001    /*--------------------------------------------------------------------------+
002    $Id: Digester.java 26268 2010-02-18 10:44:30Z juergens $
003    |                                                                          |
004    | Copyright 2005-2010 Technische Universitaet Muenchen                     |
005    |                                                                          |
006    | Licensed under the Apache License, Version 2.0 (the "License");          |
007    | you may not use this file except in compliance with the License.         |
008    | You may obtain a copy of the License at                                  |
009    |                                                                          |
010    |    http://www.apache.org/licenses/LICENSE-2.0                            |
011    |                                                                          |
012    | Unless required by applicable law or agreed to in writing, software      |
013    | distributed under the License is distributed on an "AS IS" BASIS,        |
014    | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
015    | See the License for the specific language governing permissions and      |
016    | limitations under the License.                                           |
017    +--------------------------------------------------------------------------*/
018    package edu.tum.cs.commons.digest;
019    
020    import java.security.MessageDigest;
021    import java.security.NoSuchAlgorithmException;
022    import java.util.Collection;
023    import java.util.List;
024    
025    import edu.tum.cs.commons.collections.CollectionUtils;
026    import edu.tum.cs.commons.error.EnvironmentError;
027    import edu.tum.cs.commons.string.StringUtils;
028    
029    /**
030     * Utility functions for creation of digests.
031     * 
032     * @author juergens
033     * @author $Author: juergens $
034     * @version $Rev: 26268 $
035     * @levd.rating GREEN Hash: 3CB40A00577C76D6DEA0297E30EEA8F7
036     */
037    public class Digester {
038    
039            /** Digester used to create Digester hashes. */
040            private static MessageDigest digester = getMD5();
041    
042            /**
043             * Computes an MD5 hash for a string. The fingerprint is the digester hash
044             * of the string. It is always 32 characters long and only uses characters
045             * from [0-9A-F].
046             */
047            public static String createMD5Digest(String base) {
048                    digester.reset();
049                    digester.update(base.getBytes());
050    
051                    return StringUtils.encodeAsHex(digester.digest());
052            }
053    
054            /**
055             * Computes an MD5 hash for a collection of strings. The strings are sorted
056             * before MD5 computation, so that the resulting MD5 hash is independent of
057             * the order of the strings in the collection.
058             */
059            public static String createMD5Digest(Collection<String> bases) {
060                    List<String> sortedBases = CollectionUtils.sort(bases);
061                    return createMD5Digest(StringUtils.concat(sortedBases,
062                                    StringUtils.EMPTY_STRING));
063            }
064    
065            /**
066             * Returns Digester digester or throws an AssertionError if the Digester
067             * could not be located.
068             */
069            public static MessageDigest getMD5() {
070                    try {
071                            return MessageDigest.getInstance("MD5");
072                    } catch (NoSuchAlgorithmException e) {
073                            throw new EnvironmentError(
074                                            "MD5 algorithm found. Please check your JRE installation",
075                                            e);
076                    }
077    
078            }
079    
080    }