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/instrument/ClassDefinition.java

/*
 * Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.lang.instrument;

/*
 * Copyright 2003 Wily Technology, Inc.
 */

/**
 * This class serves as a parameter block to the <code>Instrumentation.redefineClasses</code> method.
 * Serves to bind the <code>Class</code> that needs redefining together with the new class file bytes.
 *
 * @see     java.lang.instrument.Instrumentation#redefineClasses
 * @since   1.5
 */
public final class ClassDefinition {
    /**
     *  The class to redefine
     */
    private final Class<?> mClass;

    /**
     *  The replacement class file bytes
     */
    private final byte[]   mClassFile;

    /**
     *  Creates a new <code>ClassDefinition</code> binding using the supplied
     *  class and class file bytes. Does not copy the supplied buffer, just captures a reference to it.
     *
     * @param theClass the <code>Class</code> that needs redefining
     * @param theClassFile the new class file bytes
     *
     * @throws java.lang.NullPointerException if the supplied class or array is <code>null</code>.
     */
    public
    ClassDefinition(    Class<?> theClass,
                        byte[]  theClassFile) {
        if (theClass == null || theClassFile == null) {
            throw new NullPointerException();
        }
        mClass      = theClass;
        mClassFile  = theClassFile;
    }

    /**
     * Returns the class.
     *
     * @return    the <code>Class</code> object referred to.
     */
    public Class<?>
    getDefinitionClass() {
        return mClass;
    }

    /**
     * Returns the array of bytes that contains the new class file.
     *
     * @return    the class file bytes.
     */
    public byte[]
    getDefinitionClassFile() {
        return mClassFile;
    }
}

⏎ java/lang/instrument/ClassDefinition.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 

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