J%: Integrating Domain Specific Languages with Java

The idea

Embedding SQL in Java through JDBC means handing the compiler a string. It cannot check the query, so syntax errors surface at run time as a driver exception — and if the test suite never executes that query, they do not surface at all. J% closes that gap generically: a DSL is embedded in a special class declaration, keeps its own syntax, and comes with a library used both at compile time and at run time.

What J% adds to Java

• Compile-time syntax checking of the embedded DSL, with errors reported as compile errors.
• Static typing across the boundary — Java variables appearing inside DSL code are type-checked.
• Metaprogramming rather than translation. The DSL is not rewritten into Java; it generates code that drives the existing Java API for that domain — JDBC for SQL. This is the main difference from metaprogramming systems that transform the DSL into the host language.
• One container for every DSL, so the grammar does not grow a custom extension per language.
• Extensibility — a developer can add a DSL module through a defined API.

The prototype was a pre-processor translating J% source into ordinary Java, following the extensible compiler/interpreter pattern; the Java 1.5 grammar gains one new class form and a notation for external references.

Where it sits

The system is J%, developed at length in the thesis and in the 2015 journal paper.