There are many different things that can be extended in ExtendJ. Consequently, it may be difficult to figure out where to make changes in order to implement your extension. This page tries to help by giving a short introduction to different extension points in ExtendJ.
For a high-level overview of the design of ExtendJ, and possible extension points, take a look at these presentation slides.
If you are completely new to ExtendJ it is recommended that you first take a look at the SPLASH 2015 tutorial for ExtendJ. It is a good idea to return to this page after you have followed the tutorial.
This page is a work in progress. More information will be added over time. Please feel free to contribute - use the "View Source" link at the bottom of this page.
There exist a few minimal demo projects for how to build ExtendJ extensions:
Getting used to JastAdd
ExtendJ is built using JastAdd, and working with ExtendJ requires knowing a bit about how JastAdd attributes work. There are many resources for learning about JastAdd. Here are a few good references:
The notation for JastAdd attributes is documented in the JastAdd reference manual.
The following sections of this page show different kinds of extension points that your extension can hook into.
In general, there are extension for these parts of the compiler:
The scanner and parser are quite limited in their extensibility. Much more freedom is available for modifying the analysis and code generation. The goal is to show multiple different kinds of extensions below. More are to be added soon!
Most language extensions add some new syntax elements such as new operators. Adding syntax requires modifying the scanner and/or parser specifications.
Here is an example of how to add scanning and parsing for a simple version of Groovy's spread operator to ExtendJ:
Scanner file (
Parser file (
Terminals (also called tokens) are implicitly generated from the parser
specification. Any identifier in a parser rule that is not matched by a
nonterminal will be added to the terminals set. The unique identifier for each
terminal is accessed in the scanner via the class
Note that the semantic actions in the parser file build a new AST node called
Abstract grammar (
More additions need to be made to handle type analysis and error checking for the new operator. Examples of how to handle this will be shown below in more detail (TBD).
Extending Type Analysis
Adding a spread expression to Java requires defining the type of a spread
expression so that it can work in the existing type analysis framework. The
type of each expression in ExtendJ is defined by the
The type of a spread expression is a collection type containing elements of the type of the variable or method on the right hand side of the spread operator. Here is an example of computing this using an attribute:
The code uses a helper attribute
The other attributes used in