Java - 静态方法的线程安全

Question

我的应用程序并行处理一堆文件，并且由于每个文件的名称中都有删除文件的日期，因此我使用以下方法来提取删除日期:

public static Optional<Date> getFileDropDate(String filename) {
        /* Example: my_file-1_20171002.xml */
        if (StringUtils.isEmpty(filename) || !filename.matches(Constants.FILE_DATE_PATTERN)) {
            return Optional.empty();
        }
        String[] segments = filename.split(UNDERSCORE);
        try {
            // Filename definitely ends with _yyyyMMdd.xml as it matches the pattern. Split by underscore, get the last segment and take out the .xml
            return Optional.of(yyyymmdd.parse(segments[segments.length - 1].replace(Constants.XML_EXTENSION, "")));
        } catch (ParseException e) {
            logger.error("Unable to parse filename to find load date");
            return Optional.empty();
        }
    }

虽然功能非常可靠并且我的测试全部成功，但当我在多线程设置中运行它时，它会因日期解析异常而失败，并且该方法尝试解析的字符串完全奇怪。

例如:

java.lang.NumberFormatException: For input string: "E.21616"
    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:2162)
    at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
    at java.text.DateFormat.parse(DateFormat.java:364)
    at mycompany.importer.parser.ParserUtil.getFileDropDate(ParserUtil.java:308)
    at mycompany.dataprocessors.FileProcessor.lambda$importData$2(FileProcessor.java:181)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:291)
    at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
 2017-10-21 22:11:48.550 [ForkJoinPool.commonPool-worker-2] ERROR a.c.t.m.m.a.m.i.d.FileProcessor - Error in parsing files and building datamap. Feed file is: /files/my_file_1-db_20171016.xml
java.lang.NumberFormatException: For input string: "E.21616E2"
    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:2162)
    at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
    at java.text.DateFormat.parse(DateFormat.java:364)
    at mycompany.importer.parser.ParserUtil.getFileDropDate(ParserUtil.java:308)
    at mycompany.dataprocessors.FileProcessor.lambda$importData$2(FileProcessor.java:181)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:291)
    at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
 2017-10-21 22:11:48.550 [ForkJoinPool.commonPool-worker-3] ERROR a.c.t.m.m.a.m.i.d.FileProcessor - Error in parsing files and building datamap. Feed file is: /files/my_file_2-db_20171020.xml
java.lang.NumberFormatException: For input string: "20172017E4"
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Long.parseLong(Long.java:589)
    at java.lang.Long.parseLong(Long.java:631)
    at java.text.DigitList.getLong(DigitList.java:195)
    at java.text.DecimalFormat.parse(DecimalFormat.java:2051)
    at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1867)
    at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
    at java.text.DateFormat.parse(DateFormat.java:364)
    at mycompany.importer.parser.ParserUtil.getFileDropDate(ParserUtil.java:308)
    at mycompany.dataprocessors.FileProcessor.lambda$importData$2(FileProcessor.java:181)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:291)
    at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

使方法同步解决了问题，但考虑到该方法不使用或修改任何实例变量(如果不明显，常量中的所有变量都是 public static final)，并且每个线程在技术上都有自己的局部变量堆栈，为什么这个方法不是线程安全的？

Answer 1

原因是这个yyyymmdd不是线程安全的。来自 documentation :

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.

不要在线程之间共享单个 SimpleDateFormat，因为它不是线程安全的。在方法中创建一个新的 SimpleDateFormat，或者使用 ThreadLocal，或者同步对 yyyymmdd 的所有访问，或者更好的是，使用线程安全的 java.time 类。