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Norbert
Norbert

Reputation: 254

File.canWrite() does not work as expected

I found the same question 7 years ago, but cannot find a solution.

I create a file "readonlyfile.txt" with denied write permission. Windows does not allow to edit and store this file - ok- expected.What i do

However the following canWrite() code says that this file can be written.

Path readonlyfile = Paths.get("C:\\temp_\\__readonlyfile.txt"); 
boolean canwrite = false;
try
{   File f = readonlyfile.toFile();
    //File f = new File("C:/temp_/__readonlyfile.txt");
    //File f = new File("C:\\temp_\\__readonlyfile.txt"); 
    canwrite = f.canWrite();   
} catch (Exception e)
{  System.out.println("canWrite()" + e);
}
System.out.println("---canWrite: " + canwrite);  
//---canWrite: true
BufferedWriter bw = null; 
try
{    
bw = Files.newBufferedWriter(readonlyfile);
bw.write("foo");
System.out.println("write() succeeds");

} catch (Exception e) 
{   System.out.println("---cannot open: " + e);
//---cannot open: java.nio.file.AccessDeniedException: C:\temp_\__readonlyfile.txt
} finally
{ try { bw.close(); } catch (Exception ee) {}       
}

Upvotes: 3

Views: 635

Answers (1)

Norbert
Norbert

Reputation: 254

Thanks boot-and-bonnet: Files.isWritable() is more reliable than File.canWrite() however returns always false if a file does not exist. So I create some code 'isWritable() in MyFile class (see below) to overcome this problem. Test code is:

canwrite = MyFile.isWritable(readonlyfile);
System.out.println("canWrite: " + canwrite + " : " + readonlyfile);  

canwrite = MyFile.isWritable(notexists);
System.out.println("canWrite: " + canwrite + " : " + notexists); 

canwrite = MyFile.isWritable(readonlydir);
System.out.println("canWrite: " + canwrite + " : " + readonlydir);

canwrite = MyFile.isWritable(dir); // existing dir
System.out.println("canWrite: " + canwrite + " : " + dir);  

//    canWrite: false : C:\temp_\__readonlyfile.txt
//    canWrite: true : C:\temp_\~~~notexists.txt
//    canWrite: false : C:\temp_\read-only_dir
//    canWrite: true : C:\temp_

/**
 * Similar to Files.isWritable() - however also works for a non-existing file/directory.
 * In this case, the creating of 'path' as a file is simulated to check write access. 
 * Note: Files.isWritable() mistakenly returns false for a non-existing file. 
 * @param path The file or directory to be examined
 * @return true if the caller has write permission to the existing file/directory or could create this non-existing 
 * 'path' as a file, false otherwise
 */
public static boolean isWritable(Path path)
{
    if (path == null) return false;     
try
{
    if (!Files.exists(path))
    {   try 
        { Files.createFile(path); //atomic
          try { Files.delete(path); } catch (Exception e) {}
          return true;
        } catch (AccessDeniedException e) 
        {  return false;
        } catch (Exception e) 
        { logger.log(Level.INFO,"Files.createFile({0}): {1}",new Object[] {path,e}); } // let isWritable() decide
    }       
    return Files.isWritable(path); // works fine for an existing directory too, e.g. read-only-dir

} catch (Exception e)
{  logger.log(Level.WARNING,"MyFile.isWritable(): {0}",e.toString());
}
    return false;
} //--------- end of isWritable()

Upvotes: 1

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