Code Generation

Why Build Time

Joist’s code generation is well suited to build-time code generation because it:

• Uses relatively static meta-data, the database schema, so does not need to run continuously
• Uses non-code meta-data, the database schema, that has a small, non-zero gathering cost
• Changes the local database schema so should only happen at the developer’s discretion

Why Not Run Time

Joist avoids runtime code generation because it:

• Delays generation until runtime which makes artifacts unavailable for compile-time type safety
• Makes debugging painful, stepping through dynamic proxies and CGLIB-generated classes
• Slows down startup time, sometimes considerably (see performance)
• Is more complex than generating plain Java source files

Approach

• Joist uses sourcegen, a Java code generation DSL.

  sourcegen provides an abstraction layer of GClass, GMethod, and GField objects for generating source code into an object tree instead of a raw template, mostly ignoring tedious issues like class-level formatting, organizing imports, etc.

  sourcegen also allows decomposition of the generation logic, as a GClass can be passed to different modules for each to decorate, instead of all the logic for a class being commingled in a single template.

• Joist generates meta-data that drives the ORM implementation at runtime, so it avoids generating low-level persistence code.

  This avoids complexities like directly generating SQL strings, but still provides a type-safe meta-data basis for the rest of the application and Joist implementation to leverage.