How to download and setup undertow
Open terminal and run command
                                            git clone https://github.com/undertow-io/undertow.git
                                        
                                        git clone is used to create a copy or clone of undertow repositories.
                                        You pass git clone a repository URL. it supports a few different network protocols and corresponding URL formats.
Also you may download zip file with undertow https://github.com/undertow-io/undertow/archive/master.zip
Or simply clone undertow with SSH
                                            [email protected]:undertow-io/undertow.git                                    
                                    If you have some problems with undertow
You may open issue on undertow support forum (system) here: https://github.com/undertow-io/undertow/issuesSimilar to undertow repositories
Here you may see undertow alternatives and analogs
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