Why is Java's SimpleDateFormat not thread-safe? [duplicate]

Answer 1

Here is the example which results in a strange error. Even Google gives no results:

public class ExampleClass {

private static final Pattern dateCreateP = Pattern.compile("Дата подачи:\\s*(.+)");
private static final SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss dd.MM.yyyy");

public static void main(String[] args) {
    ExecutorService executor = Executors.newFixedThreadPool(100);
    while (true) {
        executor.submit(new Runnable() {
            @Override
            public void run() {
                workConcurrently();
            }
        });
    }
}

public static void workConcurrently() {
    Matcher matcher = dateCreateP.matcher("Дата подачи: 19:30:55 03.05.2015");
    Timestamp startAdvDate = null;
    try {
        if (matcher.find()) {
            String dateCreate = matcher.group(1);
            startAdvDate = new Timestamp(sdf.parse(dateCreate).getTime());
        }
    } catch (Throwable th) {
        th.printStackTrace();
    }
    System.out.print("OK ");
}
}

And result :

OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK OK java.lang.NumberFormatException: For input string: ".201519E.2015192E2"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043)
at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
at java.lang.Double.parseDouble(Double.java:538)
at java.text.DigitList.getDouble(DigitList.java:169)
at java.text.DecimalFormat.parse(DecimalFormat.java:2056)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.nonscalper.webscraper.processor.av.ExampleClass.workConcurrently(ExampleClass.java:37)
at com.nonscalper.webscraper.processor.av.ExampleClass$1.run(ExampleClass.java:25)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)