If you grew up playing Snake on a Nokia, Brick Game on a Chinese clone, or using custom dictionary apps on a low-cost touchscreen phone, you have likely encountered these file formats. Today, as retro computing and feature phones (dumbphones) make a comeback, the need to convert online has resurfaced.
If the developer possesses the original source code (typically Java for J2ME or C/C++), the process is one of migration : jar to vxp converter online
Bringing Java Apps to Your Feature Phone: The JAR to VXP Guide If you grew up playing Snake on a
This is the most common "online converter" you will find. ; it bundles the JAR inside a VXP that contains a Java emulator. Brick Game on a Chinese clone