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Why do my instance variables have to be private?

In Tapestry 5.3.1 and earlier all instance variables must be private. Starting in version 5.3.2 instance variables can also be protected or package private (that is, not public), or they can even be public if final or annotated with deprecated @Retain.

Tapestry does a large amount of transformation to your simple POJO classes as it loads them into memory. In many cases, it must locate every read or write of an instance variable and change its behavior; for example, reading a field that is a component parameter will cause a property of the containing page or component to be read.

Limiting fields to private means that Tapestry can Restricting the scope of fields allows Tapestry to do the necessary processing one class at a time, as needed, at runtime. More complex Aspect Orient Programming systems such as AspectJ can perform similar transformations (and much more complex ones), but requires they require a dedicated build step (or the introduction of a JVM agent).

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