Code Generation

Overview

Joist uses two types of code generation:

• joist.domain uses “build time” code generation ran manually by the developer
• joist.web (via bindgen) uses “design time” code generation ran automatically by the IDE

Why Build Time

joist.domain’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 Design Time

joist.web’s code generation is well suited to design-time code generation because it:

• Uses quickly changing meta-data, the source code, so needs to run continuously
• Uses codebase meta-data from the APT API, so has very little gathering cost

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

• Both joist.domain and joist.web (via bindgen) use joist.util’s sourcegen.

  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.domain generates meta-data that drives the joist.domain 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.domain implementation to leverage.