在Java对象上同步文件访问

问题描述

我有一个对象负责将JTable状态持久化到磁盘.它保存/加载可见列，它们的大小，位置等.下面是其类定义中的一些有趣的内容.

I have an object responsible for persisting JTable state to disk. It saves/loads visible columns, their size, position etc. A few interesting bits from its class definition are below.

class TableSaver {
    Timer timer = new Timer(true);

    TableSaver() {
        timer.schedule(new TableSaverTimerTask(), 15000, SAVE_STATE_PERIOD);
    }

    synchronized TableColumns load(PersistentTable table) {
        String xml = loadFile(table.getTableKey());
        // parse XML, return
    }

    synchronized void save(String key, TableColumns value) {
        try {
            // Some preparations
            writeFile(app.getTableConfigFileName(key), xml);
        } catch (Exception e) {
            // ... handle
        }
    }

    private class TableSaverTimerTask extends TimerTask {
        @Override
        public void run() {
            synchronized (TableSaver.this) {
                Iterator<PersistentTable> iterator = queue.iterator();
                while (iterator.hasNext()) {
                    PersistentTable table = iterator.next();
                    if (table.getTableKey() != null) {
                        save(table.getTableKey(), dumpState(table));
                    }
                    iterator.remove();
                }
            }
        }
    }
}

• 永远只有一个TableSaver实例.
• load()可以从许多线程中调用.计时器显然是另一个线程.
• loadFile()和writeFile()不会保留打开的文件流-它们使用功能强大，经过良好测试且用途广泛的库，该库始终使用try ... finally关闭流.
• There only exists one instance of TableSaver, ever.
• load() can be called from many threads. Timer clearly is another thread.
• loadFile() and writeFile() do not leave open file streams - they use a robust, well tested and broadly used library which always closes the streams with try ... finally.

有时这会失败，并出现以下异常:

Sometimes this fails with an exception like:

java.lang.RuntimeException: java.io.FileNotFoundException: C:\path\to\table-MyTable.xml (The requested operation cannot be performed on a file with a user-mapped section open)
    at package.FileUtil.writeFile(FileUtil.java:33)
    at package.TableSaver.save(TableSaver.java:175)
    at package.TableSaver.access$600(TableSaver.java:34)
    at package.TableSaver$TableSaverTimerTask.run(TableSaver.java:246)
    at java.util.TimerThread.mainLoop(Unknown Source)
    at java.util.TimerThread.run(Unknown Source)
Caused by: java.io.FileNotFoundException: C:\path\to\table-MyTable.xml (The requested operation cannot be performed on a file with a user-mapped section open)
    at java.io.FileOutputStream.open(Native Method)
    at java.io.FileOutputStream.<init>(Unknown Source)
    at java.io.FileOutputStream.<init>(Unknown Source)
    at package.FileUtilWorker.writeFile(FileUtilWorker.java:57)
    ... 6 more

所以我有两个问题:

1. 这种同步如何失败?请注意，我确定只有TableSaver的一个实例.
2. 堆栈跟踪中的这是什么:package.TableSaver.access$600(TableSaver.java:34)?第34行是带有class TableSaver {的行.这可能是同步无法正常工作的原因吗?
1. How can this kind of synchronization fail? Note that I am sure there only is one instance of TableSaver.
2. What is this thing in the stacktrace: package.TableSaver.access$600(TableSaver.java:34)? Line 34 is the line with class TableSaver {. Can this be the reason why the synchronization is not working?

推荐答案

Google 告诉我，这似乎是Windows特定的.这是错误6354433 的摘录:

Google learns me that this seems to be Windows specific. Here's an extract of Bug 6354433:

您使用的是哪个Java/Windows版本?它有最新的更新吗?

What Java/Windows version are you using? Does it have the latest updates?

还有另外两个相关的错误，并提供了一些有用的见解:

Here are two other related bugs with some useful insights:

• 错误4715154 -无法删除内存映射文件.
• 错误4469299 -内存映射文件未进行GC处理.
• Bug 4715154 - Memory mapped file cannot be deleted.
• Bug 4469299 - Memory mapped files are not GC'ed.

关于第二个问题，那只是内部或匿名类的自动生成的类名.

As to your second question, that's just the autogenerated classname of an inner or anonymous class.

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