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/ConnectException.java

/*
 * Copyright (c) 1996, 2008, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.net;

/**
 * Signals that an error occurred while attempting to connect a
 * socket to a remote address and port.  Typically, the connection
 * was refused remotely (e.g., no process is listening on the
 * remote address/port).
 *
 * @since   JDK1.1
 */
public class ConnectException extends SocketException {
    private static final long serialVersionUID = 3831404271622369215L;

    /**
     * Constructs a new ConnectException with the specified detail
     * message as to why the connect error occurred.
     * A detail message is a String that gives a specific
     * description of this error.
     * @param msg the detail message
     */
    public ConnectException(String msg) {
        super(msg);
    }

    /**
     * Construct a new ConnectException with no detailed message.
     */
    public ConnectException() {}
}

⏎ java/net/ConnectException.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

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