Introduction
The ServletContext#getRealPath() is intented to convert a web content path (the path in the expanded WAR folder structure on the server’s disk file system) to an absolute disk file system path.
The "https://stackoverflow.com/" represents the web content root. I.e. it represents the web folder as in the below project structure:
YourWebProject
|-- src
| :
|
|-- web
| |-- META-INF
| | `-- MANIFEST.MF
| |-- WEB-INF
| | `-- web.xml
| |-- index.jsp
| `-- login.jsp
:
So, passing the "https://stackoverflow.com/" to getRealPath() would return you the absolute disk file system path of the /web folder of the expanded WAR file of the project. Something like /path/to/server/work/folder/some.war/ which you should be able to further use in File or FileInputStream.
Note that most starters don’t seem to see/realize that you can actually pass the whole web content path to it and that they often use
String absolutePathToIndexJSP = servletContext.getRealPath("https://stackoverflow.com/") + "index.jsp"; // Wrong!
or even
String absolutePathToIndexJSP = servletContext.getRealPath("") + "index.jsp"; // Wronger!
instead of
String absolutePathToIndexJSP = servletContext.getRealPath("/index.jsp"); // Right!
Don’t ever write files in there
Also note that even though you can write new files into it using FileOutputStream, all changes (e.g. new files or edited files) will get lost whenever the WAR is redeployed; with the simple reason that all those changes are not contained in the original WAR file. So all starters who are attempting to save uploaded files in there are doing it wrong.
Moreover, getRealPath() will always return null or a completely unexpected path when the server isn’t configured to expand the WAR file into the disk file system, but instead into e.g. memory as a virtual file system.
getRealPath() is unportable; you’d better never use it
Use getRealPath() carefully. There are actually no sensible real world use cases for it. Based on my 20 years of Java EE experience, there has always been another way which is much better and more portable than getRealPath().
If all you actually need is to get an InputStream of the web resource, better use ServletContext#getResourceAsStream() instead, this will work regardless of the way how the WAR is expanded. So, if you for example want an InputStream of index.jsp, then do not do:
InputStream input = new FileInputStream(servletContext.getRealPath("/index.jsp")); // Wrong!
But instead do:
InputStream input = servletContext.getResourceAsStream("/index.jsp"); // Right!
Or if you intend to obtain a list of all available web resource paths, use ServletContext#getResourcePaths() instead.
Set<String> resourcePaths = servletContext.getResourcePaths("https://stackoverflow.com/");
You can obtain an individual resource as URL via ServletContext#getResource(). This will return null when the resource does not exist.
URL resource = servletContext.getResource(path);
Or if you intend to save an uploaded file, or create a temporary file, then see the below “See also” links.