The code below demonstarates the problem: java 7 interfaces for watching file changes report opening of file, but don’t report actual change of content.
Is there a way to detect content change? My program needs to read new content as soon as is available.
The only length reported by event listener is 0 (right after opening of the file).
Compilable source to reproduce the problem:
import java.io.Writer;
import java.nio.charset.Charset;
import java.nio.file.ClosedWatchServiceException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.nio.file.attribute.FileAttribute;
class NioModifiedProblem {
public static void println(String str) {
System.out.println(str);
}
public static void printFileInfo(Path path) {
try {
println(String.format("File %s, size %d, modified %s", path, Files.size(path), Files.getLastModifiedTime(path)));
} catch (Throwable e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
String data = "Some not too long string goes here. Goes. Goes.";
try {
final Path path = Files.createTempFile("nioProblem", ".tmp", new FileAttribute[0]);
path.toFile().deleteOnExit();
println("Created");
printFileInfo(path);
Thread thread = new Thread() {
public void run() {
try {
final Path parent = path.getParent();
final WatchService service = parent.getFileSystem().newWatchService();
WatchKey key = parent.register(service, StandardWatchEventKinds.ENTRY_MODIFY);
try {
while (true) {
for (WatchEvent<?> event : service.take().pollEvents()){
Path modifiedPath = parent.resolve((Path)event.context());
println("Path "+modifiedPath+" modified EVENT."); // This is printed only once, on file opening.
printFileInfo(modifiedPath);
}
}
} catch (ClosedWatchServiceException e) {
println("Service closed");
}
} catch (Throwable e) {
e.printStackTrace();
} finally {
println("Watcher thread exiting");
}
}
};
thread.setDaemon(true);
thread.start();
Thread.sleep(1000);
Writer fw = Files.newBufferedWriter(path, Charset.forName("UTF-8"));
println("Opened");
printFileInfo(path);
Thread.sleep(1000);
fw.write(data);
println("Written");
printFileInfo(path);
fw.close();
println("Closed");
printFileInfo(path);
Thread.sleep(1000);
println("Sleeped");
printFileInfo(path);
return;
} catch (Throwable e) {
e.printStackTrace();
}
}
}
Output on Java(TM) SE Runtime Environment (build 1.7.0-b147):
Created
File C:\Users\b\AppData\Local\Temp\nioProblem190636654560972941.tmp, size 0, modified 2011-09-14T16:20:06.782Z
Opened
Path C:\Users\b\AppData\Local\Temp\nioProblem190636654560972941.tmp modified EVENT.
File C:\Users\b\AppData\Local\Temp\nioProblem190636654560972941.tmp, size 0, modified 2011-09-14T16:20:07.807Z
File C:\Users\b\AppData\Local\Temp\nioProblem190636654560972941.tmp, size 0, modified 2011-09-14T16:20:07.807Z
Written
File C:\Users\b\AppData\Local\Temp\nioProblem190636654560972941.tmp, size 0, modified 2011-09-14T16:20:07.807Z
Closed
File C:\Users\b\AppData\Local\Temp\nioProblem190636654560972941.tmp, size 47, modified 2011-09-14T16:20:08.81Z
Sleeped
File C:\Users\b\AppData\Local\Temp\nioProblem190636654560972941.tmp, size 47, modified 2011-09-14T16:20:08.81Z
The problem with the output – event is raised only once on file opening, and is never raised on actual writes or file closing.
WatchedKey should be reset after pollEvents() to be able to accept futher events.