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ת×Ô £º http://hi.baidu.com/samuel_vong/blog/item/95471f7e2cdab33d0cd7dad1.html
/**
   * ²úÉúËæ»ú×Ö·û´®
   * */
private static Random randGen = null;
private static char[] numbersAndLetters = null;
private static Object initLock = new Object();
public static final String randomString(int length) {
         if (length < 1) {
             return null;
         }
         if (randGen == null) {
             synchronized (initLock) {
                 if (randGen == null) {
                     randGen = new Random();
                     numbersAndLetters = ("0123456789abcdefghijklmnopqrstuvwxyz" +
                     "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
                     //numbersAndLetters = ("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
                 }
             }
         }
         char [] randBuffer = new char[length];
         for (int i=0; i<randBuffer.length; i++) {
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