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();
}
}
}
}
}
- There only exists one instance of
TableSaver, ever. load()can be called from many threads. Timer clearly is another thread.loadFile()andwriteFile()do not leave open file streams - they use a robust, well tested and broadly used library which always closes the streams withtry ... finally.
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
So I have two questions:
- How can this kind of synchronization fail? Note that I am sure there only is one instance of
TableSaver. - What is this thing in the stacktrace:
package.TableSaver.access$600(TableSaver.java:34)? Line 34 is the line withclass TableSaver {. Can this be the reason why the synchronization is not working?