java - 测试 SimpleDateFormat 的多线程调用

Question

我想测试使用 SimpleDateFormat 作为静态类变量的修复。我正在得到

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，我遇到了异常

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
   }
}

我如何测试第二个是否是正确的修复？JUnits 有没有办法测试多线程？

Answer 1

SimpleDateFormat 不是线程安全的，如 javadoc 中所述。 :

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 externally.

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(就像您之后所做的那样)是一个好方法。本地实例不在线程之间共享，并且不会干扰多线程。

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

您还可以使用ThreadLocal如果您只想为每个线程创建 1 个实例，则使用变量:

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
}