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 javax.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ javax/imageio/IIOException.java

/*
 * Copyright (c) 2000, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package javax.imageio;

import java.io.IOException;

/**
 * An exception class used for signaling run-time failure of reading
 * and writing operations.
 *
 * <p> In addition to a message string, a reference to another
 * <code>Throwable</code> (<code>Error</code> or
 * <code>Exception</code>) is maintained.  This reference, if
 * non-<code>null</code>, refers to the event that caused this
 * exception to occur.  For example, an <code>IOException</code> while
 * reading from a <code>File</code> would be stored there.
 *
 */
public class IIOException extends IOException {

    /**
     * Constructs an <code>IIOException</code> with a given message
     * <code>String</code>.  No underlying cause is set;
     * <code>getCause</code> will return <code>null</code>.
     *
     * @param message the error message.
     *
     * @see #getMessage
     */
    public IIOException(String message) {
        super(message);
    }

    /**
     * Constructs an <code>IIOException</code> with a given message
     * <code>String</code> and a <code>Throwable</code> that was its
     * underlying cause.
     *
     * @param message the error message.
     * @param cause the <code>Throwable</code> (<code>Error</code> or
     * <code>Exception</code>) that caused this exception to occur.
     *
     * @see #getCause
     * @see #getMessage
     */
    public IIOException(String message, Throwable cause) {
        super(message);
        initCause(cause);
    }
}

⏎ javax/imageio/IIOException.java

Or download all of them as a single archive file:

File name: jre-rt-javax-1.8.0_191-src.zip
File size: 5381005 bytes
Release date: 2018-10-28
Download 

⇒ JRE 8 rt.jar - org.* Package Source Code

⇐ JRE 8 rt.jar - java.* Package Source Code

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