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

⏎ java/nio/charset/UnmappableCharacterException.java

/*
 * Copyright (c) 2001, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.nio.charset;


/**
 * Checked exception thrown when an input character (or byte) sequence
 * is valid but cannot be mapped to an output byte (or character)
 * sequence.
 *
 * @since 1.4
 */

public class UnmappableCharacterException
    extends CharacterCodingException
{

    private static final long serialVersionUID = -7026962371537706123L;

    private int inputLength;

    /**
     * Constructs an {@code UnmappableCharacterException} with the
     * given length.
     * @param inputLength the length of the input
     */
    public UnmappableCharacterException(int inputLength) {
        this.inputLength = inputLength;
    }

    /**
     * Returns the length of the input.
     * @return the length of the input
     */
    public int getInputLength() {
        return inputLength;
    }

    /**
     * Returns the message.
     * @return the message
     */
    public String getMessage() {
        return "Input length = " + inputLength;
    }

}

⏎ java/nio/charset/UnmappableCharacterException.java

Or download all of them as a single archive file:

File name: jre-rt-java-1.8.0_191-src.zip
File size: 6664831 bytes
Release date: 2018-10-28
Download 

⇒ JRE 8 rt.jar - javax.* Package Source Code

⇐ JRE 8 plugin.jar - Java Deploy Control Panel Plugin

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)