     *
     * @param type  the type being accessed
     */
    public MirroredTypeException(TypeMirror type) {
        super("Attempt to access Class object for TypeMirror " + type.toString(), type);
        this.type = type;
    }

    /**
     * Returns the type mirror corresponding to the type being accessed.
     * The type mirror may be unavailable if this exception has been
     * serialized and then read back in.
     *
     * @return the type mirror, or {@code null} if unavailable
     */
    public TypeMirror getTypeMirror() {
        return type;
    }

    /**
     * Explicitly set all transient fields.
     */
    private void readObject(ObjectInputStream s)
        throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        type = null;
        types = null;
    }
}

⏎ javax/lang/model/type/MirroredTypeException.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)