优化代码性能和可读性，修复潜在的多线程问题

src/main/java/cc/baka9/catseedlogin/util/Util.java

@@ -1,48 +1,44 @@
 package cc.baka9.catseedlogin.util;
 
+import java.security.SecureRandom;
 import java.text.SimpleDateFormat;
 import java.util.Date;
-import java.util.Properties;
 import java.util.Random;
 import java.util.regex.Pattern;
 
 public class Util {
-    private static Pattern passwordDifficultyRegex = Pattern.compile("^(?![0-9]+$)(?![a-zA-Z]+$)[0-9A-Za-z]{6,16}$");
+    private static final Pattern passwordDifficultyRegex = Pattern.compile("^(?![0-9]+$)(?![a-zA-Z]+$)[0-9A-Za-z]{6,16}$");
+    private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
+    private static final Random random = new SecureRandom();
 
-    public static boolean passwordIsDifficulty(String pwd){
+    public static boolean passwordIsDifficulty(String pwd) {
         return passwordDifficultyRegex.matcher(pwd).find();
     }
 
-    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
-
-    public static String time2Str(long time){
-        return sdf.format(new Date(time));
+    public static String time2Str(long time) {
+        synchronized (sdf) {
+            return sdf.format(new Date(time));
+        }
     }
 
-    public static boolean checkMail(String e_mail){
+    public static boolean checkMail(String e_mail) {
         return e_mail.matches("[a-zA-Z0-9_]+@[a-zA-Z0-9_]+(\\.[a-zA-Z0-9_]+)+");
     }
 
-    public static String randomStr(){
+    public static String randomStr() {
         int leftLimit = 97; // letter 'a'
         int rightLimit = 122; // letter 'z'
         int targetStringLength = 10;
-        Random random = new Random();
         StringBuilder buffer = new StringBuilder(targetStringLength);
         for (int i = 0; i < targetStringLength; i++) {
-            int randomLimitedInt = leftLimit + (int)
-                    (random.nextFloat() * (rightLimit - leftLimit + 1));
+            int randomLimitedInt = leftLimit + random.nextInt(rightLimit - leftLimit + 1);
             buffer.append((char) randomLimitedInt);
         }
-
         return buffer.toString();
     }
 
-    public static boolean isOSLinux(){
-        Properties prop = System.getProperties();
-        String os = prop.getProperty("os.name");
+    public static boolean isOSLinux() {
+        String os = System.getProperty("os.name");
         return os != null && os.toLowerCase().contains("linux");
     }
-
-
-}
+}