java - 未抛出 FileAlreadyExistsException 并覆盖文件

Question

这段代码:

File tmpFile = File.createTempFile(PREFIX_TMP, null, new File(reportPath));
        logger.debug("The report will be saved in the temporary file '{}'", tmpFile.getName());

        reportWriter.write(tmpFile, report);

        Calendar calendar = Calendar.getInstance();

        boolean isFileMoved = false;
        while (!isFileMoved) {
            String reportName = String.format(report.getName(), calendar.getTime());
            File reportFile = new File(reportPath, reportName);

            try {
                Files.move(tmpFile.toPath(), reportFile.toPath());
                isFileMoved = true;

                logger.info("The temporary file '{}' is renamed to '{}'", tmpFile.getName(), reportFile.getName());
            } catch (FileAlreadyExistsException e) {
                logger.warn("The file '{}' already exists in the folder '{}': adding a second to generation time",
                        reportName, reportPath);
            }

            calendar.add(Calendar.SECOND, 1);
        }

生成以下不可能的日志语句:

2016-10-04 10:35:11,674 [WARN ] [_Executor-1] a.b.c.MyGenerator - The file 'myFile_01092016103511.csv' already exists in the folder '/myDir': adding a second to generation time

2016-10-04 10:35:11,677 [WARN ] [_Executor-3] a.b.c.MyGenerator - The file 'myFile_01092016103511.csv' already exists in the folder '/myDir': adding a second to generation time

2016-10-04 10:35:11,677 [INFO ] [_Executor-1] a.b.c.MyGenerator - The temporary file 'tmp2103892505851032137.tmp' is renamed to 'myFile_01092016103512.csv'

2016-10-04 10:35:11,680 [INFO ] [_Executor-3] a.b.c.MyGenerator - The temporary file 'tmp6843688962754506611.tmp' is renamed to 'myFile_01092016103512.csv'

Executor 3 覆盖文件，即使它应该抛出 FileAlreadyExistsException。

没有抛出异常，文件的数据已被覆盖。

Answer 1

Executor 3 overwrites the file, even though it should throw a FileAlreadyExistsException.

您需要使用 ATOMIC_MOVE option如果您希望 Files.move(...) 以原子方式执行。默认情况下，它首先测试目标是否存在，然后执行易受 thread race conditions 影响的移动。 .这是 javadocs for Files.move(...) :

ATOMIC_MOVE – Performs the move as an atomic file operation. If the file system does not support an atomic move, an exception is thrown. With an ATOMIC_MOVE you can move a file into a directory and be guaranteed that any process watching the directory accesses a complete file.

如果您必须有 FileAlreadyExistsException 支持，那么您将需要使用 synchronized block 来保护移动，以便一次只允许一个线程运行它。如果您在此 block 中执行 IO，synchronized 应该几乎没有性能损失。