为JwtUtil添加从jwt直接获取username的功能

src/main/java/org/cmh/backend/Utils/JwtUtil.java

@@ -29,6 +29,10 @@ public class JwtUtil {
                 .getBody();
     }
 
+    public static String extractUsername(String token) {
+        return extractClaims(token).getSubject();
+    }
+
     public static boolean isTokenValid(String token, String username) {
         return username.equals(extractClaims(token).getSubject()) && !isTokenExpired(token);
     }