Thread synchronization issues when using SimpleDateFormat. . .

wrong usage:

 public static final SimpleDateFormat DAY_UI_MONTH_DAY_FORMAT = new SimpleDateFormat("MM-dd");
 public static final SimpleDateFormat WEEK_FORMAT = new SimpleDateFormat("EEE", Locale.US);

Ali programming specification tips:

SimpleDateFormat is a thread-unsafe class. Generally, it should not be defined as a static variable. If it is defined as static, it must be locked, or the DateUtils tool class should be used. 
Note: If it is an application of JDK8, you can use instant instead of Date, LocalDateTime instead of Calendar, and DateTimeFormatter instead of SimpleDateFormat. 
The official explanation is: simple beautiful strong immutable thread- safe.
            
Positive example 1：
    private static final String FORMAT = "yyyy-MM-dd HH:mm:ss";
    public String getFormat(Date date){
        SimpleDateFormat dateFormat = new SimpleDateFormat(FORMAT);
        return sdf.format(date);
    }
            
Positive example 2：
    private static final SimpleDateFormat SIMPLE_DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    public void getFormat(){
        synchronized (sdf){
        sdf.format(new Date());
        ….;
    }
            
Positive example 3：
    private static final ThreadLocal<DateFormat> DATE_FORMATTER = new ThreadLocal<DateFormat>() {
        @Override
        protected DateFormat initialValue() {
            return new SimpleDateFormat("yyyy-MM-dd");
        }
    };