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 com.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ com/sun/javadoc/FieldDoc.java

/*
 * Copyright (c) 1998, 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package com.sun.javadoc;

/**
 * Represents a field in a java class.
 *
 * @see MemberDoc
 *
 * @since 1.2
 * @author Robert Field
 */
public interface FieldDoc extends MemberDoc {

    /**
     * Get type of this field.
     */
    Type type();

    /**
     * Return true if this field is transient
     */
    boolean isTransient();

    /**
     * Return true if this field is volatile
     */
    boolean isVolatile();

    /**
     * Return the serialField tags in this FieldDoc item.
     *
     * @return an array of <tt>SerialFieldTag</tt> objects containing
     *         all <code>@serialField</code> tags.
     */
    SerialFieldTag[] serialFieldTags();

    /**
     * Get the value of a constant field.
     *
     * @return the value of a constant field. The value is
     * automatically wrapped in an object if it has a primitive type.
     * If the field is not constant, returns null.
     */
    Object constantValue();

    /**
     * Get the value of a constant field.
     *
     * @return the text of a Java language expression whose value
     * is the value of the constant. The expression uses no identifiers
     * other than primitive literals. If the field is
     * not constant, returns null.
     */
    String constantValueExpression();
}

⏎ com/sun/javadoc/FieldDoc.java

Or download all of them as a single archive file:

File name: jre-rt-com-1.8.0_191-src.zip
File size: 8099783 bytes
Release date: 2018-10-28
Download 

⇒ Backup JDK 8 Installation Directory

⇐ JRE 8 rt.jar - org.* Package Source Code

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)