JDK8/Java8源码在线阅读

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code 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 General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.security;

import java.nio.ByteBuffer;

import sun.security.jca.JCAUtil;

/**
 * This class defines the <i>Service Provider Interface</i> (<b>SPI</b>)
 * for the {@code MessageDigest} class, which provides the functionality
 * of a message digest algorithm, such as MD5 or SHA. Message digests are
 * secure one-way hash functions that take arbitrary-sized data and output a
 * fixed-length hash value.
 *
 * <p> All the abstract methods in this class must be implemented by a
 * cryptographic service provider who wishes to supply the implementation
 * of a particular message digest algorithm.
 *
 * <p> Implementations are free to implement the Cloneable interface.
 *
 * @author Benjamin Renaud
 *
 *
 * @see MessageDigest
 */

public abstract class MessageDigestSpi {

    // for re-use in engineUpdate(ByteBuffer input)
    private byte[] tempArray;

    /**
     * Returns the digest length in bytes.
     *
     * <p>This concrete method has been added to this previously-defined
     * abstract class. (For backwards compatibility, it cannot be abstract.)
     *
     * <p>The default behavior is to return 0.
     *
     * <p>This method may be overridden by a provider to return the digest
     * length.
     *
     * @return the digest length in bytes.
     *
     * @since 1.2
     */
    protected int engineGetDigestLength() {
        return 0;
    }

    /**
     * Updates the digest using the specified byte.
     *
     * @param input the byte to use for the update.
     */
    protected abstract void engineUpdate(byte input);

    /**
     * Updates the digest using the specified array of bytes,
     * starting at the specified offset.
     *
     * @param input the array of bytes to use for the update.
     *
     * @param offset the offset to start from in the array of bytes.
     *
     * @param len the number of bytes to use, starting at
     * {@code offset}.
     */
    protected abstract void engineUpdate(byte[] input, int offset, int len);

    /**
     * Update the digest using the specified ByteBuffer. The digest is
     * updated using the {@code input.remaining()} bytes starting
     * at {@code input.position()}.
     * Upon return, the buffer's position will be equal to its limit;
     * its limit will not have changed.
     *
     * @param input the ByteBuffer
     * @since 1.5
     */
    protected void engineUpdate(ByteBuffer input) {
        if (input.hasRemaining() == false) {
            return;
        }
        if (input.hasArray()) {
            byte[] b = input.array();
            int ofs = input.arrayOffset();
            int pos = input.position();
            int lim = input.limit();
            engineUpdate(b, ofs + pos, lim - pos);
            input.position(lim);
        } else {
            int len = input.remaining();
            int n = JCAUtil.getTempArraySize(len);
            if ((tempArray == null) || (n > tempArray.length)) {
                tempArray = new byte[n];
            }
            while (len > 0) {
                int chunk = Math.min(len, tempArray.length);
                input.get(tempArray, 0, chunk);
                engineUpdate(tempArray, 0, chunk);
                len -= chunk;
            }
        }
    }

    /**
     * Completes the hash computation by performing final
     * operations such as padding. Once {@code engineDigest} has
     * been called, the engine should be reset (see
     * {@link #engineReset() engineReset}).
     * Resetting is the responsibility of the
     * engine implementor.
     *
     * @return the array of bytes for the resulting hash value.
     */
    protected abstract byte[] engineDigest();

    /**
     * Completes the hash computation by performing final
     * operations such as padding. Once {@code engineDigest} has
     * been called, the engine should be reset (see
     * {@link #engineReset() engineReset}).
     * Resetting is the responsibility of the
     * engine implementor.
     *
     * This method should be abstract, but we leave it concrete for
     * binary compatibility.  Knowledgeable providers should override this
     * method.
     *
     * @param buf the output buffer in which to store the digest
     *
     * @param offset offset to start from in the output buffer
     *
     * @param len number of bytes within buf allotted for the digest.
     * Both this default implementation and the SUN provider do not
     * return partial digests.  The presence of this parameter is solely
     * for consistency in our API's.  If the value of this parameter is less
     * than the actual digest length, the method will throw a DigestException.
     * This parameter is ignored if its value is greater than or equal to
     * the actual digest length.
     *
     * @return the length of the digest stored in the output buffer.
     *
     * @exception DigestException if an error occurs.
     *
     * @since 1.2
     */
    protected int engineDigest(byte[] buf, int offset, int len)
                                                throws DigestException {

        byte[] digest = engineDigest();
        if (len < digest.length)
                throw new DigestException("partial digests not returned");
        if (buf.length - offset < digest.length)
                throw new DigestException("insufficient space in the output "

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