目的:为了生成一个不重复的主键，主键生成通过客户端程序生成。

规则主要是根据机器的网络接口信息、线程信息、时间和随机数生成一个不会重复的主键。

详细类如下，改了下名字，根据自己理解加了些注释

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/**
 *      Copyright (C) 2008 10gen Inc.
 *
 *   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.
 */



import java.net.*;
import java.nio.*;
import java.util.*;
import java.util.concurrent.atomic.*;
import java.util.logging.*;

/**
 * 全局对象唯一标示符
 * A globally unique identifier for objects.
 * <p>Consists of 12 bytes, divided as follows:
 * <blockquote><pre>
 * <table border="1">
 * <tr><td>0</td><td>1</td><td>2</td><td>3</td><td>4</td><td>5</td><td>6</td>
 *     <td>7</td><td>8</td><td>9</td><td>10</td><td>11</td></tr>
 * <tr><td colspan="4">time</td><td colspan="3">machine</td>
 *     <td colspan="2">pid</td><td colspan="3">inc</td></tr>
 * </table>
 * </pre></blockquote>
 *
 * @dochub CreateGuIds
 */
public class CreateGuId implements Comparable<CreateGuId> , java.io.Serializable {

    private static final long serialVersionUID = -4415279469780082174L;

    static final Logger LOGGER = Logger.getLogger( "com.easy.todo.util" );

    public static  void main(String args[]){
        System.out.println(CreateGuId.get());
    }

    /** 获取一个新的对象id
     * Gets a new object id.
     * @return the new id
     */
    public static CreateGuId get(){
        return new CreateGuId();
    }

    /** Checks if a string could be an <code>CreateGuId</code>.
     * @return whether the string could be an object id
     */
    public static boolean isValid( String s ){
        if ( s == null )
            return false;

        final int len = s.length();
        if ( len != 24 )
            return false;

        for ( int i=0; i<len; i++ ){
            char c = s.charAt( i );
            if ( c >= '0' && c <= '9' )
                continue;
            if ( c >= 'a' && c <= 'f' )
                continue;
            if ( c >= 'A' && c <= 'F' )
                continue;

            return false;
        }

        return true;
    }

    /** Turn an object into an <code>CreateGuId</code>, if possible.
     * Strings will be converted into <code>CreateGuId</code>s, if possible, and <code>CreateGuId</code>s will
     * be cast and returned.  Passing in <code>null</code> returns <code>null</code>.
     * @param o the object to convert
     * @return an <code>CreateGuId</code> if it can be massaged, null otherwise
     */
    public static CreateGuId massageToCreateGuId( Object o ){
        if ( o == null )
            return null;

        if ( o instanceof CreateGuId )
            return (CreateGuId)o;

        if ( o instanceof String ){
            String s = o.toString();
            if ( isValid( s ) )
                return new CreateGuId( s );
        }

        return null;
    }

    public CreateGuId( Date time ){
        this(time, _genmachine, _nextInc.getAndIncrement());//原子操作加1
    }

    public CreateGuId( Date time , int inc ){
        this( time , _genmachine , inc );
    }

    public CreateGuId( Date time , int machine , int inc ){