Java File handling – when is a file actually saved

Consider the following code….

File ff=File.createTempFile(“page”,”.bin”, new File(ObjectStore.temp_dir));

BufferedOutputStream to = new BufferedOutputStream(new FileOutputStream(ff));

to.write(currentDisplay.serializeToByteArray(null));         

to.flush();           

to.close();

pagesOnDisk.put(key,ff.getAbsolutePath());

It stores a serialised Java Object (currentDisplay) on disk and then stores the file location so we can reuse it. So in theory,  if the value is in the Map pagesOnDisk, we should be able to retrieve the data and reuse it…

Unfortunately, that is not always the case. While Java may think the file has been written out, an attempt to immediately reuse it results in alsorts or errors arising from trying to read a File which has not fully been written out to disk.  At the OS system level, the file has been Buffered and is still being written out.

So be aware of this ‘gotcha’ in Java and either ensure that there is a sufficient time delay to allow the data to be written out, or include some check to make sure the data is valid.