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/lang/UnsupportedClassVersionError.java

/*
 * Copyright (c) 1998, 2008, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.lang;

/**
 * Thrown when the Java Virtual Machine attempts to read a class
 * file and determines that the major and minor version numbers
 * in the file are not supported.
 *
 * @since   1.2
 */
public
class UnsupportedClassVersionError extends ClassFormatError {
    private static final long serialVersionUID = -7123279212883497373L;

    /**
     * Constructs a <code>UnsupportedClassVersionError</code>
     * with no detail message.
     */
    public UnsupportedClassVersionError() {
        super();
    }

    /**
     * Constructs a <code>UnsupportedClassVersionError</code> with
     * the specified detail message.
     *
     * @param   s   the detail message.
     */
    public UnsupportedClassVersionError(String s) {
        super(s);
    }
}

⏎ java/lang/UnsupportedClassVersionError.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

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