Problem while opening project in Eclipse STS

I have Eclipse STS installed on my windows pc. I am new to using STS. I have worked with VS code earlier.

In VS code, we directly open the folder containing the project and work on it. I know that we have to import it in the workspace and then work on it. But I want to know is it a thumb rule to always import the projects in the workspace while working with eclipse STS or we can also open the project folder directly like we do in vs code ?

I have tried opening my project folder but it is not working.

This is not really about Spring Tools but about how Eclipse works in general. Spring tools is just a plugin (or set of plugins) you can install into Eclipse (like you can with VSC), you just have the additional option to install an Eclipse version/product that comes with Spring Tools.

TL;DR

If you just have a few Java files you want to edit/run without creating much, select File > New > Java Project, uncheck Use default location and enter the path.

If the folder already uses a build tool and has an appropriate config file, use File > Import > Maven > Existing Maven Projects or File > Import > Gradle > Existing Gradle Project.

You can also create new Maven/Gradle projects in existing folders with File > New > Maven Project or similar.

If you just want to edit Java files with content assist and run them, use File > New > Java Project.

Eclipse is workspace-based

While VSC is (mostly) based on folders you can open, the Eclipse IDE is project based. There isn't really a concept of opening a currently opened folder but you have a workspace consisting of projects.

A project being in a workspace means that the directory (folder) corresponding with the project has a valid .project file and that it is registered as an opened project in the workspace. These projects can be located whereever you like but they are included in your workspace.

"Opening" a (non-Java) folder in Eclipse

So, if you want to "open" a folder in Eclipse, you first need to get a workspace and then you somehow need to get a project set up in that workspace. If the folder is already an Eclipse project, you can use File > Import... > General > Existing Projects into Workspace.

You can then select the folder with your project and import it.

If you don't have a project already, you can use File > Open Projects from File System:

After "opening" a folder that way, you have an empty project (possibly with some files):

You can open the files there, create new files, edit them or whatever. But for Eclipse, this is just a project without anything and you will have limited abilities.

In the same way (or if the previous option is not available in your installation), you can use File > New > Project (or File > New > Other > General > Project) and create a blank project. This allows you to select an existing folder for the project.

Java

A "normal" Eclipse project doesn't have to do anything with Java. It won't give you content assist, you can't run the project, it won't tell you about errors, you don't have build tools.

If you want to create/"open" a Java project, you can use File > New > Java Project (and select an existing folder if you like).

That then gives you access to the Java support Eclipse provides.

Technically you could also add the "Java" nature to a non-Java project using Project > Properties > Project Natures > Add > Java and Right Click on the project > Build Path > Add Libraries > JRE System Library but I wouldn't recommend doing that.

Build tools

In more complex projects (for example Spring projects), you will want to use a build tool (probably Maven or Gradle, other build tools are more or less irrelevant).

If you already have a Java project but that isn't using a build tool, you can convert it to a Maven project by right-clicking on the project and selecting Configure > Convert to Maven project:

However, you should make sure you have source folders set up properly before doing that (i.e. you should have your source code in src/main/java).

You can also create a Maven project and specify an existing folder as the location by deselecting Use default Workspace location:

This would then automatically create the Maven project structure and you might need to move your classes there.

Of course, you can do the same with Gradle.

Spring

All of this is unrelated to Spring except that Spring projects are Java projects. When working with Spring, you probably want to use Maven or Gradle.

What you can do if you already know you are working with Spring Tools is creating a Spring project using File > New > Spring Starter project and it will set up a Spring project for you. However, this will only work for new or empty folders. You cannot use existing folders for that.

Summary

So yes, you can "open" folders in Eclipse but it's called differently and actually creates a new project. But when you do, you need to make a choice on whether you want to open an existing project, create a new plain project without Java or a new Java project.

In most cases, you would want a folder structure with distinct source and output folders but this isn't required for projects that don't use a build tool.

But in most cases, I would just create a proper project or import it using the appropriate mechanism if the project is set up properly.