mini-java

A MiniJava compiler implemented in Clojure. This was a semester-long project for the Fall 2014 section of CSC444 at SUNY Oswego. Any MiniJava program according to the grammar specification given by Cambridge University can be compiled down to JVM bytecode. In addition to the Cambridge grammar, my own personal addition to the language -- a recur statement -- is correctly parsed and compiled.

API documentation is provided here.

Running

To run from the JAR, simply download the JAR from the final release, and run with java -jar mini-javac.jar --help.

To build from source, first ensure that Leiningen 2.0 is installed and on your classpath, then run as follows:

$ lein antlr4
$ lein uberjar
$ java -jar target/mini-javac.jar --help
Usage: mini-javac [options] filename

Options:
  -d, --directory DIR     .  Destination directory for class files
      --syntax               Stop after syntax checking
      --static-semantics     Stop after static semantics checking
  -h, --help

Sample MiniJava programs are provided in the samples/ directory. Run the sample-run.sh bash script to compile and run all samples.

Implementation

Source code parsing is accomplished using ANTLR4 to create an AST, according to the grammar specified in src/antlr/MiniJava.g4. Useful messages are output in the event of parse errors, indicating the location of the error and providing some insight into the possible cause, much like javac. A correctly parsed AST is then transformed into a more desirable form using the functions in the mini-java.ast namespace.

Static semantics (type checking and name resolution) was performed on the AST using functions I wrote in the mini-java.static-semantics namespace. This ensures that no errors beyond mere syntax errors survived until compilation time. Errors are reported in a manner that is almost consistent with javac.

After static semantics checking succeeds, the AST is transformed once again, in the mini-java.code-gen namespace, this time into valid JVM bytecode. This process was aided by the use of the robust ASM 5 library. Valid Java .class files are output at the end of this process, which can be run using java. This has been tested on OpenJDK 1.7.0, but in theory it should work on any standard Java implementation starting with JDK 1.1.0.

Language Feature Addition

As part of the project, we were required to choose one addition to make to the language. I chose to make a recur statement for handling tail recursion. The statement resembles the ternary operator from C-family languages, but uses it in a very different way. An example usage of the statement is given in the factorial program below:

public int factorial(int n) {
    return this.factorial_iter(n, 1);
}

public int factorial_iter(int n, int result) {
    recur 0 < n ? (n-1, n*result) : result;
}

The statement is broken up into 3 parts. The first is the predicate, 0 < n, which determines which of the two following parts are to be evaluated. If the predicate is true, the recursive case is evaluated, (n-1, n*result), and the method recurs with the given bindings. If the predicate is false, the base case is evaluated, result, and returned. Since MiniJava only allows return statements at the end of a method, this made it simple to implement tail call optimization. As a result, any use of the recur statement is automatically tail call optimized.

Credit

Daniel Wysocki © 2014 - MIT License

Special thanks goes to Dr. Doug Lea, the instructor who helped make this possible. I'd also like to thank Terence Parr for developing the ANTLR parsing tool which I wouldn't have survived without, the folks from the ObjectWeb consortium for developing ASM which made bytecode generation a breeze, and the Clojure community for creating the language which made writing this compiler a joy.