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

⏎ com/sun/org/apache/xerces/internal/impl/dv/dtd/StringDatatypeValidator.java

/*
 * Copyright (c) 2007, 2018, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
/*
 * Copyright 1999-2002,2004 The Apache Software Foundation.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.sun.org.apache.xerces.internal.impl.dv.dtd;

import com.sun.org.apache.xerces.internal.impl.dv.*;

/**
 * <P>StringValidator validates that XML content is a W3C string type.</P>
 * <P>The string datatype represents character strings in XML. The
 * value space of string is the set of finite-length sequences
 * of characters (as defined in [XML 1.0 Recommendation
 * (Second Edition)]) that match the Char production
 * from [XML 1.0 Recommendation (Second Edition)].
 * A character is an atomic unit of communication; it
 * is not further specified except to note that every
 * character has a corresponding Universal Code Set
 * code point ([ISO 10646],[Unicode] and [Unicode3]),
 * which is an integer.</P>
 *
 * @xerces.internal
 *
 */
public class StringDatatypeValidator implements DatatypeValidator {

    // construct a string datatype validator
    public StringDatatypeValidator() {
    }

    /**
     * Checks that "content" string is valid string value.
     * If invalid a Datatype validation exception is thrown.
     *
     * @param content       the string value that needs to be validated
     * @param context       the validation context
     * @throws InvalidDatatypeException if the content is
     *         invalid according to the rules for the validators
     * @see InvalidDatatypeValueException
     */
    public void validate(String content, ValidationContext context) throws InvalidDatatypeValueException {
    }

}

⏎ com/sun/org/apache/xerces/internal/impl/dv/dtd/StringDatatypeValidator.java

Or download all of them as a single archive file:

File name: jre-rt-com-1.8.0_191-src.zip
File size: 8099783 bytes
Release date: 2018-10-28
Download 

⇒ Backup JDK 8 Installation Directory

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

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)