No, not all .jar files are harmful. They are just simple Java files. However, some files might get corrupt if designed to harm your device. It depends on the source code in it. If your device shows an error that says it cannot open the jar file or something similar keeps happening, make sure the file is safe and does not contain any virus. Before opening files you do not create, make sure you trust the sender. This will prevent your data from being corrupt and save your files. Sometimes, jar files can be prone to this, so you need to be careful. Sometimes people don’t know how to run and open .jar files windows 10 and trust some other sources to open them, which are not reliable so beware of them.