About Question enthuware.ocpjp.v8.2.1673 :

[gubble]

Isn't it possible, considering the lag of WatchService, that the key that we get contains both the ENTRY_CREATE and ENTRY_DELETE events? Especially if the file is cerated and deleted in a very short time frame

[admin]

No, until you reset the key, you will not get the second event.

[dcolvin]

To the contrary, I believe it is indeed possible for the key to get both events. Per the WatchKey Javadocs, "Events are retrieved by invoking the key's pollEvents method. This method retrieves and removes all events accumulated for the object."

To show how both events could accumulate, I tried adding the following code just before the while loop:

Code: Select all

Path file = Paths.get(directoryPath, "testFile.txt");
Files.createFile(file);
Files.delete(file);

I get the output:
true
sun.nio.fs.AbstractWatchKey$Event@7ea987ac
sun.nio.fs.AbstractWatchKey$Event@12a3a380

and for clarity, changing println(watchEvent) to println(kind + " " + watchEvent):
true
ENTRY_CREATE sun.nio.fs.AbstractWatchKey$Event@7ea987ac
ENTRY_DELETE sun.nio.fs.AbstractWatchKey$Event@12a3a380

confirming the two events gotten.

One more consideration is that a file creation could possibly result in one or more ENTRY_MODIFY events in addition to the ENTRY_CREATE event, depending on how the file is created. For example, when I instead create a file in the watched directory at the command line (e.g., "echo > testFile" or "copy file1 file2") sometimes I get:
true
ENTRY_CREATE sun.nio.fs.AbstractWatchKey$Event@7ea987ac
ENTRY_MODIFY sun.nio.fs.AbstractWatchKey$Event@12a3a380

So in this way as well, more than one event could be received even without calling reset() on the key.

[admin]

You are right.
The issue is actually with the event that happens after pollEvents has been called but reset has not yet been called. During this time, if you call pollEvent again, you will get that event but if you call reset, then that event will be lost. So the point is that there is a small duration between pollEvents and reset during which an event can get lost.

In the given question, it is possible to get two events if they happen very quickly (I have updated the problem statement to make that clear) but in normal course, the second event will not be printed.

HTH,
Paul.