B4J Code Snippet B4J Single Instance checker (Windows & Linux) irrespective of the Java version and packager used

This method was tested and works on my setup, even if the jar or exe is copied or renamed:
- Windows 10, B4J 9.10 (and later), OpenJDK 11
- Linux Mint Cinnamon 21.1, B4J 9.80, OpenJDK 11

#AdditionalJar: sqlite-jdbc-3.36.0.1

Sub AppStart (Form1 As Form, Args() As String)

    MainForm = Form1
    MainForm.Show

    If ImAlreadyRunning(File.DirData("ThisProgram"), "lock.sqlite") Then
        fx.Msgbox(MainForm, "This program is already running", "No such luck")
        ExitApplication
        Return
    End If

'...

End Sub

Sub ImAlreadyRunning(dir As String, fileName As String) As Boolean

    If File.Exists(dir, fileName) Then
       ' Start of Linux-specific block. Turn it into comment if you don't need it.
       ' Requires the jShell library to be added to the project, and /usr/bin/lsof must exist.
        Dim os As String = GetSystemProperty("os.name", "").ToLowerCase
        If (os.Contains("win") = False) And (os.Contains("mac") = False) And File.Exists("/usr/bin/lsof", "") Then
            ' Linux. As per https://stackoverflow.com/questions/57986206/how-sqlite-can-be-used-after-deleting-local-database-file :
            ' "On Linux or Unix systems, deleting a file via rm or through a file manager application will unlink the file from the
            ' file system's directory structure; however, if the file is still open (in use by a running process) it will still be
            ' accessible to this process and will continue to occupy space on disk."
            '
            ' Therefore, we're going to use the lsof command to check if the file is in use.
            Try
                Dim lsofShl As Shell
                lsofShl.Initialize("", "/usr/bin/lsof", Array As String(File.Combine(dir, fileName)))
                Dim lsofShlResult As ShellSyncResult = lsofShl.RunSynchronous(-1)
                If lsofShlResult.Success And (lsofShlResult.ExitCode = 0) And lsofShlResult.StdOut.Contains(fileName) Then Return True
            Catch
                Log("ImAlreadyRunning - lsof failed: " & LastException)
            End Try
        End If
        ' End of Linux-specific block.
        Try
            ' Note: at least on Windows 10, no exception is thrown if the sqlite file is in use by
            ' another process. That's why the below 'Try' block with the SQL operations is there,
            ' and why it doesn't just open the database. A successful File.Delete is inconclusive.
            File.Delete(dir, fileName)
        Catch
            Return True
        End Try
    End If

    Try
        Dim sql1 As SQL
        sql1.InitializeSQLite(dir, fileName, True)
        sql1.BeginTransaction
        sql1.ExecNonQuery("CREATE TABLE IF NOT EXISTS TblLock(ColInt int)")
        sql1.ExecNonQuery("INSERT INTO TblLock (ColInt) VALUES (1)")
        sql1.TransactionSuccessful ' This will cause exception "java.sql.SQLException: database is locked" if another instance was already running
        ' IMPORTANT: do NOT close rs! (that's the whole point)
        Dim rs As ResultSet = sql1.ExecQuery("SELECT * FROM TblLock")
    Catch
        Return True
    End Try

    Return False

End Sub

Dependencies:
- Internal library: jSQL
- Additional jar: sqlite-jdbc-... : the latest one is 3.36.0.1 as per @Claudio Oliveira 's post: https://www.b4x.com/android/forum/threads/new-sqlite-jdbc-version-3-36-0-1-2021-06-30.132348/

EDIT: added the 'Note' comment.
EDIT 2023-07-04: added provisions for Linux.

 

[Erel]

Where is the code that creates lock.sqlite?
Where is the code that deletes lock.sqlite?
How do you handle cases where the computer is turned off abruptly while the app is running?

 

[walt61]

If you scroll down in the code block you'll see 'Sub ImAlreadyRunning' And there you'll see that the sqlite db is deleted (if not in use at that time) and re-created.

 

[walt61]

I should probably add this for clarification: I observed (on Win 10) that the 'File.Delete' doesn't throw an exception if the db is already in use by another program. That's why the second 'Try' block is there with the sql operations, which do work properly.

 

[Erel]

Missed the scrollbar. Sorry...

 

[walt61]

 

[Cableguy]

Can you explain a use case for such code... some of us are just hobbyists with no formal background in software engineering

 

[walt61]

I use it to make sure my 'client' for my home automation stuff is running just once on a PC (while it's ok to be running on multiple devices, but just one instance at a time on each device), as it uses/updates certain resources (files, databases) and would get confused if that's happening multiple times on the same PC. I could of course change my code to take multiple instances into account, but for this case that would be pointless.

Another example could be a program that shows itself in the system tray and you only want to see its icon there once.

Basically I'd say any program that requires exclusive access to certain resources could check if it's already running. Launch4J (the B4J packager I used with Java 8) has a 'single instance' option but as I moved to Java 11 and the integrated B4Jpackager11, I wanted that functionality too. A forum search showed that others had come up with solutions as well, but I could imagine situations where they wouldn't be bullet proof - I think mine is

 

[Cableguy]

I use it to make sure my 'client' for my home automation stuff is running just once on a PC (while it's ok to be running on multiple devices, but just one instance at a time on each device), as it uses/updates certain resources (files, databases) and would get confused if that's happening multiple times on the same PC. I could of course change my code to take multiple instances into account, but for this case that would be pointless.

Another example could be a program that shows itself in the system tray and you only want to see its icon there once.

Basically I'd say any program that requires exclusive access to certain resources could check if it's already running. Launch4J (the B4J packager I used with Java 8) has a 'single instance' option but as I moved to Java 11 and the integrated B4Jpackager11, I wanted that functionality too. A forum search showed that others had come up with solutions as well, but I could imagine situations where they wouldn't be bullet proof - I think mine is

Thank you for your explanation, very clear and I can now see the usefullness of this snippet

 

[walt61]

Added a Linux-specific block to method 'ImAlreadyRunning'.