在使用SimpleDateFormat的經常會有一些錯誤的用法，例如如下方式：
public?class?TestDateFormat
{
?????private?SimpleDateFormat?sdf?=?new?SimpleDateFormat("yyyy-MM-dd");
?????public?void?method1(){
?????????sdf.format(new?Date());
?????}
?????public?void?method2(){
?????????sdf.format(new?Date());
?????}
?)為了漸少new 的次數而把SimpleDateFormat做成成員或者靜態成員，但這樣的做法是隱含著錯誤的，是不
?安全的。如下給出證明：
?import?java.text.SimpleDateFormat;
import?java.util.Date;
import?java.util.HashMap;
import?java.util.Map;
public?class?TestDateFormat{
????private?SimpleDateFormat?sdf?;
????public?static?void?main(String[]?args)?{
????????SimpleDateFormat?sdf=?new?SimpleDateFormat("yyyy-MM-dd");
????????Date?date1?=?new?Date();
????????Date?date2?=?new?Date(date1.getTime()+1000*60*60*24);
????????System.out.println(date1);
????????System.out.println(date2);
????????Thread?thread1?=?new?Thread(new?Thread1(sdf,date1));
????????thread1.start();
????????Thread?thread2?=?new?Thread(new?Thread2(sdf,date2));
????????thread2.start();
????}
????
}
class?Thread1?implements?Runnable{
????private?SimpleDateFormat?sdf;
????private?Date?date;
????public?Thread1(SimpleDateFormat?sdf,Date?date){
????????this.sdf?=?sdf;
????????this.date?=?date;
????}
????public?void?run()?{
????????for(;;){
????????????String?strDate?=?sdf.format(date);
????????????if("2007-02-27".equals(strDate)){
????????????????System.err.println("Error:?date1="+strDate);
????????????????System.exit(0);
????????????}
????????}
????}
}
class?Thread2?implements?Runnable{
????private?SimpleDateFormat?sdf;
????private?Date?date;
????public?Thread2(SimpleDateFormat?sdf,Date?date){
????????this.sdf?=?sdf;
????????this.date?=?date;
????}
????public?void?run()?{
????????for(;;){
????????????String?strDate?=?sdf.format(date);
????????????if("2007-02-26".equals(strDate)){
????????????????System.err.println("Error:?date1="+strDate);
????????????????System.exit(0);
????????????}
????????}
????}
}很快，基本幾十次就會出現錯誤。錯誤原因：
?因為SimpleDateFormat處理復雜，Jdk的實現中使用了成員變量來傳遞參數，這個就造成在多線程的時候
?會出現錯誤。上邊的程序證明了這個問題。
?
?再看看SimpleDateFormat的Jdk 的Source，有這么一段注釋，說明它不是線程安全的。
?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
?
?繼續看看Sun自己的網站上。在sun的bug database中
?Bug ID:? 4228335? 講的就是這個問題。
?SimpleDateFormat is not threadsafe (one more try)
?其中有這么幾段話值得關注：
?1，
?Aside from the obvious, there are two reasons why this fix should be made:
- The documentation for DateFormat states that a DateFormat object should be used
multiple times, implying that construction is expensive.? Furthermore,
?no mention is made of SimpleDateFormat's lack of thread-safety.?
?Since for most applications the date formats remain constant,
?it is very easy to conclude that DateFormats should be application globals.
?But SimpleDateFormat produces incorrect results when used in this way.
- Bug 4101500, a similar problem with NumberFormat, was fixed.
建議修改這個問題，而且NumberFormat已經修改，解決了這個問題。簡單測試了一下NumberFormat確是不出錯
2,
Use of a cloned Calendar object in format(), as suggested in JDC comments,
slows down the execution 30-50%. And it potentially affects footprint because
of cloned objects. Another approach would be to have a Calendar instance per
thread. However, this approach will cause problems at the API semantic level due
to the get/setCalendar methods.
這一段解釋了為什么沒修改這個問題，一個是保持API不變，另一個是因為Clone比new慢，會損失效率
關于clone與new的快慢，看這里http://www.aygfsteel.com/dreamstone/archive/2007/02/26/100761.html。
其它的還有好多，感興趣的自己看看吧。
結論：每次使用它的時候現new，或者用一個同步函數把new好的包裝起來使用吧。