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/io/DeleteOnExitHook.java

/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */
package java.io;

import java.util.*;
import java.io.File;

/**
 * This class holds a set of filenames to be deleted on VM exit through a shutdown hook.
 * A set is used both to prevent double-insertion of the same file as well as offer
 * quick removal.
 */

class DeleteOnExitHook {
    private static LinkedHashSet<String> files = new LinkedHashSet<>();
    static {
        // DeleteOnExitHook must be the last shutdown hook to be invoked.
        // Application shutdown hooks may add the first file to the
        // delete on exit list and cause the DeleteOnExitHook to be
        // registered during shutdown in progress. So set the
        // registerShutdownInProgress parameter to true.
        sun.misc.SharedSecrets.getJavaLangAccess()
            .registerShutdownHook(2 /* Shutdown hook invocation order */,
                true /* register even if shutdown in progress */,
                new Runnable() {
                    public void run() {
                       runHooks();
                    }
                }
        );
    }

    private DeleteOnExitHook() {}

    static synchronized void add(String file) {
        if(files == null) {
            // DeleteOnExitHook is running. Too late to add a file
            throw new IllegalStateException("Shutdown in progress");
        }

        files.add(file);
    }

    static void runHooks() {
        LinkedHashSet<String> theFiles;

        synchronized (DeleteOnExitHook.class) {
            theFiles = files;
            files = null;
        }

        ArrayList<String> toBeDeleted = new ArrayList<>(theFiles);

        // reverse the list to maintain previous jdk deletion order.
        // Last in first deleted.
        Collections.reverse(toBeDeleted);
        for (String filename : toBeDeleted) {
            (new File(filename)).delete();
        }
    }
}

⏎ java/io/DeleteOnExitHook.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)