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/imageio/plugins/bmp/BMPConstants.java

/*
 * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package com.sun.imageio.plugins.bmp;

public interface BMPConstants {
    // bmp versions
    static final String VERSION_2 = "BMP v. 2.x";
    static final String VERSION_3 = "BMP v. 3.x";
    static final String VERSION_3_NT = "BMP v. 3.x NT";
    static final String VERSION_4 = "BMP v. 4.x";
    static final String VERSION_5 = "BMP v. 5.x";

    // Color space types
    static final int LCS_CALIBRATED_RGB = 0;
    static final int LCS_sRGB = 1;
    static final int LCS_WINDOWS_COLOR_SPACE = 2;
    static final int PROFILE_LINKED = 3;
    static final int PROFILE_EMBEDDED = 4;

    // Compression Types
    static final int BI_RGB = 0;
    static final int BI_RLE8 = 1;
    static final int BI_RLE4 = 2;
    static final int BI_BITFIELDS = 3;
    static final int BI_JPEG = 4;
    static final int BI_PNG = 5;
}

⏎ com/sun/imageio/plugins/bmp/BMPConstants.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)