How to generate unique filenames??

i need to be able to generate unique files from a servlet..
my initial instinct was to use the seesion id as part of the filename, however as this file will be embedded in the responding html, this is not safe, as the user will only have to look at the html source to view a session id value.
i am now considering to use the date/time of the creation of a session as the unique identifier for the file, however this will not work if two sessions can be created at the same time.
my question thefore is if no two sessions can have the same date and time?
if not.. can anyone give me an idea as how to produce unique filenames?

If you are actually creating a file, why not usethe
java.io.File.createTempFile() method?how does that help with prducing a unique value to use
as the name of a file?
with that method i still need to supply the filename
as one of its arguments!
No you don't. You provide a prefix and suffix and it fills in the middle with something guaranteed to be unique, in the directory you specify. Here is the javadoc:
createTempFile
public static File createTempFile(String prefix,
String suffix,
File directory)
throws IOException
Creates a new empty file in the specified directory, using the given prefix and suffix strings to generate its name. If this method returns successfully then it is guaranteed that:
1. The file denoted by the returned abstract pathname did not exist before this method was invoked, and
2. Neither this method nor any of its variants will return the same abstract pathname again in the current invocation of the virtual machine.
This method provides only part of a temporary-file facility. To arrange for a file created by this method to be deleted automatically, use the deleteOnExit() method.
The prefix argument must be at least three characters long. It is recommended that the prefix be a short, meaningful string such as "hjb" or "mail". The suffix argument may be null, in which case the suffix ".tmp" will be used.
To create the new file, the prefix and the suffix may first be adjusted to fit the limitations of the underlying platform. If the prefix is too long then it will be truncated, but its first three characters will always be preserved. If the suffix is too long then it too will be truncated, but if it begins with a period character ('.') then the period and the first three characters following it will always be preserved. Once these adjustments have been made the name of the new file will be generated by concatenating the prefix, five or more internally-generated characters, and the suffix.
If the directory argument is null then the system-dependent default temporary-file directory will be used. The default temporary-file directory is specified by the system property java.io.tmpdir. On UNIX systems the default value of this property is typically "/tmp" or "/var/tmp"; on Microsoft Windows systems it is typically "c:\\temp". A different value may be given to this system property when the Java virtual machine is invoked, but programmatic changes to this property are not guaranteed to have any effect upon the the temporary directory used by this method.
Parameters:
prefix - The prefix string to be used in generating the file's name; must be at least three characters long
suffix - The suffix string to be used in generating the file's name; may be null, in which case the suffix ".tmp" will be used
directory - The directory in which the file is to be created, or null if the default temporary-file directory is to be used
Returns:
An abstract pathname denoting a newly-created empty file
Throws:
IllegalArgumentException - If the prefix argument contains fewer than three characters
IOException - If a file could not be created
SecurityException - If a security manager exists and its SecurityManager.checkWrite(java.lang.String) method does not allow a file to be created
Since:
1.2

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