Java, Using a newly-created Text File

Question

The code below describes a Java situation. I used some psudocode in the getRuntime().exec function; but, the main point is that a new file is created.

Although the second line causes an error the first time it is run I can see the new file has been created. Also, the 2nd time I run it, it works; I mean to say the 2nd time it is run, it can read the file created on the previous run. So, the best I can figure is that the 2nd line needs to wait until the new file is created (~1,000 lines of text).

Runtime.getRuntime().exec(new String[] {"Do something that writes a new file",  "c:/"+newFileName+".xml"});

...

 File fileToParse = new File("c:/"+newFileName+".xml");

Why not use File to create the file on your system? That way you know it is done when the method returns. How about: File#createNewFile() — Nate W.
– Nate W., Commented Nov 7, 2011 at 20:00

JB Nizet · Accepted Answer · 2011-11-07 20:00:23Z

4

Wait for the process to terminate using the waitFor method on the Process returned by Runtime.exec. Else, the second line of code executes while the external process is running and hasn't created the file already.

answered Nov 7, 2011 at 20:00

JB Nizet

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Community · Accepted Answer · 2017-05-23 11:55:50Z

1

Creating files can be done natively in Java using the File.createNewFile() method. That is much nicer and clearer.

File fileThatShouldBeCreated = new File("C:\\" + newFileName + ".xml");
fileThatShouldBeCreated.createNewFile(); // Create it!
System.out.println(fileThatShouldBeCreated.exists()); // Check if it exists.

Since you told us that you use a utility which doesn't simply create a file but also write data to it, this doesn't answer the question, of course.

In addition to JB Nizet's answer, I want to tell that I had some problems with the waitFor() method. It didn't want to return. I guess it was because of I didn't read all the output of the process (by stdout). What do others think about this? Is this a relevant conclusion? I just read the first couple of lines of the output and called waitFor(), but there was still a lot of other output after those couple of lines.

edited May 23, 2017 at 11:55

CommunityBot

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answered Nov 7, 2011 at 20:03

Martijn Courteaux

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3 Comments

JB Nizet Over a year ago

Yes, you need to read the output of the program. Else it hangs trying to write more until the output is read.

Martijn Courteaux Over a year ago

@JBNizet: Is this also needed when you even don't call Process.getInputStream()?

JB Nizet Over a year ago

Yes. The stream exists, whether you get it or not. Read javaworld.com/javaworld/jw-12-2000/jw-1229-traps.html?page=2

Mike Thomsen · Accepted Answer · 2011-11-07 19:57:54Z

1

You'd probably want to use the new fork/join approach to concurrency.

answered Nov 7, 2011 at 19:57

Mike Thomsen

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John Vint · Accepted Answer · 2011-11-07 20:34:59Z

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If you can write this under Java 7 then you can use the new Path and WatchService functionalities. You would point to the path and watch it to notify you when it is created

Path path = new File("c:/"+newFileName+".xml").toPath()
WatchService watcher = FileSystems.getDefault().newWatchService();
path.register(watcher, ENTRY_CREATE);

WatchKey key = watcher.take();//will block here until the file is created

answered Nov 7, 2011 at 20:34

John Vint

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