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/Externalizable.java

/*
 * Copyright (c) 1996, 2004, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.io;

import java.io.ObjectOutput;
import java.io.ObjectInput;

/**
 * Only the identity of the class of an Externalizable instance is
 * written in the serialization stream and it is the responsibility
 * of the class to save and restore the contents of its instances.
 *
 * The writeExternal and readExternal methods of the Externalizable
 * interface are implemented by a class to give the class complete
 * control over the format and contents of the stream for an object
 * and its supertypes. These methods must explicitly
 * coordinate with the supertype to save its state. These methods supersede
 * customized implementations of writeObject and readObject methods.<br>
 *
 * Object Serialization uses the Serializable and Externalizable
 * interfaces.  Object persistence mechanisms can use them as well.  Each
 * object to be stored is tested for the Externalizable interface. If
 * the object supports Externalizable, the writeExternal method is called. If the
 * object does not support Externalizable and does implement
 * Serializable, the object is saved using
 * ObjectOutputStream. <br> When an Externalizable object is
 * reconstructed, an instance is created using the public no-arg
 * constructor, then the readExternal method called.  Serializable
 * objects are restored by reading them from an ObjectInputStream.<br>
 *
 * An Externalizable instance can designate a substitution object via
 * the writeReplace and readResolve methods documented in the Serializable
 * interface.<br>
 *
 * @author  unascribed
 * @see java.io.ObjectOutputStream
 * @see java.io.ObjectInputStream
 * @see java.io.ObjectOutput
 * @see java.io.ObjectInput
 * @see java.io.Serializable
 * @since   JDK1.1
 */
public interface Externalizable extends java.io.Serializable {
    /**
     * The object implements the writeExternal method to save its contents
     * by calling the methods of DataOutput for its primitive values or
     * calling the writeObject method of ObjectOutput for objects, strings,
     * and arrays.
     *
     * @serialData Overriding methods should use this tag to describe
     *             the data layout of this Externalizable object.
     *             List the sequence of element types and, if possible,
     *             relate the element to a public/protected field and/or
     *             method of this Externalizable class.
     *
     * @param out the stream to write the object to
     * @exception IOException Includes any I/O exceptions that may occur
     */
    void writeExternal(ObjectOutput out) throws IOException;

    /**
     * The object implements the readExternal method to restore its
     * contents by calling the methods of DataInput for primitive
     * types and readObject for objects, strings and arrays.  The
     * readExternal method must read the values in the same sequence
     * and with the same types as were written by writeExternal.
     *
     * @param in the stream to read data from in order to restore the object
     * @exception IOException if I/O errors occur
     * @exception ClassNotFoundException If the class for an object being
     *              restored cannot be found.
     */
    void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;
}

⏎ java/io/Externalizable.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)