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Java File lock

Posted on 2009-03-31
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Last Modified: 2013-11-24
I am using the following method to marshal user specific data to an XML file using Castor.  I would like to insure that multiple-threads are NOT saving the same user file at the same time..

Any suggestions would be appreciated

Thanks,
public boolean saveUserFilterFile() {
	Debug.debug(
		"WebViewUser.saveUserFilterFile() - entry",
		Debug.LEVEL_TRACE);
	if ((isCreateNewFilter()|| isAdministrator()) && saveFilter) {
		FilterGroupList expFl = new FilterGroupList();
	
		FileWriter writer  = null;
		String filename =
			ExpAppProperties.getExpFilterPath() + getUserName() + ".xml";
		String tempFileName = ExpAppProperties.getExpFilterPath() + "~~" + getUserName() + ".xml";
		try {
			//---( Clear current FilterGroupList in expUserProperties )---
			//---( Step through consolidated filter list copying user groups to expUserProperties )--
			for (Enumeration fl = allFilterGroupList.enumerateFilterGroup(); fl.hasMoreElements(); ) {
				FilterGroup fg = (FilterGroup) fl.nextElement();
				if ( fg.getUserFilterGroup() ) {
					expFl.addFilterGroup(fg);
				}
			}
			expUserProperties.setFilterGroupList(expFl);
 
			//---( Marshal expUserProperties )---
			writer = new FileWriter(tempFileName);
			Marshaller marshaller = new Marshaller(writer);
			marshaller.setNamespaceMapping("xsi","http://www.w3.org/2001/XMLSchema-instance");
			marshaller.setSchemaLocation("http://www.S4iSystems.com xsd/wvUserFilterFileSchema.xsd");
			marshaller.marshal(expUserProperties);
			if (writer != null ) {
				writer.close();
				// Rename temporary file to user file
				File tf = new File(tempFileName);
				File of = new File(filename);
				of.delete();
				tf.renameTo(of);
			}
		} catch (Exception e) {
			if (writer != null ) {
				try {
					writer.close();
				} catch (IOException e1) {
					e1.printStackTrace();
				}
			} else {
				e.printStackTrace();
			}
				Debug.debug(
					"Critical Error! WebViewUser.saveUserFilterFile(" +
						filename + 
						") - Save failed. " + e.getMessage(),
						Debug.LEVEL_ERROR);
				System.out.println(
					"Critical Error! WebViewUser.saveUserFilterFile(" +
						filename + 
						") - Save failed. " + e.getMessage());
				return false;
			}
		}
		return true;
}//--( end saveUserFilterFile() )--

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Question by:sol400
8 Comments
 
LVL 17

Assisted Solution

by:Thomas4019
Thomas4019 earned 800 total points
ID: 24033970
You could use java syncronization to do this. Basically you need one object that never changes in you code, it doesnt matter what it is. Java will make sure it is only in 1 synchronized block at any given time. Note that each thread must synchronize around the same integer.

Here is a link explaining sychronization.
http://java.sun.com/docs/books/tutorial/essential/concurrency/locksync.html
Integer holder = 0;
 
public void write()
{
   synchronized(holder)
   {
       //IO CODE
   }
}
 
public void read()
{
   synchronized(holder)
   {
       //IO CODE
   }
}

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LVL 5

Expert Comment

by:avya2k
ID: 24037565
you can have private static String object with any string and use it in synchronized block as follows
say private static String myStr="";
public boolean saveUserFilterFile() {
    synchronized(myStr){
       //method body
    }
}

This would ensure that at a time only one thread is using ur method
0
 

Author Comment

by:sol400
ID: 24038269
Hi,
I am NOT trying to single thread the method BUT only single thread multiple saves of the same file.  I can have end users saving unique files, multiple users saving the same file and a single user saving the same file in multiple threads.

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LVL 12

Accepted Solution

by:
basav_com earned 1200 total points
ID: 24040810
Create a lock file ( tmp file ) to check that no one is using.
0
 

Author Comment

by:sol400
ID: 24041121
Hi,
So are you saying that the solution would be to create a temporary file with the same name of every file I save and then check for it's existance before starting the save. I would then delay and loop on the existance check. then delete the file when I exit.. This seems like a lot of overhead.
0
 
LVL 17

Expert Comment

by:Thomas4019
ID: 24043937
Will the file only be edited by one program with multiple threads. Or is it possible there might be multiple Java programs running at the same time changing the files.
0
 

Author Comment

by:sol400
ID: 24044178
This is a web aplication where the xml file is used to contain single or group of end user specific persistance data. Each end user will have the file unmarshaled into their session object when they login to the web application.

I think I have solved the problem (I am testing now) based on the suggestion from basav.com.  I marshal the data into a temporary file then delete the orginal file & rename the temporary file.  It looks like all I have to do is check for the temporary files existance.
e.g.:
String tempFileName = ExpAppProperties.getExpFilterPath() + "~~" + getUserName() + ".xml";
File ckTemp = new File(tempFileName);
for ( int x = 0; x < 3; x++) {
      if (!ckTemp.exists()) {
            break;
      }
      try {
            Thread.sleep(2000);
      } catch (InterruptedException e) {
      }
}
      if (ckTemp.exists()) {
            ckTemp.delete();
}

0
 
LVL 92

Expert Comment

by:objects
ID: 24044322
all you need is to synchronize Thomas4019 suggested earlier.

> I am NOT trying to single thread the method BUT only single thread multiple saves of the same file.

so you just synch the block of code that does the save

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