转自：http://blog.csdn.net/jeamking/article/details/7183958
有时我们在同一个类中都是使用同一种日期格式，又或者为了减少new SimpleDateFormat次数，自然而然的就会出现如下代码：
private static SimpleDateFormat sdf = newSimpleDateFormat("yyyy-MM-dd HH:mm:ss");
但是这样做在多线程并发下会存在安全隐患。SimpleDateFormat 类并不是线程同步的，JDK中我们可以看到如下描述：
* Date formats are not synchronized.
* Itis recommended to create separate format instances for each thread.
* Ifmultiple threads access a format concurrently, it must be synchronized
*externally.
下面通过一个简单程序来进行测试：
public class DateUtil {
privatestatic SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-ddHH:mm:ss");
publicDate parse(String str) throws ParseException{
returnsdf.parse(str);
}
}
public class Test {
/**
* @param args
* @throws InterruptedException
*/
publicstatic void main(String[] args) throws InterruptedException {
SdfRunnablett = new SdfRunnable();
for(inti=0;i<10;i++){
newThread(tt).start();
}
}
}
输出结果：
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Exception in thread "Thread-1"java.lang.NumberFormatException: multiple points
atsun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
atjava.lang.Double.parseDouble(Unknown Source)
atjava.text.DigitList.getDouble(Unknown Source)
atjava.text.DecimalFormat.parse(Unknown Source)
atjava.text.SimpleDateFormat.subParse(Unknown Source)
atjava.text.SimpleDateFormat.parse(Unknown Source)
atjava.text.DateFormat.parse(Unknown Source)
atDateUtil.parse(DateUtil.java:8)
atSdfRunnable.run(SdfRunnable.java:8)
atjava.lang.Thread.run(Unknown Source)
多运行几次，并不每次都输出这个错误信息，还可能输出如下错误信息：
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Wed Sep 23 12:23:35 CST 2201
Fri Dec 23 12:23:35 CST 2011
Exception in thread "Thread-6"java.lang.NumberFormatException: empty String
atsun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
atjava.lang.Double.parseDouble(Unknown Source)
atjava.text.DigitList.getDouble(Unknown Source)
atjava.text.DecimalFormat.parse(Unknown Source)
atjava.text.SimpleDateFormat.subParse(Unknown Source)
atjava.text.SimpleDateFormat.parse(Unknown Source)
atjava.text.DateFormat.parse(Unknown Source)
atDateUtil.parse(DateUtil.java:8)
atSdfRunnable.run(SdfRunnable.java:8)
atjava.lang.Thread.run(Unknown Source)
Exception in thread "Thread-1"java.lang.NumberFormatException: For input string: ""
atjava.lang.NumberFormatException.forInputString(Unknown Source)
atjava.lang.Long.parseLong(Unknown Source)
atjava.lang.Long.parseLong(Unknown Source)
atjava.text.DigitList.getLong(Unknown Source)
atjava.text.DecimalFormat.parse(Unknown Source)
atjava.text.SimpleDateFormat.subParse(Unknown Source)
atjava.text.SimpleDateFormat.parse(Unknown Source)Fri Dec 23 12:23:35 CST 2011
atjava.text.DateFormat.parse(Unknown Source)
atDateUtil.parse(DateUtil.java:8)
atSdfRunnable.run(SdfRunnable.java:8)
atjava.lang.Thread.run(Unknown Source)
在以上示例中我们把SimpleDateFormat定义为静态成员的。接下来我们把SimpleDateFormat定义为类普通成员。
public class DateUtil {
privateSimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
publicDate parse(String str) throws ParseException{
returnsdf.parse(str);
}
}
再次运行程序，结果如下：
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
Fri Dec 23 12:23:35 CST 2011
程序运行正常。并没有出现错误信息。那么，是不是意味着SimpleDateFormat作为普通成员就没有安全隐患呢，请看如下示例：
public class SdfRunnable implementsRunnable {
privateSimpleDateFormat sdf;
privateString dateStr;
publicSdfRunnable(SimpleDateFormat sdf,String dateStr){
this.sdf= sdf;
this.dateStr= dateStr;
}
publicvoid run() {
try{
System.out.println(this.sdf.parseObject(this.dateStr));
}catch (ParseException e) {
e.printStackTrace();
}
}
}
public class Test {
/**
* @param args
* @throws InterruptedException
*/
publicstatic void main(String[] args) throws InterruptedException {
SimpleDateFormatsdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
SdfRunnablett = new SdfRunnable(sdf,"2011-12-34 12:23:11");
for(inti=0;i<10;i++){
newThread(tt).start();
}
}
}
运行输出结果如下：
Tue Jan 03 12:23:11 CST 2012
Tue Jan 03 12:23:11 CST 2012
Tue Jan 03 12:23:11 CST 2012
Tue Jan 03 12:23:11 CST 2012
Tue Jan 03 12:23:11 CST 2012
Thu Jan 01 00:00:11 CST 1970
Sun Jan 03 12:23:11 CST 1971
Tue Jan 03 12:23:11 CST 2012
Exception in thread "Thread-9"java.lang.NumberFormatException: multiple points
atsun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
atjava.lang.Double.parseDouble(Unknown Source)
atjava.text.DigitList.getDouble(Unknown Source)
atjava.text.DecimalFormat.parse(Unknown Source)
atjava.text.SimpleDateFormat.subParse(Unknown Source)
atjava.text.SimpleDateFormat.parse(Unknown Source)
atjava.text.DateFormat.parseObject(Unknown Source)
atjava.text.Format.parseObject(Unknown Source)
atSdfRunnable.run(SdfRunnable.java:14)
atjava.lang.Thread.run(Unknown Source)
由此可见，不管是SimpleDateFormat作为静态成员还是成员，在多线程并发下都会存在安全隐患。所以应该在需要使用的地方再new SimpleDateFormat()。
关于SimpleDateFormat非多线程安全问题，在sun的bug database（http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4228335）中有更多的描述。