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Simpledateformat thread safety issues
2022-06-23 07:20:00 【51CTO】
Why are threads unsafe ？
It mainly contains two blocks parse and format unsafe .
You can see , Multiple threads share variables calendar, And modify it calendar. So in a multithreaded environment , When multiple threads use the same SimpleDateFormat object （ Such as static modification ） Words , Such as calling format When the method is used , Multiple threads call at the same time calender.setTime Method , Lead to time Modified by another thread , So threads are not safe .
SimpleDateFormat Of format Method thread insecurity
this.format It uses SimpleDateFormat Of format Method
private StringBuffer format(Date arg0, StringBuffer arg1, FieldDelegate arg2) {
this.calendar.setTime(arg0);
boolean arg3 = this.useDateFormatSymbols();
int arg4 = 0;
while (arg4
<
this.compiledPattern.length)
{
int
arg5 =
this.compiledPattern[arg4]
>>> 8;
int arg6 = this.compiledPattern[arg4++]
& 255;
if (arg6 == 255) {
arg6 = this.compiledPattern[arg4++]
<
< 16;
arg6 |= this.compiledPattern[arg4++];
}
switch (arg5) {
case 100 :
arg1.append((char) arg6);
break;
case 101 :
arg1.append(this.compiledPattern, arg4, arg6);
arg4 += arg6;
break;
default :
this.subFormat(arg5, arg6, arg2, arg1, arg3);
}
}
return arg1;
}
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Different threads
It will still lead to thread insecurity .
SimpleDateFormat It's inheritance DateFormat class ,DateFormat Maintain one in the class Calendar object
SimpleDateFormat Inherit DateFormat , The use of calendar Parent class DateFormat Medium
public class SimpleDateFormat extends DateFormat {}
DateFormat Of calendar Used for date - Time calculation , It's also used for format Methods are also used for parse Method
public abstract class DateFormat extends Format {
protected Calendar calendar;
}
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Parse Resulting in thread safety problems
SimpleDateFormat Of parse Method
public Date parse(String arg0, ParsePosition arg1) {
this.checkNegativeNumberExpression();
int arg2 = arg1.index;
int arg3 = arg2;
int arg4 = arg0.length();
boolean[] arg5 = new boolean[]{false};
CalendarBuilder arg6 = new CalendarBuilder();
int arg7 = 0;
label82 : while (arg7
<
this.compiledPattern.length)
{
int
arg8 =
this.compiledPattern[arg7]
>>> 8;
int arg9 = this.compiledPattern[arg7++]
& 255;
if (arg9 == 255) {
arg9 = this.compiledPattern[arg7++]
<
< 16;
arg9 |= this.compiledPattern[arg7++];
}
switch (arg8) {
case 100 :
if (arg2
<
arg4
&&
arg0.charAt(arg2) =
=
(char)
arg9)
{
++arg2;
break;
}
arg1.index =
arg3;
arg1.errorIndex =
arg2;
return
null;
case
101
:
while
(true)
{
if
(arg9--
<= 0) {
continue label82;
}
if (arg2 >= arg4
|| arg0.charAt(arg2) != this.compiledPattern[arg7++]) {
arg1.index = arg3;
arg1.errorIndex = arg2;
return null;
}
++arg2;
}
default :
boolean arg10 = false;
boolean arg11 = false;
if (arg7
<
this.compiledPattern.length)
{
int
arg12 =
this.compiledPattern[arg7]
>>> 8;
if (arg12 != 100
&
& arg12 != 101) {
arg10 = true;
}
if (this.hasFollowingMinusSign
&
& (arg12 == 100 || arg12 == 101)) {
int arg13;
if (arg12 == 100) {
arg13 = this.compiledPattern[arg7]
& 255;
} else {
arg13 = this.compiledPattern[arg7 + 1];
}
if (arg13 == this.minusSign) {
arg11 = true;
}
}
}
arg2 = this.subParse(arg0, arg2, arg8, arg9, arg10, arg5,
arg1, arg11, arg6);
if (arg2
<
0)
{
arg1.index =
arg3;
return
null;
}
}
}
arg1.index =
arg2;
try
{
Date
arg15 =
arg6.establish(this.calendar).getTime();
if
(arg5[0]
&&
arg15.before(this.defaultCenturyStart))
{
arg15 =
arg6.addYear(100).establish(this.calendar).getTime();
}
return
arg15;
}
catch
(IllegalArgumentException
arg14)
{
arg1.errorIndex =
arg2;
arg1.index =
arg3;
return
null;
}
}
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Key points
Date arg15 = arg6.establish(this.calendar).getTime();
The inside Of establish Method .establish yes CalendarBuilder Methods
Calendar establish(Calendar arg0) {
boolean arg1 = this.isSet(17)
&
& this.field[17] > this.field[1];
if (arg1
&
& !arg0.isWeekDateSupported()) {
if (!this.isSet(1)) {
this.set(1, this.field[35]);
}
arg1 = false;
}
arg0.clear();
int arg2;
int arg3;
for (arg2 = 2; arg2
<
this.nextStamp;
++arg2)
{
for
(arg3 =
0;
arg3
<= this.maxFieldIndex; ++arg3) {
if (this.field[arg3] == arg2) {
arg0.set(arg3, this.field[18 + arg3]);
break;
}
}
}
if (arg1) {
arg2 = this.isSet(3) ? this.field[21] : 1;
arg3 = this.isSet(7) ? this.field[25] : arg0.getFirstDayOfWeek();
if (!isValidDayOfWeek(arg3)
&
& arg0.isLenient()) {
if (arg3 >= 8) {
--arg3;
arg2 += arg3 / 7;
arg3 = arg3 % 7 + 1;
} else {
while (arg3
<
=
0)
{
arg3
+=
7;
--arg2;
}
}
arg3 =
toCalendarDayOfWeek(arg3);
}
arg0.setWeekDate(this.field[35],
arg2,
arg3);
}
return
arg0;
}
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It mainly depends on
This meeting will calendar Remove , And no new value is set
You know SimpleDateFormat Maintained for format and parse Method to calculate the date - The time of the calendar It's emptied , If the thread at this time A take calendar Empty and no time to set the new value , Threads B Also enter parse The method uses SimpleDateFormat Object calendar object , At this point, thread safety problems will arise ！
Solution ：
1、 take SimpleDateFormat Defined as a local variable
2、 Add a thread synchronization lock ：synchronized(lock)
3、 Use ThreadLocal, Each thread has its own SimpleDateFormat Object copy
Solution chestnut ：threadLocal
class ThreadLocalSimpleFormatDateUtil {
private static final String date_format = "yyyy-MM-dd HH:mm:ss";
private static ThreadLocal
<
DateFormat
> threadLocal = new ThreadLocal
<
DateFormat
>();
public static DateFormat getDateFormat() {
DateFormat df = threadLocal.get();
if (df == null) {
df = new SimpleDateFormat(date_format);
threadLocal.set(df);
}
return df;
}
public static String formatDate(Date date) throws ParseException {
return getDateFormat().format(date);
}
public static Date parse(String strDate) throws ParseException {
return getDateFormat().parse(strDate);
}
}
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Official account ： The community of programmers and developers
Test code
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
class MySimpleDateFormatThread extends Thread {
private SimpleDateFormat sdf;
private String dateString;
public MySimpleDateFormatThread(SimpleDateFormat sdf, String dateString) {
this.sdf = sdf;
this.dateString = dateString;
}
@Override
public void run() {
try {
Date date = sdf.parse(dateString);
String dateStr = sdf.format(date);
if(!dateStr.equals(dateString)) {
System.out.println("ThreadName=" + this.getName() + " Wrong report , Date string ：" + dateString + ", Date string converted to ：" + dateStr);
} else {
System.out.println("ThreadName=" + this.getName() + " success , Date string ：" + dateString);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
public class TestSimpleDateFormat {
public static void main(String[] args) {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
String[] dateString = {"2017-11-05","2017-11-06","2017-11-07","2017-11-08","2017-11-09","2017-11-10","2017-11-11","2017-11-12","2017-11-13","2017-11-14"};
Thread[] threads = new Thread[10];
for (int i = 0; i
<
threads.length;
i++)
{
threads[i] =
new
MySimpleDateFormatThread(sdf,
dateString[i]);
}
for
(int
i =
0;
i
< threads.length; i++) {
threads[i].start();
}
}
}
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There will be an error as follows ：
ThreadName=Thread-3 Wrong report , Date string ：2017-11-08, Date string converted to ：2016-12-08
ThreadName=Thread-2 Wrong report , Date string ：2017-11-07, Date string converted to ：2016-12-08
ThreadName=Thread-7 Wrong report , Date string ：2017-11-12, Date string converted to ：2016-12-08
ThreadName=Thread-4 Wrong report , Date string ：2017-11-09, Date string converted to ：2200-11-09
ThreadName=Thread-8 success , Date string ：2017-11-13java.lang.NumberFormatException: multiple points
ThreadName=Thread-9 success , Date string ：2017-11-14
at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
at sun.misc.FloatingDecimal.parseDouble(Unknown Source)
at java.lang.Double.parseDouble(Unknown Source)
at java.text.DigitList.getDouble(Unknown Source)
at java.text.DecimalFormat.parse(Unknown Source)
at java.text.SimpleDateFormat.subParse(Unknown Source)
at java.text.SimpleDateFormat.parse(Unknown Source)
at java.text.DateFormat.parse(Unknown Source)
at JavaThread.MySimpleDateFormatThread.run(TestSimpleDateFormat.java:34)
java.lang.NumberFormatException: multiple points
at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
at sun.misc.FloatingDecimal.parseDouble(Unknown Source)
at java.lang.Double.parseDouble(Unknown Source)
at java.text.DigitList.getDouble(Unknown Source)
at java.text.DecimalFormat.parse(Unknown Source)
at java.text.SimpleDateFormat.subParse(Unknown Source)
at java.text.SimpleDateFormat.parse(Unknown Source)
at java.text.DateFormat.parse(Unknown Source)
at JavaThread.MySimpleDateFormatThread.run(TestSimpleDateFormat.java:34)
java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(Unknown Source)
at java.lang.Long.parseLong(Unknown Source)
at java.lang.Long.parseLong(Unknown Source)
at java.text.DigitList.getLong(Unknown Source)
at java.text.DecimalFormat.parse(Unknown Source)
at java.text.SimpleDateFormat.subParse(Unknown Source)
at java.text.SimpleDateFormat.parse(Unknown Source)
at java.text.DateFormat.parse(Unknown Source)
at JavaThread.MySimpleDateFormatThread.run(TestSimpleDateFormat.java:34)
java.lang.ArrayIndexOutOfBoundsException: -1
at java.text.DigitList.fitsIntoLong(Unknown Source)
at java.text.DecimalFormat.parse(Unknown Source)
at java.text.SimpleDateFormat.subParse(Unknown Source)
at java.text.SimpleDateFormat.parse(Unknown Source)
at java.text.DateFormat.parse(Unknown Source)
at JavaThread.MySimpleDateFormatThread.run(TestSimpleDateFormat.java:34)
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