java - 证明 SimpleDateFormat 不是线程安全的

Question

我想通过一个简单的 JUnit 测试向同事证明 SimpleDateFormat 不是线程安全的。下面的类没有表达我的观点(在多线程环境中重用 SimpleDateFormat)，我不明白为什么。你能发现是什么阻止了我使用 SDF 时抛出运行时异常吗？

public class SimpleDateFormatThreadTest
{
    @Test
    public void test_SimpleDateFormat_MultiThreaded() throws ParseException{
        Date aDate = (new SimpleDateFormat("dd/MM/yyyy").parse("31/12/1999"));
        DataFormatter callable = new DataFormatter(aDate);

        ExecutorService executor = Executors.newFixedThreadPool(1000);
        Collection<DataFormatter> callables = Collections.nCopies(1000, callable);

        try{
            List<Future<String>> futures = executor.invokeAll(callables);
            for (Future f : futures){
                try{
                    assertEquals("31/12/1999", (String) f.get());
                }
                catch (ExecutionException e){
                    e.printStackTrace();
                }
            }
        }
        catch (InterruptedException e){
            e.printStackTrace();
        }
    }
}

class DataFormatter implements Callable<String>{
    static SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");

    Date date;

    DataFormatter(Date date){
        this.date = date;
    }

    @Override
    public String call() throws RuntimeException{
        try{
            return sdf.format(date);
        }
        catch (RuntimeException e){
            e.printStackTrace();
            return "EXCEPTION";
        }
    }
}

Answer 1

缺乏线程安全并不一定意味着代码会抛出异常。 Andy Grove 的文章 SimpleDateFormat and Thread Safety 对此进行了解释，不再在线提供。在其中，他展示了 SimpleDateFormat 缺乏线程安全性，方法是展示给定不同的输入，输出并不总是正确的。

When I run this code, I get the following output:

    java.lang.RuntimeException: date conversion failed after 3 iterations.
    Expected 14-Feb-2001 but got 01-Dec-2007

Note that "01-Dec-2007" isn't even one of the strings in the test data. It is actually a combination of the dates being processed by the other two threads!

虽然原始文章不再在线提供，但以下代码说明了这个问题。它是根据似乎是基于 Andy Grove 最初文章的文章创建的。

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.List;
import java.util.Locale;

public class SimpleDateFormatThreadSafety {
    private final SimpleDateFormat dateFormat = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);

    public static void main(String[] args) {
        new SimpleDateFormatThreadSafety().dateTest(List.of("01-Jan-1999", "14-Feb-2001", "31-Dec-2007"));
    }

    public void dateTest(List<String> testData) {
        testData.stream()
                .map(d -> new Thread(() -> repeatedlyParseAndFormat(d)))
                .forEach(Thread::start);
    }

    private void repeatedlyParseAndFormat(String value) {
        for (int i = 0; i < 1000; i++) {
            Date d = tryParse(value);
            String formatted = dateFormat.format(d);
            if (!value.equals(formatted)) {
                throw new RuntimeException("date conversion failed after " + i
                        + " iterations. Expected " + value + " but got " + formatted);
            }
        }
    }

    private Date tryParse(String value) {
        try {
            return dateFormat.parse(value);
        } catch (ParseException e) {
            throw new RuntimeException("parse failed");
        }
    }
}

有时此转换会因返回错误的日期而失败，有时会因 NumberFormatException 而失败:

java.lang.NumberFormatException: For input string: ".E2.31E2"