Why is Java s SimpleDateFormat not thread-safe

Question

Please  tell with a code example why is SimpleDateFormat not threadsafe  What is the problem in this class  Is The problem with format function of SimpleDateFormat   Please give a code which demonstrates this fault in class   FastDateFormat is threadsafe  Why  what is the difference b w the SimpleDateFormat and FastDateFormat   Please explain with a code which demonstrates this issue

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SimpleDateFormat stores intermediate results in instance fields  So if one instance is used by two threads they can mess each other s results   Looking at the source code reveals that there is a Calendar instance field  which is used by operations on DateFormat   SimpleDateFormat   For example parse     calls calendar clear   initially and then calendar add      If another thread invokes parse     before the completion of the first invocation  it will clear the calendar  but the other invocation will expect it to be populated with intermediate results of the calculation   One way to reuse date formats without trading thread-safety is to put them in a ThreadLocal - some libraries do that  That s if you need to use the same format multiple times within one thread  But in case you are using a servlet container  that has a thread pool   remember to clean the thread-local after you finish   To be honest  I don t understand why they need the instance field  but that s the way it is  You can also use joda-time DateTimeFormat which is threadsafe

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Release 3 2 of commons-lang will have FastDateParser class that is a thread-safe substitute of SimpleDateFormat for Gregorian calendar  See LANG-909 for more information

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Here   s an example defines a SimpleDateFormat object as a static field  When two or more threads access    someMethod    concurrently with different dates  they can mess with each other   s results       public class SimpleDateFormatExample            private static final SimpleDateFormat simpleFormat   new SimpleDateFormat  yyyy-MM-dd HH mm ss SSS              public String someMethod Date date                return simpleFormat format date                      You can create a service like below and use jmeter to simulate concurrent users using the same SimpleDateFormat object formatting different dates and their results will be messed up   public class FormattedTimeHandler extends AbstractHandler    private static final String OUTPUT TIME FORMAT    yyyy-MM-dd HH mm ss SSS   private static final String INPUT TIME FORMAT    yyyy-MM-ddHH mm ss   private static final SimpleDateFormat simpleFormat   new SimpleDateFormat OUTPUT TIME FORMAT      apache commons lang3 FastDateFormat is threadsafe private static final FastDateFormat fastFormat   FastDateFormat getInstance OUTPUT TIME FORMAT    public void handle String target  Request baseRequest  HttpServletRequest request  HttpServletResponse response          throws IOException  ServletException        response setContentType  text html charset utf-8        response setStatus HttpServletResponse SC OK       baseRequest setHandled true        final String inputTime   request getParameter  time        Date date   LocalDateTime parse inputTime  DateTimeFormat forPattern INPUT TIME FORMAT   toDate         final String method   request getParameter  method        if   SimpleDateFormat  equalsIgnoreCase method                use SimpleDateFormat as a static constant field  not thread safe         response getWriter   println simpleFormat format date          else if   FastDateFormat  equalsIgnoreCase method                use apache commons lang3 FastDateFormat  thread safe         response getWriter   println fastFormat format date          else              create new SimpleDateFormat instance when formatting date  thread safe         response getWriter   println new SimpleDateFormat OUTPUT TIME FORMAT  format date             public static void main String   args  throws Exception          embedded jetty configuration  running on port 8090  change it as needed      Server server   new Server 8090       server setHandler new FormattedTimeHandler          server start        server join           The code and jmeter script can be downloaded here

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If you want to use the same date format among multiple threads  declare it as a static and synchronize on the instance variable when using it     static private SimpleDateFormat sdf   new SimpleDateFormat           synchronized sdf          use the instance here to format a date        The above makes it thread safe

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SimpleDateFormat is a concrete class for formatting and parsing dates in a locale-sensitive manner      From the JavaDoc        But Date formats are not synchronized  It is recommended to create   separate format instances for each thread  If multiple threads access   a format concurrently  it must be synchronized externally      To make the SimpleDateFormat class thread-safe  look at the following approaches     Create a new SimpleDateFormat instance each time you need to use one  Although this is thread safe  it is the slowest possible approach    Use synchronization  This is a bad idea because you should never choke-point your threads on a server    Use a ThreadLocal  This is the fastest approach of the 3  see http   www javacodegeeks com 2010 07 java-best-practices-dateformat-in html

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DateTimeFormatter in Java 8 is immutable and thread-safe alternative to SimpleDateFormat

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Here is a code example that proves the fault in the class  I ve checked  the problem occurs when using parse and also when you are only using format

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ThreadLocal   SimpleDateFormat    SimpleDateFormatThreadSafe   package com foocoders text   import java text AttributedCharacterIterator  import java text DateFormatSymbols  import java text FieldPosition  import java text NumberFormat  import java text ParseException  import java text ParsePosition  import java text SimpleDateFormat  import java util Calendar  import java util Date  import java util Locale  import java util TimeZone   public class SimpleDateFormatThreadSafe extends SimpleDateFormat        private static final long serialVersionUID   5448371898056188202L      ThreadLocal lt SimpleDateFormat gt  localSimpleDateFormat       public SimpleDateFormatThreadSafe             super            localSimpleDateFormat   new ThreadLocal lt SimpleDateFormat gt                  protected SimpleDateFormat initialValue                     return new SimpleDateFormat                                        public SimpleDateFormatThreadSafe final String pattern            super pattern           localSimpleDateFormat   new ThreadLocal lt SimpleDateFormat gt                  protected SimpleDateFormat initialValue                     return new SimpleDateFormat pattern                                       public SimpleDateFormatThreadSafe final String pattern  final DateFormatSymbols formatSymbols            super pattern  formatSymbols           localSimpleDateFormat   new ThreadLocal lt SimpleDateFormat gt                  protected SimpleDateFormat initialValue                     return new SimpleDateFormat pattern  formatSymbols                                       public SimpleDateFormatThreadSafe final String pattern  final Locale locale            super pattern  locale           localSimpleDateFormat   new ThreadLocal lt SimpleDateFormat gt                  protected SimpleDateFormat initialValue                     return new SimpleDateFormat pattern  locale                                       public Object parseObject String source  throws ParseException           return localSimpleDateFormat get   parseObject source              public String toString             return localSimpleDateFormat get   toString               public Date parse String source  throws ParseException           return localSimpleDateFormat get   parse source              public Object parseObject String source  ParsePosition pos            return localSimpleDateFormat get   parseObject source  pos              public void setCalendar Calendar newCalendar            localSimpleDateFormat get   setCalendar newCalendar              public Calendar getCalendar             return localSimpleDateFormat get   getCalendar               public void setNumberFormat NumberFormat newNumberFormat            localSimpleDateFormat get   setNumberFormat newNumberFormat              public NumberFormat getNumberFormat             return localSimpleDateFormat get   getNumberFormat               public void setTimeZone TimeZone zone            localSimpleDateFormat get   setTimeZone zone              public TimeZone getTimeZone             return localSimpleDateFormat get   getTimeZone               public void setLenient boolean lenient            localSimpleDateFormat get   setLenient lenient              public boolean isLenient             return localSimpleDateFormat get   isLenient               public void set2DigitYearStart Date startDate            localSimpleDateFormat get   set2DigitYearStart startDate              public Date get2DigitYearStart             return localSimpleDateFormat get   get2DigitYearStart               public StringBuffer format Date date  StringBuffer toAppendTo  FieldPosition pos            return localSimpleDateFormat get   format date  toAppendTo  pos              public AttributedCharacterIterator formatToCharacterIterator Object obj            return localSimpleDateFormat get   formatToCharacterIterator obj              public Date parse String text  ParsePosition pos            return localSimpleDateFormat get   parse text  pos              public String toPattern             return localSimpleDateFormat get   toPattern               public String toLocalizedPattern             return localSimpleDateFormat get   toLocalizedPattern               public void applyPattern String pattern            localSimpleDateFormat get   applyPattern pattern              public void applyLocalizedPattern String pattern            localSimpleDateFormat get   applyLocalizedPattern pattern              public DateFormatSymbols getDateFormatSymbols             return localSimpleDateFormat get   getDateFormatSymbols               public void setDateFormatSymbols DateFormatSymbols newFormatSymbols            localSimpleDateFormat get   setDateFormatSymbols newFormatSymbols              public Object clone             return localSimpleDateFormat get   clone               public int hashCode             return localSimpleDateFormat get   hashCode               public boolean equals Object obj            return localSimpleDateFormat get   equals obj              https   gist github com pablomoretti 9748230

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Here is the example which results in a strange error  Even Google gives no results   public class ExampleClass    private static final Pattern dateCreateP   Pattern compile                s         private static final SimpleDateFormat sdf   new SimpleDateFormat  HH mm ss dd MM yyyy     public static void main String   args        ExecutorService executor   Executors newFixedThreadPool 100       while  true            executor submit new Runnable                  Override             public void run                     workConcurrently                                       public static void workConcurrently         Matcher matcher   dateCreateP matcher               19 30 55 03 05 2015        Timestamp startAdvDate   null      try           if  matcher find                  String dateCreate   matcher group 1               startAdvDate   new Timestamp sdf parse dateCreate  getTime                     catch  Throwable th            th printStackTrace              System out print  OK           And result    OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK java lang NumberFormatException  For input string    201519E 2015192E2  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 1869  at java text SimpleDateFormat parse SimpleDateFormat java 1514  at java text DateFormat parse DateFormat java 364  at com nonscalper webscraper processor av ExampleClass workConcurrently ExampleClass java 37  at com nonscalper webscraper processor av ExampleClass 1 run ExampleClass java 25  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 1142  at java util concurrent ThreadPoolExecutor Worker run ThreadPoolExecutor java 617  at java lang Thread run Thread java 745

[java] Why is Java's SimpleDateFormat not thread-safe?

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