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Minijava compiler Javabien

The minijavac compiler.

A compilation project for Third year students of Telecom Bretagne.

'ocamlbuild Main.byte' (or native) to build the compiler. The main file is Main/, it should not be modified. It opens the given file, creates a lexing buffer, initializes the location and call the compile function of the module Main/ It is this function that you should modify to call your parser.

'ocamlbuild Main.byte -- ' (or native) to build and then execute the compiler on the file given. By default, the program searches for file with the extension .java and append it to the given filename if it does not end with it.

If you want to reuse an existing ocaml library. Start by installing it with opam. For example, to use colored terminal output you use 'opam install ANSITerminal'. Then you must inform ocamlbuild to use the ocamlfind tool: ocamlbuild -use-ocamlfind Main.byte -- tests/ and you must modify your _tags file to declare the library: true: package(ANSITerminal)

The Lexer/Parser is incomplete but should be ok for phase2. It contains a remaining conflict: a conflict between expression and declaration of variable in statements that could be solved at the price of a much more complex grammar... Here the behavior of choosing shift should be ok.


The project can be quickly built with the provided Makefile:

Build the Main binary:


Some toy programs are available under the Test/programs/ folder they should all run with the Javabien compiler, but not necessary with javac compiler.

# HelloWorld
./Main.byte Test/programs/

# Implementation of a stack using a linked list:
./Main.byte Test/programs/

# Program that does a linear regression on a given method:
./Main.byte Test/programs/

# Reimplementation of the `cat` gnu util:
./Main.byte Test/programs/ Makefile

# Simple HttpServer that serves the content of a folder
./Main.byte Test/programs/ 8000 ~/mywebsite

# See Test/programs/ for the full list


Run crafted test suite:

make test-all

To pin-point a specific problem, tests can be run individually, the list of available tests recipes are available using the following command:

make test-list

Design notes

Memory management


For each program the interpreter creates a stack of memory which allow the java program to store and retrieve variables. A memory is built around three main structures:

  • The data_store: This is an unstructured key-value map that associate an arbitrary key to a memory object. That object can be a Java object, a Java class, a Java variable, a method, a Java array...
  • The reference_store: this structure specifies the association between a Java name (class name, object name...) with an identifier in the data_store. This structure defines if a variable is in scope or not.
  • The names: which is essentially a stack of reference_stores

Both structures are built around an Ocaml Hashtbl, and the implementation is done in a module details located in the in the Utils/ file.

In order to define scopes for variables, the names is built as a stack of reference_stores, when the program enters a new bloc, a new reference_store is pushed on top of the stack and all new variables names will be referenced there.

When searching for a name in scope, the function Memory.get_address_from_name will look through each reference_store in the names stack starting from the top. We call this names stack a transparent stack as it can be written only on top but it can be peaked through.

In order to handle function call, we create another stack of names that is opaque: only the top item can be written and read. In short, the memory model can be summarized using the following diagram:

Program memory model

The following diagram illustrate an example of memory binding:

Program memory model example

Garbage collection

The garbage collection is made by building a seen/not seen tree of objects with the memory stacks as root. We iterate on each name in the stacked memories and reference_stores and set an object as seen or not seen. Once this pass is done each object in the data_store that is not marked as seen is deleted.

This method is slow and have a complexity around O(N) with N equal to the number of objects in the datastore but is good enough for our use case.

The garbage collector is called after each computation of an expression. In order to keep good performances, garbage is actually collected only if there is at least twice as many objects in the data_store than at the end of the last garbage collection.

Performance notices

This memory model has one main caveat: for each memory object retrieved, created or modified we need to make an indirection from the names store to the data_store. This can lead to some penalties.

This is even worse for Arrays in which each element of an array is a reference to an object in the data_store. In which case, iterating over an array require resolving each object which are not necessarily stored in continuous parts of the memory.

Natives and stdlib

When starting a program, the interpreter will load into memory an environment of existing classes. Those classes are described in the stdlib/ folder. We tried to follow the jdk classes, but they are nowhere as complete as the jdk.

In order to handle such classes, we have implemented the Java method modifier: native. When a native method is met, the interpreter will search in an Hashtbl if the method is not referenced in Ocaml.

One example of Class with native methods is the Debug class which is used to debug the interpreter. The static java method Debug.dumpMemory() will print the state of the program memory in place. The static java method Debug.debug(var) will print a java name as it is seen in memory. Both methods can be used anywhere in a program.

Typing tests

There are two recipes dedicated to the typing suite:

  • make test-ShouldWorkTypeTest.byte: this suite will try to type the files located in the Test/typing/should_work folder and verify that every file here are correctly typed.
  • make test-ShouldFailTypeTest.byte: this suite will try to type the files located in the Test/typing/should_fail folder and check that each file fail the typing phase, however it will not check why the test failed.

Evaluation tests

Using the recipe make test-EvalUnitTest.byte it is possible to run a unit test suite against a list of files located in the Test/eval_units/ folder. These tests will check that the outputs of Debug.debug(Object o) matches with the comment lines starting with //: located at the top of the file. For example the following test will pass:

//: 1

class Main {
        static void main() {

While the following one will fail

//: 5

class Main {
        static void main() {

This test suite allows us to verify that the output of a script matches the expected one.

To launch a specific unit test, the main-class argument can be used:

./Main.byte -skip-tc -main-class Main Test/eval_units/

Features implemented

Here are the lists of features that we planned and/or implemented. We do not aim to implement them all but this list works well as a TODO list of features we might want to integrate in JavaBien.

AST evaluation


  • Primitives types
    • int
    • bool
    • float
    • double
    • Char
    • String
  • Simple arithmetic operations
  • Simple logic operations
  • Postfix operations
  • Prefix operations
  • Variable declaration
  • Variable assignation
  • null element
  • Arrays
    • Creation
    • Assignation
    • Element access
  • Casting
  • Exceptions
  • Object unboxing

Class language

  • Class declaration
  • Method declaration
  • Attributes declaration
  • Attribute Access
  • Static Attribute declaration
  • Static Attribute Access
  • Inheritance
  • Native Method declaration
  • Custom class constructor
  • Subclasses

Control flow

  • If, else if, else
  • Block
  • while loop
  • For loop
  • Method call
  • Method return
  • Method overload
  • Inheritance


  • Simple memory heap
  • Simple memory stack
  • Unnamed pointers
  • Garbage collection


  • File open, read, write
  • Socket create, bind, accept, read, write


  • Inline printing
  • Inline memory dump
  • Program arguments
  • Simple STD
  • Threads and multithreading control
  • Imports
  • Informative errors

Type checking

You can find a non-exhaustive list of the compile-time errors cases specified by the specification in the file TypeErrors and whether we handle them or not. Note that this file may not be up to date with our current implementation.

Our type-checking environment uses two HashTbl (the one provided in Utils/ It is defined as:

    classes_env = classes_env;
    exec_env = Env.initial();
    current_class = "";
    current_method = None
  • classes_env is a HashTbl where classes info is stored as javaclass with their name as key. This Class environment is populated in TypingEnv. When populating the environment, some checks are done on different things like modifiers combination for instance.

  • The exec_env contains the environment of the method that is being checked, as string,type pairs, each pair corresponding to a local variable, the string being its name and the type its type.

  • current_class is the name of the class being checked.

  • current_method is the javamethod (see in Typing/ being checked.

In Typing/ you will find the different exceptions that are raised in a case where a compile-time error should occur.

We have not implemented type checking for the Throw and Try statements. As for expressions, we do not handle casts, condop and classof.

We do not handle the static contexts and accesibility for the classes. We handle basic inheritance and private attributes.

Parser bugs and fixes

We have noticed some errors in the built AST, they are listed below and are fixed when we needed. Some patches might be merged back upstream for next years.

  • b = true && true; (also <= < > >=) This doesn't respect operators’ precedence. AST:
└─ Op
   ├─ Expression
   │  └─ AssignExp
   │     ├─ Expression
   │     │  └─ Name
   │     │     └─ b
   │     ├─ =
   │     └─ Expression
   │        └─ Val
   │           └─ Boolean
   │              └─ true
   ├─ &&
   └─ Expression
      └─ Val
         └─ Boolean
            └─ true

Expected AST:

└─ AssignExp
   ├─ Expression
   │  └─ Name
   │     └─ b
   ├─ =
   └─ Expression
      └─ Op
         ├─ Expression
         │  └─ Val
         │     └─ Boolean
         │        └─ true
         ├─ &&
         └─ Expression
            └─ Val
               └─ Boolean
                  └─ true
  • int notseenasanarray[]; this should be of type int[]. But int[] array; works Fixed in 386aa089a259ed950709cd292c15254a9872b851
├─ Modifiers :
├─ notseenasanarray
├─ int
└─ none
  • two[] = one; This should not be parsed; The expression between [] should be mandatory Fixed in 386aa089a259ed950709cd292c15254a9872b851

  • one = two[]; Same as above Fixed in 386aa089a259ed950709cd292c15254a9872b851

  • emptyInit = new int[][1][]{1, 2}; This should not be parsed. Even -v output is wrong Fixed in 88aa9f0265a5274cef0a1768755d673411428791

arr = new int[2][][4]{3, 4};

-v output:
arr = {3,4}.int[2][][4];
  • Char literal are not parsed correctly Fixed in 5475cacae3030bd5bc3c96b0b3387fb2038f475a (not fixed: escaped chars like \n)

  • double d = -2.0 + 4.0; This doesn't respect operators precedence. AST :

└ VarDecl
   └ Element
      ├ double
      ├ d
      └ Expression
         └ Pre
            ├ -
            └ Expression
               └ Op
                  ├ Expression
                  │  └ Val
                  │     └ Float
                  │        └ 2.0
                  ├  +
                  └ Expression
                     └ Val
                        └ Float
                           └ 4.0

Expected AST :

└ VarDecl
   └ Element
      ├ double
      ├ d
      └ Expression
         └ Op
            ├ Expression
            │   └ Pre
            │      ├ -
            │      └ Expression
            │         └ Val
            │            └ Float
            │               └ 2.0
            ├  +
            └ Expression
               └ Val
                  └ Float
                     └ 4.0