public class DateUtilTest {
    
    public static class TestSimpleDateFormatThreadSafe extends Thread {
        @Override
        public void run() {
            while(true) {
                try {
                    this.join(2000);
                } catch (InterruptedException e1) {
                    e1.printStackTrace();
                }
                try {
                    System.out.println(this.getName()+":"+DateUtil.parse("2013-05-24 06:02:20"));
                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        }    
    }
    
    
    public static void main(String[] args) {
        for(int i = 0; i < 3; i++){
            new TestSimpleDateFormatThreadSafe().start();
        }
            
    }
}

Exception in thread "Thread-1" java.lang.NumberFormatException: multiple points
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1082)
    at java.lang.Double.parseDouble(Double.java:510)
    at java.text.DigitList.getDouble(DigitList.java:151)
    at java.text.DecimalFormat.parse(DecimalFormat.java:1302)
    at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
    at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1311)
    at java.text.DateFormat.parse(DateFormat.java:335)
    at com.peidasoft.orm.dateformat.DateNoStaticUtil.parse(DateNoStaticUtil.java:17)
    at com.peidasoft.orm.dateformat.DateUtilTest$TestSimpleDateFormatThreadSafe.run(DateUtilTest.java:20)
Exception in thread "Thread-0" java.lang.NumberFormatException: multiple points
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1082)
    at java.lang.Double.parseDouble(Double.java:510)
    at java.text.DigitList.getDouble(DigitList.java:151)
    at java.text.DecimalFormat.parse(DecimalFormat.java:1302)
    at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
    at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1311)
    at java.text.DateFormat.parse(DateFormat.java:335)
    at com.peidasoft.orm.dateformat.DateNoStaticUtil.parse(DateNoStaticUtil.java:17)
    at com.peidasoft.orm.dateformat.DateUtilTest$TestSimpleDateFormatThreadSafe.run(DateUtilTest.java:20)
Thread-2:Mon May 24 06:02:20 CST 2021
Thread-2:Fri May 24 06:02:20 CST 2013

private StringBuffer format(Date date, StringBuffer toAppendTo,
                                FieldDelegate delegate) {
        // Convert input date to time field list
        calendar.setTime(date);

    　　boolean useDateFormatSymbols = useDateFormatSymbols();

        for (int i = 0; i < compiledPattern.length; ) {
            int tag = compiledPattern[i] >>> 8;
        　　int count = compiledPattern[i++] & 0xff;
        　　if (count == 255) {
            　　count = compiledPattern[i++] << 16;
        　　　　count |= compiledPattern[i++];
        　　}

        　　switch (tag) {
        　　　　case TAG_QUOTE_ASCII_CHAR:
        　　　　　　toAppendTo.append((char)count);
        　　　　　　break;

        　　　　case TAG_QUOTE_CHARS:
        　　　　　　toAppendTo.append(compiledPattern, i, count);
        　　　　　　i += count;
        　　　　　　break;

        　　　　default:
                　 subFormat(tag, count, delegate, toAppendTo, useDateFormatSymbols);
        　　　　　　break;
        　　}
    　　}
        return toAppendTo;
    }

public class DateSyncUtil {

    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
      
    public static String formatDate(Date date)throws ParseException{
        synchronized(sdf){
            return sdf.format(date);
        }  
    }
    
    public static Date parse(String strDate) throws ParseException{
        synchronized(sdf){
            return sdf.parse(strDate);
        }
    } 
}

public class DateUtils extends org.apache.commons.lang3.time.DateUtils {

    public static final String PATTERN_DATE_TIME = "yyyy-MM-dd HH:mm:ss";
      
    private  static Map<String, ThreadLocal<DateFormat>> dateFormatMap;

    /**
     * 創建單例(li)
     * 沒(mei)有使(shi)用(yong)(yong)單例(li)模式(shi)的話(hua),spring在注入數據時(shi)如果有邏輯調用(yong)(yong)了(le)getDateFormat(pattern),會因為(wei)dateFormatMap尚未初始化而(er)報NullPoint
     * @return
     */
    private static Map<String, ThreadLocal<DateFormat>> getDateFormatMap() {
        if (dateFormatMap == null) {
            synchronized (DateUtils.class) {
                if (dateFormatMap == null) {
                    dateFormatMap = new ConcurrentHashMap<>();
                }
            }
        }
        return dateFormatMap;
    }
    
    /**
     * 獲取線程安全的(de)DateFormat
     * SimpleDateFormat有兩個問(wen)(wen)題(ti)(ti):
     * 1.非線程安全,僅僅是聲明為static,使用時(shi)不(bu)上鎖在并(bing)發(fa)狀況下調用parse()有可(ke)(ke)能得(de)到(dao)錯誤(wu)的(de)時(shi)間
     * 2.頻繁實例化(hua)有可(ke)(ke)能導致內存溢出
     * 使用ThreadLocal將(jiang)DateFormat變(bian)為線程獨享,既可(ke)(ke)以避免并(bing)發(fa)問(wen)(wen)題(ti)(ti),又可(ke)(ke)以減少反(fan)復(fu)創(chuang)建實例的(de)開銷
     * @param pattern
     * @return
     */
    public  static DateFormat getDateFormat(final String pattern) {
        ThreadLocal<DateFormat> dateFormat = getDateFormatMap().get(pattern);
        if (dateFormat == null) {
            dateFormat = new ThreadLocal<DateFormat>() {
                @Override
                protected DateFormat initialValue() {
                    return new SimpleDateFormat(pattern);
                }
            };
            dateFormatMap.put(pattern, dateFormat);
        }
        return dateFormat.get();
    }

    /**
     * Date格式(shi)化 "yyyy-MM-dd HH:mm:ss"字符串
     * 
     * @param date
     * @return
     */
    public static String DateFormatToString(Date date) {
        DateFormat sdf = getDateFormat(PATTERN_DATE_TIME);
        return sdf.format(date);
    }
}