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/awt/HeadlessException.java

/*
 * Copyright (c) 2000, 2004, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.awt;

/**
 * Thrown when code that is dependent on a keyboard, display, or mouse
 * is called in an environment that does not support a keyboard, display,
 * or mouse.
 *
 * @since 1.4
 * @author  Michael Martak
 */
public class HeadlessException extends UnsupportedOperationException {
    /*
     * JDK 1.4 serialVersionUID
     */
    private static final long serialVersionUID = 167183644944358563L;
    public HeadlessException() {}
    public HeadlessException(String msg) {
        super(msg);
    }
    public String getMessage() {
        String superMessage = super.getMessage();
        String headlessMessage = GraphicsEnvironment.getHeadlessMessage();

        if (superMessage == null) {
            return headlessMessage;
        } else if (headlessMessage == null) {
            return superMessage;
        } else {
            return superMessage + headlessMessage;
        }
    }
}

⏎ java/awt/HeadlessException.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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