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/ObjectInputValidation.java

/*
 * Copyright (c) 1996, 1999, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.io;

/**
 * Callback interface to allow validation of objects within a graph.
 * Allows an object to be called when a complete graph of objects has
 * been deserialized.
 *
 * @author  unascribed
 * @see     ObjectInputStream
 * @see     ObjectInputStream#registerValidation(java.io.ObjectInputValidation, int)
 * @since   JDK1.1
 */
public interface ObjectInputValidation {
    /**
     * Validates the object.
     *
     * @exception InvalidObjectException If the object cannot validate itself.
     */
    public void validateObject() throws InvalidObjectException;
}

⏎ java/io/ObjectInputValidation.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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