What is the JAD file format?
A .jad file - short for Java Application Descriptor - is used to describe its companion JAR archive. Both the .jad file and the .jar file are required by the Java Micro Edition (ME) virtual machine to run Java applications and games on mobile devices and cell phones. The .jar file contains the actual bytecode and resources of the Java application package, while the .jad file holds the technical information needed to locate, verify, and install the .jar.
The .jad is saved in plain-text format and can be opened with any text editor. It is structured as a list of Key: Value attribute lines - one per line - describing the MIDlet suite. Mandatory attributes include MIDlet-Name, MIDlet-Version, MIDlet-Vendor, MIDlet-Jar-URL, MIDlet-Jar-Size, MicroEdition-Profile, and MicroEdition-Configuration. A device reads the small .jad first during an over-the-air (OTA) download, displays the app name, vendor, and file size to the user, then fetches and installs the .jar only if the user confirms. If the MIDlet-Jar-Size value in the .jad does not match the actual .jar, installation fails.
The format was introduced around 2000-2002 as part of the MIDP 1.0 specification by Sun Microsystems and was widely used on Nokia S40, Sony Ericsson, and similar feature phones throughout the 2000s. Its official MIME type is text/vnd.sun.j2me.app-descriptor. J2ME's decline around 2012, displaced by Android APK apps and iOS app bundles, rendered .jad files largely obsolete - though emulators such as J2ME Loader and FreeJ2ME still support them today.
Security & safety
RISK: LOWA JAD is a tiny plain-text descriptor and is not executable, so the file itself is harmless to open and read. The risk lies in what it installs: the companion .jar is real code, and on old phones signed/unsigned MIDlets had permission prompts (network, messaging) that some malware abused to send premium-rate SMS. So treat the .jar from an untrusted JAD with the same caution as any downloaded app. Also avoid sketchy "JAD to APK converter" sites - the conversion isn't real and the portals are typically ad/scam traps; use a reputable open-source J2ME emulator instead.
Format details
in a nutshellPrograms that open JAD files
Windows4 apps
Any text editor (Notepad, Notepad++)
Open-source Open the .jad as text to read its attributes (name, vendor, jar URL/size). This reads the descriptor; it does not run the app.
KEmulator (J2ME emulator)
Free Load the .jad (with its .jar in the same folder) to run the J2ME app/game on Windows. Popular legacy MIDlet emulator.
Java ME SDK / Wireless Toolkit
Free Oracle's J2ME emulator/SDK runs and tests MIDlet suites (open the JAD); aimed at developers.
Eclipse (+ MTJ / dev tools)
Open-source For developers building/editing MIDlets; not an end-user runner. Use an emulator to just play an app. 
Linux1 app
FreeJ2ME
Open-source Open-source J2ME emulator (also runs on Windows/Mac via Java); load the .jad/.jar to run the MIDlet. 
Android1 app
J2ME Loader
Open-source Install the app, then open the .jad/.jar to run old J2ME games on Android. The practical modern way to play them. Technical details
deep spec| MIME type | text/vnd.sun.j2me.app-descriptor |
| Character encoding | UTF-8 plain text |
| File structure | Key: Value attribute lines, one per line; no sections or nesting |
| Binary signature | None - plain text with no magic bytes; typically begins with a MIDlet- or MicroEdition- attribute line |
| Typical file size | Under 1 KB to a few KB |
| Mandatory attributes | MIDlet-Name, MIDlet-Version, MIDlet-Vendor, MIDlet-Jar-URL, MIDlet-Jar-Size, MicroEdition-Profile, MicroEdition-Configuration |
| Integrity verification | MIDlet-Jar-Size cross-checks the companion .jar file size; signed suites add MIDlet-Certificate and MIDlet-Jar-RSA-SHA1 attributes in the .jad |
| Compression | None - the .jad is uncompressed plain text |
| Companion file | Always paired with a .jar containing the MIDlet suite bytecode and resources; the two files must match on Jar-URL and Jar-Size |
| Runtime declaration | MicroEdition-Profile (e.g. MIDP-2.0) and MicroEdition-Configuration (e.g. CLDC-1.1) specify the required J2ME runtime |
| Installation protocol | OTA (over-the-air): device downloads the small .jad first, presents name/vendor/size to user, then fetches and installs .jar on confirmation |
| Specification owner | Sun Microsystems (Java ME / MIDP specification); maintained by Oracle after acquisition |
| Superseded by | Android .apk and iOS app bundles following J2ME's decline circa 2012; no direct successor format |
| Associated platforms | J2ME feature phones (Nokia S40, Sony Ericsson, Motorola); Windows/desktop via Java ME SDK emulators and KEmulator |
| Released | 2000-2002 (Java ME / MIDP 1.0; J2ME mobile applications) |
| Latest version | MIDP 2.1 era; format effectively frozen after the decline of J2ME (~2012) |
| Open standard | Yes · royalty-free |
| Specification | docs.oracle.com |
JAD conversions
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