JJTree

JJTree is a preprocessor used with the Java Compiler Compiler (JavaCC) to facilitate the construction of Abstract Syntax Trees (ASTs) during parsing. It provides a separate tree grammar language that lets developers specify the desired AST structure independently from the underlying lexical and parsing rules. When JJTree processes a tree grammar, it translates it into a standard JavaCC grammar file and injects code to build the AST according to the defined tree layout.

Historically, JJTree originated as part of the JavaCC project, developed by Terence Parr in the 1990s. It is distributed alongside JavaCC and has evolved with various releases of the toolchain. JJTree is widely used in compiler construction, interpreters, and other language-processing tasks, where clear and maintainable AST construction is important.

In practice, a developer writes a tree grammar that describes how tokens and productions should appear in the AST, including node labeling and the relationships between parent and child nodes. JJTree then generates the necessary JavaCC-based parser code and supporting classes to produce an AST during parsing. Common features include labeled tree nodes, handling of optional and repeated children, and integration with the generated node classes.

The generated AST typically relies on a base node class, such as SimpleNode, with concrete node types defined by the grammar. This separation of concerns—the parser logic in JavaCC and the tree structure in JJTree—helps reduce boilerplate and clarifies the organization of linguistic data. JJTree remains a recognized approach in educational contexts and in projects that require structured, navigable parse trees.