Libgdx Lwjgl3 Desktop

libGDX LWJGL3 Desktop Backend

Reference for the LWJGL3 desktop launcher, window configuration, fullscreen/windowed management, desktop lifecycle behavior, file access, and common gotchas.

LWJGL3 is the current desktop backend. The old LwjglApplication / LwjglApplicationConfiguration classes are the legacy LWJGL2 backend — do NOT use them. Always use Lwjgl3Application and Lwjgl3ApplicationConfiguration.

Launcher Class

The desktop launcher lives in the desktop module, not in core. It contains the main() method.

package com.example.game.lwjgl3;

import com.badlogic.gdx.backends.lwjgl3.Lwjgl3Application;
import com.badlogic.gdx.backends.lwjgl3.Lwjgl3ApplicationConfiguration;
import com.example.game.MyGame;

public class DesktopLauncher {
    public static void main(String[] args) {
        Lwjgl3ApplicationConfiguration config = new Lwjgl3ApplicationConfiguration();
        config.setTitle("My Game");
        config.setWindowedMode(1280, 720);
        config.useVsync(true);
        config.setForegroundFPS(60);
        config.setWindowIcon("icons/icon128.png", "icons/icon32.png", "icons/icon16.png"); // JPEG, PNG, or BMP
        config.setIdleFPS(15); // reduce CPU when unfocused/iconified
        new Lwjgl3Application(new MyGame(), config);
    }
}

Never put main() in the core module. Core is cross-platform; the launcher is platform-specific.

Configuration Quick Reference

| Method | Purpose | |---|---| | setTitle(String) | Window title bar text | | setWindowedMode(w, h) | Initial window size in pixels | | useVsync(boolean) | Sync frame rate to monitor refresh rate | | setForegroundFPS(int) | Software cap on render loop FPS | | setWindowIcon(String...) | Window/taskbar icons — JPEG, PNG, or BMP | | setIdleFPS(int) | Render rate when unfocused/iconified (default 60) | | setResizable(boolean) | Allow user to resize window | | setDecorated(boolean) | Show/hide title bar and borders | | setFullscreenMode(DisplayMode) | Start in fullscreen | | setBackBufferConfig(r,g,b,a,depth,stencil,samples) | Color/depth/stencil bits and MSAA | | pauseWhenLostFocus(boolean) | When true, fires pause()/resume() on focus loss/gain (not just iconify). Default: false | | setWindowListener(Lwjgl3WindowListener) | Window event callbacks (focus, iconify, file drop) |

setForegroundFPS() vs useVsync()

These are independent controls:

• useVsync(true) — Syncs buffer swaps to the monitor's refresh rate (e.g., 60Hz). Eliminates screen tearing. Hardware-driven.
• setForegroundFPS(n) — Software cap: sleeps between frames to limit render() calls to n per second. Works independently of vsync.

Use both together, or either alone:

• useVsync(true) + setForegroundFPS(60) — Vsync for smoothness, FPS cap as a safety net
• useVsync(false) + setForegroundFPS(120) — No vsync, software-limited to 120fps
• useVsync(true) + setForegroundFPS(0) — Vsync only, no software cap

setForegroundFPS(0) means unlimited — the render loop spins as fast as possible, using 100% CPU. Always set a reasonable cap unless you have a specific reason not to.

setIdleFPS(n) — Controls the render rate when the window is unfocused or iconified. Default is 60. Since render() continues running even when the window is minimized, use setIdleFPS(15) or similar to reduce background CPU usage without stopping the loop entirely.

Back Buffer Configuration

setBackBufferConfig(r, g, b, a, depth, stencil, samples) controls the color depth, depth buffer, stencil buffer, and MSAA samples for the OpenGL context. The default config has no stencil buffer. If you use FrameBuffer with stencil operations, you must request stencil bits here — otherwise stencil writes silently fail:

// Request 8-bit stencil buffer (default has 0)
config.setBackBufferConfig(8, 8, 8, 8, 16, 8, 0);
//                         R  G  B  A  depth stencil MSAA

Fullscreen and Windowed Mode

Start in fullscreen:

config.setFullscreenMode(Lwjgl3ApplicationConfiguration.getDisplayMode());

Toggle at runtime:

// To fullscreen
Gdx.graphics.setFullscreenMode(Gdx.graphics.getDisplayMode());

// To windowed
Gdx.graphics.setWindowedMode(1280, 720);

Monitor enumeration:

Graphics.Monitor[] monitors = Lwjgl3ApplicationConfiguration.getMonitors();
Graphics.DisplayMode[] modes = Lwjgl3ApplicationConfiguration.getDisplayModes(monitors[0]);

Desktop Lifecycle: pause() / resume()

| Event | What happens | |---|---| | Window minimized/iconified | pause() fires | | Window restored from taskbar | resume() fires | | Window loses focus (Alt+Tab) | Nothing — pause() does NOT fire | | Window closed | pause() → dispose() | | render() while iconified | Continues running |

Key difference from Android: On desktop, pause() fires ONLY on iconification (minimize), NOT on focus loss. The render loop keeps running even when the window is iconified or unfocused. To detect focus loss, use Lwjgl3WindowListener (see below). To receive pause()/resume() on focus loss (matching Android behavior), call config.pauseWhenLostFocus(true). This is false by default.

Window Listener

Lwjgl3WindowListener provides callbacks for window events that ApplicationListener doesn't cover. Register via config.setWindowListener(). Use Lwjgl3WindowAdapter to override only what you need:

config.setWindowListener(new Lwjgl3WindowAdapter() {
    @Override public void focusLost() { paused = true; }
    @Override public void focusGained() { paused = false; }
});

Available callbacks: focusLost(), focusGained(), iconified(boolean), maximized(boolean), filesDropped(String[]), closeRequested() (return false to cancel close).

filesDropped() enables drag-and-drop file support — a desktop-only feature useful for editors and tools. Do NOT use raw GLFW calls (e.g., glfwSetWindowFocusCallback) — use Lwjgl3WindowListener instead.

Multi-Window Support

LWJGL3 supports multiple windows — each gets its own ApplicationListener:

Lwjgl3Application app = (Lwjgl3Application) Gdx.app;
Lwjgl3WindowConfiguration windowConfig = new Lwjgl3WindowConfiguration();
windowConfig.setTitle("Debug Window");
windowConfig.setWindowedMode(400, 300);
app.newWindow(new MyDebugListener(), windowConfig);

All windows update on the same thread, sequentially. Gdx.graphics / Gdx.input point to whichever window is currently being updated. This is desktop-only — do not use in cross-platform core code.

File Access on Desktop

| Method | Reads from | Writable | Notes | |---|---|---|---| | Gdx.files.internal("x") | assets/ on classpath | No | Bundled game assets | | Gdx.files.local("x") | Working directory | Yes | Depends on where JVM launched from | | Gdx.files.external("x") | User home directory | Yes | ~/ on Linux/macOS, C:\Users\\ on Windows |

Working directory gotcha: Gdx.files.local() resolves relative to the JVM working directory. In IntelliJ or Gradle run configurations, you must explicitly set the working directory to the assets/ folder (or core project root) — otherwise local file paths will resolve to unexpected locations.

Clipboard

// Read
String text = Gdx.app.getClipboard().getContents();
// Write
Gdx.app.getClipboard().setContents("copied text");

Custom Cursor

// System cursor
Gdx.graphics.setSystemCursor(Graphics.Cursor.SystemCursor.Crosshair);

// Custom cursor from Pixmap
Pixmap pm = new Pixmap(Gdx.files.internal("cursor.png"));
Cursor cursor = Gdx.graphics.newCursor(pm, hotspotX, hotspotY);
Gdx.graphics.setCursor(cursor);
pm.dispose(); // Pixmap can be disposed after creating cursor

Common Mistakes

1. Using LwjglApplication / LwjglApplicationConfiguration — Those are the legacy LWJGL2 backend. Always use Lwjgl3Application / Lwjgl3ApplicationConfiguration.
2. Putting main() in the core module — The launcher class with main() belongs in the desktop module. Core is cross-platform.
3. Assuming focus loss triggers pause() — On LWJGL3 desktop, only iconification (minimize) triggers pause(). Alt+Tab / focus loss does nothing.
4. Setting setForegroundFPS(0) without knowing the effect — Zero means unlimited. The CPU will spin at 100%. Always set a cap.
5. Expecting Gdx.files.local() to find files in the assets folder — local() uses the working directory, which may not be assets/. Set the working directory in your IDE/Gradle run config, or use internal() for read-only assets.
6. Assuming render() stops when the window is minimized — On desktop, render() continues running even when iconified. Check pause state explicitly if you need to stop updates.
7. Using LWJGL2-era Display or Monitor APIs — Use Lwjgl3ApplicationConfiguration.getMonitors() and getDisplayModes() for monitor enumeration.
8. Not setting setIdleFPS() and wondering why the game uses 100% CPU when minimized — render() keeps running at setForegroundFPS rate even when iconified. Use setIdleFPS(15) or similar to reduce background CPU usage.
9. Using raw GLFW calls or polling to detect focus loss instead of Lwjgl3WindowListener — libGDX provides focusLost() and focusGained() callbacks via Lwjgl3WindowAdapter. Don't go behind the framework with GLFW.glfwSetWindowFocusCallback() or direct LWJGL3 imports.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: kyu-n
• Source: kyu-n/gdx-claude-skills
• License: MIT

Install and usage instructions live in the source repository linked above.