[size=large]Bug: Call to method of static java.text.DateFormat
Pattern id: STCAL_INVOKE_ON_STATIC_DATE_FORMAT_INSTANCE, type: STCAL, category: MT_CORRECTNESS
As the JavaDoc states, DateFormats are inherently unsafe for multithreaded use. The detector has found a call to an instance of DateFormat that has been obtained via a static field. This looks suspicous.
For more information on this see Sun Bug #6231579 and Sun Bug #6178997.
上面的英文解释其实应该说得比较清楚，在Java文档中，已经明确说明了DateFormats 是非线程安全的，而在SimpleDateFormat的Jdk 的Source文件中，我们也找到这么一段注释，说明它不是线程安全的。
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
在Sun自己的网站上。在sun的bug database中，Sun Bug #6231579 ，Sun Bug #6178997都可以印证这个问题。
导致SimpleDateFormat出现多线程安全问题的原因，是因为：SimpleDateFormat处理复杂，Jdk的实现中使用了成员变量来传递参数，这就造成在多线程的时候会出现错误。
而Findbugs所说的“Call to static DateFormat”，其实就是一些人：为了渐少new 的次数而把SimpleDateFormat做成成员或者静态成员，上面已经说了，这样做是不安全的。
其实，出现这种问题的代码一般都长得差不多，典型的代码示例如下：
===================================================================================
package test;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
public class TestDateFormat
{
public static void main(String[] args)
{
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Date today = new Date();
Date tomorrow = new Date(today.getTime() + 1000 * 60 * 60 * 24);
System.out.println("当前日期为：" + dateFormat.format(today));
System.out.println("明天日期为：" + dateFormat.format(tomorrow));
new Thread(new TestThreadToday(dateFormat, today)).start();
new Thread(new TestThreadTomorrow(dateFormat, tomorrow)).start();
}
}
class TestThreadToday implements Runnable
{
private DateFormat dateFormat;
private Date currentDate;
public TestThreadToday(DateFormat format, Date date)
{
this.dateFormat = format;
this.currentDate = date;
}
public void run()
{
// TODO Auto-generated method stub
final String currentDateStr = dateFormat.format(currentDate);
while(true)
{
try
{
Thread.sleep(100);
} catch (InterruptedException e)
{
}
String tempDateStr = dateFormat.format(currentDate);
System.out.println(Thread.currentThread().getName() + " : 当前日期 : " + tempDateStr);
if (false == tempDateStr.equals(currentDateStr))
{
System.out.println("日期格式化出现异常！！当前日期为：" + tempDateStr);
System.exit(0);
}
}
}
}
class TestThreadTomorrow implements Runnable
{
private DateFormat dateFormat;
private Date tomorrow;
public TestThreadTomorrow(DateFormat format, Date date)
{
this.dateFormat = format;
this.tomorrow = date;
}
public void run()
{
// TODO Auto-generated method stub
final String tomorrowDateStr = dateFormat.format(tomorrow);
while(true)
{
try
{
Thread.sleep(100);
} catch (InterruptedException e)
{
}
String tempDateStr = dateFormat.format(tomorrow);
System.out.println(Thread.currentThread().getName() + " : 明天日期 : " + tempDateStr);
if (false == tempDateStr.equals(tomorrowDateStr))
{
System.out.println("日期格式化出现异常！！明天日期为：" + tempDateStr);
System.exit(0);
}
}
}
}
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