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/lang/IllegalMonitorStateException.java

/*
 * Copyright (c) 1995, 2008, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.lang;

/**
 * Thrown to indicate that a thread has attempted to wait on an
 * object's monitor or to notify other threads waiting on an object's
 * monitor without owning the specified monitor.
 *
 * @author  unascribed
 * @see     java.lang.Object#notify()
 * @see     java.lang.Object#notifyAll()
 * @see     java.lang.Object#wait()
 * @see     java.lang.Object#wait(long)
 * @see     java.lang.Object#wait(long, int)
 * @since   JDK1.0
 */
public
class IllegalMonitorStateException extends RuntimeException {
    private static final long serialVersionUID = 3713306369498869069L;

    /**
     * Constructs an <code>IllegalMonitorStateException</code> with no
     * detail message.
     */
    public IllegalMonitorStateException() {
        super();
    }

    /**
     * Constructs an <code>IllegalMonitorStateException</code> with the
     * specified detail message.
     *
     * @param   s   the detail message.
     */
    public IllegalMonitorStateException(String s) {
        super(s);
    }
}

⏎ java/lang/IllegalMonitorStateException.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)