Convert JAD to JAR

A Java ME phone or emulator may require a JAR, while a download or backup contains only its JAD descriptor. The usual task is to locate the matching JAR, not convert the JAD, because the descriptor does not contain the application itself.

What the JAD format is

JAD means Java Application Descriptor. It is a small plain-text file used by Java ME applications to store metadata such as the application name, version, vendor, required configuration, JAR location, and JAR size.

Older Nokia, Sony Ericsson, Samsung, and other Java-enabled phones use JAD files during installation; Java ME emulators and archived mobile applications may also include them. Open a JAD in a text editor and look for entries such as MIDlet-Jar-URL: game.jar and MIDlet-Jar-Size: 123456. MIDlet-Jar-URL can contain a local filename, a relative URL, or an absolute download URL.

What the JAR format is

JAR means Java Archive. It is a ZIP-based package that can contain compiled Java classes, images, application resources, a manifest at META-INF/MANIFEST.MF, and other metadata. For Java ME software, the JAR contains the executable payload; the JAD is an optional or required installation descriptor, depending on the device or emulator.

A JAR can be inspected with 7-Zip or the JDK jar utility. Opening it as a ZIP does not convert a JAD or recreate missing application files.

How to get the matching JAR

1. Open the JAD in a text editor and copy the complete value of MIDlet-Jar-URL.
2. If the value is a URL, download that resource from the original site or repository. If it is a filename, search the same download, extracted folder, device backup, or original distribution.
3. Check the downloaded file against MIDlet-Jar-Size, which specifies the expected size in bytes. On Windows, right-click the JAR, select Properties, and compare the byte count.
4. Use 7-Zip or jar tf application.jar to confirm that the archive contains application files, such as .class files and META-INF/MANIFEST.MF.
5. Copy the JAR to the phone or emulator. Keep the JAD when the installer requires it; do not rename the JAR unless the MIDlet-Jar-URL value is changed to match.

A JAR with the same name but a different application version or byte size may be incompatible. Search for the exact filename, version, and vendor when the original JAR is missing. Online repositories and web archives can help locate the original distribution, but they do not convert a descriptor into an application.

How to build a new JAR from application files

If you have the compiled classes and resources, install a Java Development Kit and place those files in their intended package directory structure. From the directory containing the build output, run:

jar cf application.jar -C build/classes .

To supply a manifest, create manifest.mf and run:

jar cfm application.jar manifest.mf -C build/classes .

This creates an archive; it does not reproduce the original Java ME build. Java ME applications may require preprocessing, obfuscation, a particular manifest, signing, resource processing, or a vendor-specific build tool. A JAD listing class names or other metadata cannot substitute for missing source code, compiled classes, resources, or signing keys.

Conversion limits and online tools

A JAD cannot generally be converted into a working JAR. It is metadata, not a container for the application payload. No reputable online converter can recover classes and resources that are absent from the JAD, so uploading the descriptor to a conversion service is unnecessary and may expose application metadata.

If you possess only source files, use the original Java ME development environment or a compatible JDK-based build process rather than an online converter. If you possess an existing JAR, use 7-Zip or the JDK jar command for inspection and repackaging; repackaging can invalidate signatures and may alter installation behavior.

Some phones require both files and verify that JAD values, manifest data, JAR size, version, and signing information agree. When installation reports a size, version, or security error, obtain the original JAD/JAR pair instead of editing metadata blindly.