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====== Java 2 MicroEdition (J2ME) ======
[[wp>Java_Platform,_Micro_Edition|Java 2 Platform, Micro Edition]] (J2ME), later renamed Java ME, is a subset of the [[wp>Java_(programming_language)|Java]] platform designed by [[wp>Sun_Microsystems|Sun Microsystems]] for embedded and mobile devices. It was introduced in 1999 and became the **dominant mobile gaming platform** of the 2000s, powering games on billions of feature phones worldwide before the smartphone era.

J2ME games are built on two layers: **CLDC** (Connected Limited Device Configuration) providing the base VM for resource-constrained devices, and **[[wp>Mobile_Information_Device_Profile|MIDP]]** (Mobile Information Device Profile) providing UI, networking, and the Game API. Applications are called **MIDlets** and are packaged as ''.jar'' files. MIDP 2.0 (2002) introduced the dedicated Game API with ''GameCanvas'', ''Sprite'', ''TiledLayer'', and ''LayerManager'' classes, enabling 2D game development.

J2ME games were distributed for a wide range of feature phones with varying screen resolutions:
  * **128x128** — Nokia Series 40v2 (3220, 6020, 6230)
  * **128x160** — Nokia 6101, Sony Ericsson K310
  * **176x220** — Nokia Series 40v3, Sony Ericsson K750/W800i
  * **240x320** (QVGA) — higher-end phones (Sony Ericsson K800, Nokia N95)

By 2013, over 3 billion Java ME-enabled phones were in circulation. Major publishers like [[wp>Gameloft|Gameloft]], [[wp>Electronic_Arts|EA Mobile]], [[wp>Digital_Chocolate|Digital Chocolate]], and [[wp>Glu_Mobile|Glu Mobile]] produced hundreds of titles. Notable games include Diamond Rush, the Asphalt series, Prince of Persia mobile, Bounce Tales, Gravity Defied, and many more.

This system scrapes metadata for the "j2me" group(s) and loads the ''j2me'' set from the currently selected theme, if available.
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==== Quick reference ====
  * **Emulator:** [[#retroarch|RetroArch]]
  * **Core:** [[#libretro:_squirreljme|libretro: squirreljme]], [[#libretro:_freej2me|libretro: freej2me]]
  * **Folder:** ''/userdata/roms/j2me''
  * **Accepted ROM formats:** ''.jar''

===== BIOS =====
No official BIOS files are tracked by Batocera's BIOS checker for J2ME.

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The **FreeJ2ME** core requires ''freej2me-lr.jar'' to be placed in ''/userdata/bios/''. This JAR acts as the Java ME runtime that the native libretro shim communicates with. Without it, the freej2me core will not function.

The **SquirrelJME** core does not require any BIOS files.
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===== ROMs =====
Place your J2ME games in ''/userdata/roms/j2me''.

J2ME games are packaged as ''.jar'' files (Java ARchive). Games were often distributed in multiple resolution variants for different phone models. If you have a choice, pick the variant that matches the screen resolution you want to emulate (e.g. 176x220 or 240x320 for best results).

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Large collections of J2ME games can be found on the [[https://archive.org/|Internet Archive]]. Many games are abandonware as their publishers no longer exist or have stopped supporting mobile Java.
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===== Emulators =====
==== RetroArch ====
[[https://docs.libretro.com/|RetroArch]] (formerly SSNES), is a ubiquitous frontend that can run multiple "cores", which are essentially the emulators themselves. The most common cores use the [[https://www.libretro.com/|libretro]] API, so that's why cores run in RetroArch in Batocera are referred to as "libretro: (core name)". RetroArch aims to unify the feature set of all libretro cores and offer a universal, familiar interface independent of platform.

=== RetroArch configuration ===
RetroArch offers a **Quick Menu** accessed by pressing ''[HOTKEY]'' + {{:wiki:south.png?nolink&20|South button (B SNES)}} which can be used to alter various things like [[:advanced_retroarch_settings|RetroArch and core options]], and [[:remapping_controls_per_emulator|controller mapping]]. Most RetroArch related settings can be altered from Batocera's EmulationStation.

=== libretro: squirreljme ===
[[https://github.com/SquirrelJME/SquirrelJME|SquirrelJME]] is a Java ME 8 Virtual Machine written entirely in Java, aiming for 99.9% compatibility with the Java ME standard. It is the default emulator for J2ME in Batocera.

  * No BIOS files required
  * Supports older J2ME software designed for feature phones
  * Pure Java implementation

=== libretro: freej2me ===
[[https://github.com/hex007/freej2me|FreeJ2ME]] is a free, open-source J2ME emulator focused on game compatibility. The libretro core uses a native C shim that communicates via pipes with ''freej2me-lr.jar'' (the actual Java emulator).

  * Requires ''freej2me-lr.jar'' in ''/userdata/bios/''
  * Requires Java runtime on the host system
  * Supports MIDP 1.0 and 2.0 games (2D)
  * Games auto-scale to window size
  * Per-game configuration (resolution, phone type) saved automatically

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[[https://github.com/TASEmulators/freej2me-plus|FreeJ2ME-Plus]] is an actively maintained fork with major improvements including preliminary M3G (Mobile 3D Graphics) support.
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===== Controls =====
J2ME games were designed for phone keypads. The default mapping translates gamepad buttons to phone keys:

^ Batocera RetroPad ^ Phone key ^
| D-pad | Arrow keys / 2-4-6-8 numpad |
| {{:wiki:south.png?nolink&20|South button (B SNES)}} | Soft key / Select / 5 |
| {{:wiki:east.png?nolink&20|East button (A SNES)}} | Soft key / Fire |
| ''[START]'' | Right soft key |
| ''[SELECT]'' | Left soft key |

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Some games may require specific phone key mappings. Use RetroArch's [[:remapping_controls_per_emulator|controller remapping]] to adjust controls per game if needed.
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===== See also =====
  * [[systems:ngage|Nokia N-Gage]] — hybrid phone/gaming device that also ran J2ME games
  * [[systems:psp|PlayStation Portable]] — competing handheld from the same era

===== Troubleshooting =====
==== FreeJ2ME: game doesn't launch ====
Make sure ''freej2me-lr.jar'' is in ''/userdata/bios/''. Also verify that a Java runtime is available on the system. If the game still doesn't work, try SquirrelJME instead.

==== Game runs at wrong resolution ====
J2ME games were made for specific screen sizes. If the game looks stretched or cropped, check the core options for resolution settings. Some games need a specific resolution like 176x220 or 240x320 to display correctly.

==== Game controls don't work ====
J2ME games used various phone key layouts. If the default mapping doesn't work for a particular game, use RetroArch's Quick Menu to remap the controls.

==== Further troubleshooting ====
For further troubleshooting, refer to the [[:support|generic support pages]].