SimpleDateFormat做成员或者静态成员多线程安全隐患

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。