kernel.jar is a component in iText Java library to provide low-level functionalities. iText Java library allows you to generate and manage PDF documents.

The Source Code files are provided together with the JAR file in the binary packge like iText7-Core-7.1.4.zip. You can download it at iText 7 Core Download site.

You can compile it to generate your JAR file, using kernel.pom as the build configuration file.

The source code of kernel-7.1.4.jar is provided below:

⏎ com/itextpdf/kernel/pdf/PdfVersion.java

/*

    This file is part of the iText (R) project.
    Copyright (c) 1998-2018 iText Group NV
    Authors: Bruno Lowagie, Paulo Soares, et al.

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU Affero General Public License version 3
    as published by the Free Software Foundation with the addition of the
    following permission added to Section 15 as permitted in Section 7(a):
    FOR ANY PART OF THE COVERED WORK IN WHICH THE COPYRIGHT IS OWNED BY
    ITEXT GROUP. ITEXT GROUP DISCLAIMS THE WARRANTY OF NON INFRINGEMENT
    OF THIRD PARTY RIGHTS

    This program is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
    or FITNESS FOR A PARTICULAR PURPOSE.
    See the GNU Affero General Public License for more details.
    You should have received a copy of the GNU Affero General Public License
    along with this program; if not, see http://www.gnu.org/licenses or write to
    the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
    Boston, MA, 02110-1301 USA, or download the license from the following URL:
    http://itextpdf.com/terms-of-use/

    The interactive user interfaces in modified source and object code versions
    of this program must display Appropriate Legal Notices, as required under
    Section 5 of the GNU Affero General Public License.

    In accordance with Section 7(b) of the GNU Affero General Public License,
    a covered work must retain the producer line in every PDF that is created
    or manipulated using iText.

    You can be released from the requirements of the license by purchasing
    a commercial license. Buying such a license is mandatory as soon as you
    develop commercial activities involving the iText software without
    disclosing the source code of your own applications.
    These activities include: offering paid services to customers as an ASP,
    serving PDFs on the fly in a web application, shipping iText with a closed
    source product.

    For more information, please contact iText Software Corp. at this
    address: sales@itextpdf.com
 */
package com.itextpdf.kernel.pdf;

import java.io.Serializable;
import com.itextpdf.io.util.MessageFormatUtil;
import java.util.ArrayList;
import java.util.List;

/**
 * This class represents all official PDF versions.
 */
public class PdfVersion implements Comparable<PdfVersion>, Serializable {

    private static final long serialVersionUID = 6168855906667968169L;

    private static final List<PdfVersion> values = new ArrayList<>();

    public static final PdfVersion PDF_1_0 = createPdfVersion(1, 0);
    public static final PdfVersion PDF_1_1 = createPdfVersion(1, 1);
    public static final PdfVersion PDF_1_2 = createPdfVersion(1, 2);
    public static final PdfVersion PDF_1_3 = createPdfVersion(1, 3);
    public static final PdfVersion PDF_1_4 = createPdfVersion(1, 4);
    public static final PdfVersion PDF_1_5 = createPdfVersion(1, 5);
    public static final PdfVersion PDF_1_6 = createPdfVersion(1, 6);
    public static final PdfVersion PDF_1_7 = createPdfVersion(1, 7);
    public static final PdfVersion PDF_2_0 = createPdfVersion(2, 0);

    private int major;
    private int minor;

    /**
     * Creates a PdfVersion class.
     * @param major major version number
     * @param minor minor version number
     */
    private PdfVersion(int major, int minor) {
        this.major = major;
        this.minor = minor;
    }

    @Override
    public String toString() {
        return MessageFormatUtil.format("PDF-{0}.{1}", major, minor);
    }

    public PdfName toPdfName() {
        return new PdfName(MessageFormatUtil.format("{0}.{1}", major, minor));
    }

    /**
     * Creates a PdfVersion class from a String object if the specified version
     * can be found.
     *
     * @param value version number
     * @return PdfVersion of the specified version
     */
    public static PdfVersion fromString(String value) {
        for (PdfVersion version : values) {
            if (version.toString().equals(value)) {
                return version;
            }
        }
        throw new IllegalArgumentException("The provided pdf version was not found.");
    }

    /**
     * Creates a PdfVersion class from a {@link PdfName} object if the specified version
     * can be found.
     *
     * @param name version number
     * @return PdfVersion of the specified version
     */
    public static PdfVersion fromPdfName(PdfName name) {
        for (PdfVersion version : values) {
            if (version.toPdfName().equals(name)) {
                return version;
            }
        }
        throw new IllegalArgumentException("The provided pdf version was not found.");
    }

    @Override
    public int compareTo(PdfVersion o) {
        int majorResult = Integer.compare(major, o.major);
        if (majorResult != 0) {
            return majorResult;
        } else {
            return Integer.compare(minor, o.minor);
        }
    }

    @Override
    public boolean equals(Object obj) {
        return getClass() == obj.getClass() && compareTo((PdfVersion) obj) == 0;
    }

    private static PdfVersion createPdfVersion(int major, int minor) {
        PdfVersion pdfVersion = new PdfVersion(major, minor);
        values.add(pdfVersion);
        return pdfVersion;
    }
}

⏎ com/itextpdf/kernel/pdf/PdfVersion.java

Or download all of them as a single archive file:

File name: kernel-7.1.4-sources.jar
File size: 1201221 bytes
Release date: 2018-10-09
Download 

⇒ iText io.jar Source Code

⇐ Download and Install iText7-Core-7.1.4.zip

⇑ Download and Install iText Java Library

⇑⇑ iText for PDF Generation