When comparing Grails vs ASP.NET Core, the Slant community recommends ASP.NET Core for most people. In the question“What are the best web frameworks to create a web REST API?” ASP.NET Core is ranked 9th while Grails is ranked 28th. The most important reason people chose ASP.NET Core is:
Thanks to breakthroughs in ROSLYN compiler and the efforts of the .NET COre developer team, code written in C# can reach speeds just a step behind C++.
Specs
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Pros
Pro Grails is highly scalable
Grails is an abstraction over Spring and Hibernate. This makes Grails applications fast and scalable. This is because both Spring and Hibernate are themselves scalable.
Pro More than 900 plugins
Grails is a full-stack web framework, not just MVC. It contains a lot of stuff out of the box, but it doesn’t enforce it. It contains over 900 plugins which provide a Groovy API for a lot of useful and well-known Java libraries. And what is more important is that they are super easy to install!
Pro Very fast setup and scaffolding
Setting up a new project is quite fast and code generation (scaffolding) saves you a lot of time. It also uses a convention over configuration principle which helps you bypass all the configuration trouble.
Grails also comes with a reloading mechanism out of the box.
Pro Easy to use
Grails is designed to be a rapid development framework with a straight learning curve. It advocates convention over configuration. Extensibility is very simple when using plugins (there is a lot of them). One command in the console – and all the dependencies and configurations are managed for you.
Pro Great UI design alternatives
Do you need something advanced in the UI? You can either create it in GSP with an addon of CSS/JavaScript or you can find a plugin that will do the work for you. Plugins include integration with Bootstrap, jQuery, Yeoman and much more.
Pro Great documentation and community
The Documentation section is actually a wiki, which can be modified by any logged in user. It has an official manual, tutorials, screencasts, a sample app and much more. If that does not do it for you, then there are countless third-party tutorials, more than 12k questions on SO and much much more
Pro Fast and getting faster
Thanks to breakthroughs in ROSLYN compiler and the efforts of the .NET COre developer team, code written in C# can reach speeds just a step behind C++.
Pro Multi platform
Can run on Windows, Linux and Mac (also Visual Studio Code editor).
Pro JSON optimization
In .NET Core 2.1 and 3.0, new APIs are added that make it possible to write JSON APIs that require less memory, using Span<T> and UTF8 strings, and improve throughput of applications like Kestrel, ASP.NET Core web server. See also Utf8JsonReader.
Pro Tutorials and documentation quality
Both microsoft and 3rd party tutorials are mostly of high quality and encourage you to use the industry best-practices.
Pro Built-in middleware
Built-in middleware featuring: Authentication, Cookie policy, Health Check, MVC, Session etc.
Pro Hosting
Ability to host on IIS, Nginx, Apache, Docker, or self-host in your own process.
Pro Ease of Use
Pro Security
It is a very secure platform.
Pro Tooling
Both VS and VSCode are powerful free IDEs that are well integrated with ASP.net Core. VS Community also allows for commercial use for projects with less than 5 developers.
Cons
Con Grails is a fairly complex framework
Grails is a pretty heavy piece of software. It's functionality is covered by GORM (Grails' Object Relational Mapping) which is a facade for hybernate and by Spring MVC.
Everything is glued by core Spring and furthermore, Grails adds another level of abstraction on top of all this. These things may create some trouble down the road when debugging.
Con Too obtuse and JVM centric
If you're a big fan of the JVM and have mastered all its goofy quirks, Grails might be a good choice, but there are other frameworks out there that are more straightforward and easier to use.