测试多线程调用SimpleDateFormat

Question

我想测试一个修复,其中SimpleDateFormat用作静态类变量.我得到了

以前我的代码是

public class Abc{
   private static SimpleDateFormat dateformatter;

   public static String method1(final Calendar calendar) {
      String thePattern = "ddMMM";
      dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something

   }

   public static String method2(final Calendar calendar) {
      String thePattern = "ddMMyyyy";
      dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something

   }
}

对于这个,我得到了以下异常

java.lang.ArrayIndexOutOfBoundsException: 965
            at sun.util.calendar.BaseCalendar.getCalendarDateFromFixedDate(BaseCalendar.java:454)
            at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2333)
            at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2248)
            at java.util.Calendar.setTimeInMillis(Calendar.java:1140)
            at java.util.Calendar.setTime(Calendar.java:1106)
            at java.text.SimpleDateFormat.format(SimpleDateFormat.java:955)
            at java.text.SimpleDateFormat.format(SimpleDateFormat.java:948)
            at java.text.DateFormat.format(DateFormat.java:336)

现在我已将其更改为:

public class Abc{  
   public static String method1(final Calendar calendar) {
      String thePattern = "ddMMM";
      SimpleDateFormat dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something
   }

   public static String method2(final Calendar calendar) {
      String thePattern = "ddMMyyyy";
      SimpleDateFormat dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something
   }
}

如何测试第二个是否正确？JUnits有什么方法可以测试多线程吗？

Answer 1

SimpleDateFormat不是线程安全的,如javadoc中所述:

日期格式未同步.建议为每个线程创建单独的格式实例.如果多个线程同时访问格式,则必须在外部进行同步.

但是在这里你甚至搞乱了引用因为:

dateformatter = new SimpleDateFormat(thePattern, Locale.US);

由于dateformatter是静态的,因此它的内容在线程之间共享.

在本地创建一个新实例(就像你之后做的那样)是一个很好的方法.线程之间不共享本地实例,也不会混淆多线程.

只是不要忘记删除,private static SimpleDateFormat dateformatter;以避免混淆.

如果要为每个线程仅创建1个实例,还可以使用ThreadLocal变量:

private static final ThreadLocal<SimpleDateFormat> dateformatter1 =
    new ThreadLocal<SimpleDateFormat>() {
         @Override protected SimpleDateFormat initialValue() {
             return new SimpleDateFormat("ddMMM");
     }
 };

private static final ThreadLocal<SimpleDateFormat> dateformatter2 =
    new ThreadLocal<SimpleDateFormat>() {
        @Override protected SimpleDateFormat initialValue() {
            return new SimpleDateFormat("ddMMyyyy");
    }
};

public static String method1(final Calendar calendar) {
    SimpleDateFormat dateformatter = dateformatter1.get();
    sPars = dateformatter.format(calendar.getTime());
    //Something
}

public static String method2(final Calendar calendar) {
    SimpleDateFormat dateformatter = dateformatter2.get();
    sPars = dateformatter.format(calendar.getTime());
    //Something
}