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

#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