Question

This is in Java 6.

I’ve seen more than once that people create temp files, do something, then rename it to the output file. Everything is wrapped in a try-finally block, where the temp file is deleted in finally in case something goes wrong in between.

try {
    //do something with tempFile
    //do something with tempFile
    //do something with tempFile
    tempFile.renameTo(outputFile);
}
finally {
    if (tempFile.exists())
        tempFile.delete()
}

I was wondering what are the benefits of doing that instead of doing something to the output file directly and delete it in case of exceptions.

try {
    //do something with outputFile
    //do something with outputFile
    //do something with outputFile
}
catch (Exception e) {
    if (outputFile.exists())
        outputFile.delete();
}

My guess is that deleting temp files in finally benefits me when the try block can throw many kinds of exceptions. Is my guess right? What else?

Answer 1

finally is always executed while the catch above is not executed for exceptions that derive from java.lang.Error plus it will delete the file also when it can’t be renamed (this operation doesn’t throw an exception when it fails … an ancient bug in Java IO).