Java-靜態方法的線程安全

Question

我的應用程序並行處理一堆文件，並且由於每個文件都有刪除文件的日期作為其名稱的一部分，因此我使用以下方法提取刪除日期：

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();
        }
    }

雖然功能相當可靠並且我的測試全部成功，但是當我在多線程安裝程序中運行它時，它由於日期解析異常而失敗，並且該方法嘗試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)

使方法同步可以解決問題，但是鑒於此方法不使用或不修改任何實例變量（並且如果不明顯，則Constant中的所有變量都是public static final），並且每個線程從技術上講都有自己的局部堆棧變量，為什么此方法不是線程安全的？

Answer 1

原因是此yyyymmdd不是線程安全的。 從文檔中 ：

日期格式不同步。 建議為每個線程創建單獨的格式實例。 如果多個線程同時訪問一種格式，則必須在外部進行同步。

不要在線程之間共享單個SimpleDateFormat，因為它不是線程安全的。 在方法中創建一個新的SimpleDateFormat，或者使用ThreadLocal，或者java.time yyyymmdd所有訪問同步，或者更好的是，使用java.time類，它們是線程安全的。