我试图编写一个示例来显示 SimpleDateFormat 是线程不安全的。但它不起作用!谁能给我一个显示 SimpleDateFormat 线程不安全的例子?
public static void main(String[] args) throws ParseException, InterruptedException {
Date aDate = (new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2016-12-15 23:59:59"));
ExecutorService executor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors() * 1000);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
DataFormatter callable = new DataFormatter(sdf, aDate);
Collection<DataFormatter> callables = Collections.nCopies(1000, callable);
executor.invokeAll(callables);
executor.shutdown();
}
private static class DataFormatter implements Callable<String> {
private SimpleDateFormat sdf;
private Date aDate;
public DataFormatter(SimpleDateFormat sdf, Date aDate) {
this.sdf = sdf;
this.aDate = aDate;
}
@Override
public String call() throws Exception {
String format = sdf.format(aDate);
Assert.assertEquals("2016-12-15 23:59:59", format);
return format;
}
}
最佳答案
当然。您的代码的问题在于您试图一遍又一遍地格式化相同日期,因此共享字段永远不会持有不同的值。如果我们查看Can anyone give me an example of showing SimpleDateFormat is thread unsafe?
SimpleDateFormat
中的代码,我们会发现它扩展了 DateFormat
,后者具有共享的 Calendar calendar
字段。这就是类的重入问题。
// shared with everyone else calling the same SimpleDateFormat
protected Calendar calendar;
...
// method from DateFormat that is extended by SimpleDateFormat
private StringBuffer format(Date date, StringBuffer toAppendTo, FieldDelegate delegate) {
calendar.setTime(date);
...
顺便说一句,还要注意它使用 StringBuffer
这意味着它使用 synchronized
方法。令人沮丧的是,我们付出了同步性能损失的代价,但我们却无法使用 SimpleDateFormat
重新进入。
这是我对如何演示它的看法。我只是在随机日期运行日期格式两次并检查结果。只有 20 个并发线程时它会立即失败。
public class SimpleDateFormatEnosafe {
private static final SimpleDateFormat format =
new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public static void main(String[] args) {
ExecutorService executor = Executors.newCachedThreadPool();
DataFormatter formatter = new DataFormatter();
for (int i = 0; i < 20; i++) {
executor.submit(formatter);
}
executor.shutdown();
// NOTE: this could never finish if all but one thread fails in the pool
}
private static class DataFormatter implements Runnable {
@Override
public void run() {
ThreadLocalRandom random = ThreadLocalRandom.current();
while (true) {
Date date = new Date(random.nextLong());
String output1 = format.format(date);
String output2 = format.format(date);
if (!output1.equals(output2)) {
System.out.println(output1 + " != " + output2);
break;
}
}
}
}
}
关于java - 谁能给我一个显示 SimpleDateFormat 线程不安全的例子?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/41158005/