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/net/CacheRequest.java

/*
 * Copyright (c) 2003, 2004, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.net;

import java.io.OutputStream;
import java.io.IOException;

/**
 * Represents channels for storing resources in the
 * ResponseCache. Instances of such a class provide an
 * OutputStream object which is called by protocol handlers to
 * store the resource data into the cache, and also an abort() method
 * which allows a cache store operation to be interrupted and
 * abandoned. If an IOException is encountered while reading the
 * response or writing to the cache, the current cache store operation
 * will be aborted.
 *
 * @author Yingxian Wang
 * @since 1.5
 */
public abstract class CacheRequest {

    /**
     * Returns an OutputStream to which the response body can be
     * written.
     *
     * @return an OutputStream to which the response body can
     *         be written
     * @throws IOException if an I/O error occurs while
     *         writing the response body
     */
    public abstract OutputStream getBody() throws IOException;

    /**
     * Aborts the attempt to cache the response. If an IOException is
     * encountered while reading the response or writing to the cache,
     * the current cache store operation will be abandoned.
     */
    public abstract void abort();
}

⏎ java/net/CacheRequest.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)