SimpleDateFormat的併發問題
摘要：
java在做日期轉換時我們會使用SimpleDateFormat做時間轉換，但其實SimpleDateFormat不是執行緒安全的，如果SimpleDateFormat用static宣告或只例項化一次被多個執行緒使用併發度高時就會出併發異常，看如下例子
public static ...
java在做日期轉換時我們會使用SimpleDateFormat做時間轉換，但其實SimpleDateFormat不是執行緒安全的，如果SimpleDateFormat用static宣告或只例項化一次被多個執行緒使用併發度高時就會出併發異常，看如下例子
public static void main(String[] args) {
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
List<String> lists = new ArrayList<String>() {
{
add("2018-11-22 01:11:11");
add("2018-11-22 02:22:22");
add("2018-11-22 03:33:33");
add("2018-11-22 04:44:44");
add("2018-11-22 03:55:55");
add("2018-11-22 04:55:56");
}
};
ExecutorService executorService = Executors.newCachedThreadPool();
//用CountDownLatch增加併發度
CountDownLatch countDownLatch = new CountDownLatch(lists.size());
for (String list : lists) {
executorService.submit(() -> {
try {
countDownLatch.await();
Date parse = format.parse(list);
System.out.println(parse.toString());
} catch (Exception e) {
e.printStackTrace();
}
});
countDownLatch.countDown();
}
}
執行日誌如下，可能每次報錯不一樣
java.lang.NumberFormatException: multiple points at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890) at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110) at java.lang.Double.parseDouble(Double.java:538) at java.text.DigitList.getDouble(DigitList.java:169) at java.text.DecimalFormat.parse(DecimalFormat.java:2056) at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869) at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514) at java.text.DateFormat.parse(DateFormat.java:364) at thread.aqs.SimpleDateFormateTest.lambda$main$0(SimpleDateFormateTest.java:42) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) java.lang.NumberFormatException: For input string: "E2E2." at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043) at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110) at java.lang.Double.parseDouble(Double.java:538) at java.text.DigitList.getDouble(DigitList.java:169) at java.text.DecimalFormat.parse(DecimalFormat.java:2056) at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162) at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514) at java.text.DateFormat.parse(DateFormat.java:364) at thread.aqs.SimpleDateFormateTest.lambda$main$0(SimpleDateFormateTest.java:42) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) java.lang.NumberFormatException: For input string: "E2E" at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043) at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110) at java.lang.Double.parseDouble(Double.java:538) at java.text.DigitList.getDouble(DigitList.java:169) at java.text.DecimalFormat.parse(DecimalFormat.java:2056) at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162) at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514) at java.text.DateFormat.parse(DateFormat.java:364) at thread.aqs.SimpleDateFormateTest.lambda$main$0(SimpleDateFormateTest.java:42) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) java.lang.NumberFormatException: For input string: "E.4220118E4"
原因
SimpleDateFormat的父類持有一個物件Calendar
parse方法流程是這樣的：
//解析字串 ... //清空Calendar Calendar.clear(); //迴圈設定每個field ...
所以在多執行緒中會出現A執行緒設定Calendar值後被B執行緒修改，
或者A和B執行緒同時設定Calendar的值，都會出現錯誤
看一下原始碼
SimpleDateFormat繼承DateFormat
public class SimpleDateFormat extends DateFormat
入口
public Date parse(String source) throws ParseException
{
ParsePosition pos = new ParsePosition(0);
Date result = parse(source, pos);
if (pos.index == 0)
throw new ParseException("Unparseable date: \"" + source + "\"" ,
pos.errorIndex);
return result;
}
parse中解析字串後的一部分
Date parsedDate;
try {
//出問題的方法
parsedDate = calb.establish(calendar).getTime();
// If the year value is ambiguous,
// then the two-digit year == the default start year
if (ambiguousYear[0]) {
if (parsedDate.before(defaultCenturyStart)) {
parsedDate = calb.addYear(100).establish(calendar).getTime();
}
}
}
// An IllegalArgumentException will be thrown by Calendar.getTime()
// if any fields are out of range, e.g., MONTH == 17.
catch (IllegalArgumentException e) {
pos.errorIndex = start;
pos.index = oldStart;
return null;
}
calb.establish方法
Calendar establish(Calendar cal) {
boolean weekDate = isSet(WEEK_YEAR)
&& field[WEEK_YEAR] > field[YEAR];
if (weekDate && !cal.isWeekDateSupported()) {
// Use YEAR instead
if (!isSet(YEAR)) {
set(YEAR, field[MAX_FIELD + WEEK_YEAR]);
}
weekDate = false;
}
//清空
cal.clear();
for (int stamp = MINIMUM_USER_STAMP; stamp < nextStamp; stamp++) {
for (int index = 0; index <= maxFieldIndex; index++) {
if (field[index] == stamp) {
//設定field
cal.set(index, field[MAX_FIELD + index]);
break;
}
}
}
if (weekDate) {
int weekOfYear = isSet(WEEK_OF_YEAR) ? field[MAX_FIELD + WEEK_OF_YEAR] : 1;
int dayOfWeek = isSet(DAY_OF_WEEK) ?
field[MAX_FIELD + DAY_OF_WEEK] : cal.getFirstDayOfWeek();
if (!isValidDayOfWeek(dayOfWeek) && cal.isLenient()) {
if (dayOfWeek >= 8) {
dayOfWeek--;
weekOfYear += dayOfWeek / 7;
dayOfWeek = (dayOfWeek % 7) + 1;
} else {
while (dayOfWeek <= 0) {
dayOfWeek += 7;
weekOfYear--;
}
}
dayOfWeek = toCalendarDayOfWeek(dayOfWeek);
}
cal.setWeekDate(field[MAX_FIELD + WEEK_YEAR], weekOfYear, dayOfWeek);
}
return cal;
}
如何解決
每次使用時都new一個SimpleDateFormat當然可以解決這個問題，但頻繁建立銷燬物件效能不高，方法上加鎖又會降低併發度
因為每個執行緒自己執行肯定是按順序執行，所以可以利用ThreadLocal
public class DateUtil{
private static ThreadLocal<DateFormat> threadLocal = ThreadLocal.withInitial(()->{
return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
});
public static Date parse(String dateStr) throws ParseException {
return threadLocal.get().parse(dateStr);
}
public static String format(Date date) {
return threadLocal.get().format(date);
}
}
每個執行緒持有自己的SimpleDateFormat，再跑測試程式碼，發現不報錯了
//輸出 Thu Nov 22 01:11:11 CST 2018 Thu Nov 22 04:55:56 CST 2018 Thu Nov 22 03:33:33 CST 2018 Thu Nov 22 03:55:55 CST 2018 Thu Nov 22 02:22:22 CST 2018 Thu Nov 22 04:44:44 CST 2018