Which framework is commonly used for Java development?

The Spring framework is commonly used for Java development because it provides a comprehensive programming and configuration model for Java applications. It facilitates the creation of enterprise-level applications by offering a wide array of features such as dependency injection, aspect-oriented programming, transaction management, and support for building web applications through Spring MVC. It promotes good design practices and helps developers to build scalable, maintainable, and flexible applications.

Unlike other choices, the Spring framework is specifically designed and optimized for Java. The Android framework, while also related to Java, is particular to mobile app development on Android devices. Flask and Django, on the other hand, are frameworks used for Python web development and do not apply to Java. Therefore, Spring is the most relevant and commonly used framework among Java developers.