Exploring QtJambi: A Java Wrapper for Qt GUI Development—Challenges and Insights

I recently experimented with QtJambi, a Java wrapper for the well-known Qt C++ library used to build GUIs. Here are some initial thoughts, remarks and observations:

• Building a QtJambi project can be somewhat challenging. It requires installing the Qt framework, configuring system paths to Qt’s native libraries, and setting proper JVM options. Although it is possible to bundle native libraries within the wrapper JARs, I haven’t tried this yet.
• The overall development approach is clean and straightforward. You create windows or dialogs, add layouts, place widgets (components or controls) into those layouts, configure widgets and then display the window or dialog to the user. This model should feel familiar to anyone with GUI experience.
• Diving deeper, QtJambi can become quite complex, comparable to usual Java Swing development. The API sometimes feels overly abstracted with many layers that could potentially be simplified.
• There is an abundance of overloaded methods and constructors, which can make it difficult to decide which ones to use. For example, the QShortcut class has 34 different constructors. This likely comes from a direct and not fully optimized mapping from the C++ Qt API.
• Like Swing, QtJambi is not thread-safe. All GUI updates must occur on the QtJambi UI thread only. Ignoring this can cause crashes, not just improper UI refresh like in Swing.
• There is no code reuse between Java Swing and QtJambi. Even concepts that appear close and reusable are not shared. QtJambi is essentially a projection of C++ Qt’s architecture and design patterns into Java, so learning it from scratch is necessary even for experienced Swing developers.
• Using AI tools to learn QtJambi can be tricky. AI often mixes Java Swing concepts with QtJambi, resulting in code that won’t compile. It can also confuse Qt’s C++ idioms when translating them directly to Java, which doesn’t always fit.
• Despite being a native wrapper, QtJambi has some integration challenges, especially on macOS. For example, handling the application Quit event works differently and only catching window-close events behaves properly out of the box. In contrast, native Java QuitHandler support is easier and more reliable there, but it doesn't work with QtJambi.
• Mixing Java AWT with QtJambi is problematic. This may leads to odd behaviors or crashes. The java.awt.Desktop class also does not function in this context.
• If you want a some times challenging Java GUI framework with crashes and quirks, QtJambi fits the bill! It brings a lot of power but also some of complexity and instability compared to standard Java UI options.
• There is a GUI builder that works with Qt, but it is possible to use its designs in QtJambi, generating source code or loading designs at runtime. The only issue: the cost starts from $600 per year for small businesses to >$5,000 per year for larger companies.

Notable Applications Built With QtJambi

Notable applications built with QtJambi are few. One example is the Interactive Brokers desktop trading platform (IBKR Desktop), which uses QtJambi for its user interface. 

Beyond this, well-known commercial or open-source projects created specifically with QtJambi are scarce and often not widely publicized. Most QtJambi usage tends to be in smaller-scale or internal tools rather than major flagship applications. This limited visibility can make it challenging to pitch QtJambi adoption to decision-makers.

QtJambi Licensing

QtJambi doesn’t have a separate commercial license; it inherits Qt’s licensing model. Qt can be used under free LGPL/GPL licenses if you comply with their terms, or under paid commercial licenses that provide additional advantages and fewer restrictions. Make sure to check your ability to comply with LGPL/GPL or your need for commercial licensing before proceeding.

Should You Consider QtJambi For Your Desktop Apps?

There are three strong contenders for desktop applications: Java Swing, JavaFX, and QtJambi. There is also SWT, but I would prefer to avoid it.

If you already have stable, well-functioning Java Swing applications and lack the resources or justification to rewrite them, staying with Swing is usually the best approach. The effort and risk of migrating large, mature codebases often outweigh the benefits unless there is a strong business case.

For new desktop projects, both JavaFX and QtJambi are worth evaluating.

JavaFX is typically safest when you want:

• A well-supported, familiar Java-based framework.
• Easier development, packaging, and deployment.
• Powerful animations, modern UIs, and broad tools and community support.
• Reliable, long-term support for business needs without high-performance or native integration demands.

QtJambi is a strong choice if your application requires:

• Superior graphics performance and efficient rendering.
• A native look and feel across platforms.
• Responsive, complex interfaces, or advanced custom widget support.

Be prepared for a steeper learning curve, more complex build processes, possible native library management, and licensing issues.

Summary

QtJambi is a performant and powerful, yet sometimes complex, Java wrapper for the Qt GUI framework, providing a native look and feel along with a wide range of advanced widgets. While it is well-suited for high-performance, native-like applications, it comes with a steep learning curve, more intricate setup requirements, and limited community support compared to JavaFX or Swing. Despite these challenges, QtJambi is worth considering for developers who need cross-platform consistency, efficient rendering, and access to Qt’s rich feature set.