Convert JAR to AAR

Some archive aar file types seem to exist, but it doesn't look like they can be created from Java archives (.jar), which means jar to aar conversion is not very likely or helpful.

To convert a .jar file to an .aar file, you need to understand that a .jar (Java Archive) file contains libraries or applications developed in the Java programming language. In contrast, one file type with the .aar file extension is an Android archive. This is specifically designed for Android Studio and contains all the components of an Android library, including compiled code, resources, and a manifest file.

It is possible to include a jar file in the Android library by creating a New library project in Android Studio. Open Android Studio and start a new Android Library project. This will be the container for your .aar file. Add the .jar file once your library project is set up, copy your .jar file into the libs folder of the project. You may need to create this folder if it doesn't already exist.

Simply put, it is impossible to convert JAR to AAR directly.
Thus, there is no so-called jar to aar converter or a free online .jar to .aar converter to be found.