Parsing some Java code with Python using ANTLR

juuso :

I want to build a Java parser using ANTLR in Python.

I downloaded the grammars from the ANTLR repository:

Lexer :https://github.com/antlr/grammars-v4/blob/master/java/java/JavaLexer.g4

Parser: https://github.com/antlr/grammars-v4/blob/master/java/java/JavaParser.g4

Then I used my script.bat to generate the python code I need :

java -jar antlr-4.8-complete.jar -Dlanguage=Python3 Java8Lexer.g4
java -jar antlr-4.8-complete.jar -Dlanguage=Python3 Java8Parser.g4

antlr-4.8-complete.jar downloaded here: https://www.antlr.org/download/antlr-4.8-complete.jar

This generated this list of files:

• Java8Lexer.interp
• Java8Lexer.py
• Java8Lexer.tokens
• Java8Parser.interp
• Java8Parser.py
• Java8Parser.tokens
• Java8ParserListener.py

Then I wrote this code to parse a java file:

import antlr4
from antlr4 import *
from java.antlr_unit2 import Java8Parser, Java8Lexer

def main():
    code = open('test.txt', 'r').read()
    lexer = Java8Lexer.Java8Lexer(antlr4.InputStream(code))
    stream = antlr4.CommonTokenStream(lexer)
    parser = Java8Parser.Java8Parser(stream)
    tree = parser.expression()
    print (tree)

if __name__ == '__main__':
    main()

My test java code test.txt is something like this:

package org.jabref.gui.fieldeditors;
import java.util.ArrayList;
/**
 * This class contains some code
 */
public class TextInputControlBehavior {

    private static final boolean SHOW_HANDLES = Properties.IS_TOUCH_SUPPORTED && !OS.OS_X;

}

Since this is too short, here is an example of code I want to parse: https://pastebin.com/KNxfasKQ

When I run this code I get this :

line 1:0 extraneous input 'package' expecting {'boolean', 'byte', 'char', 'double', 'float', 'int', 'long', 'new', 'short', 'super', 'this', 'void', IntegerLiteral, FloatingPointLiteral, BooleanLiteral, CharacterLiteral, StringLiteral, 'null', '(', '!', '~', '++', '--', '+', '-', Identifier, '@'}
[]

Am I doing something wrong? I didn't wrote the grammar, I just took it from ANTLR repo.

EDIT: Pavel Smirnov's answer helped me and now I don't get the warning. But now the program seems really slow and i get an empty tree as output.

SOLVED: I was printing tree but I had to print(tree.toStringTree(recog=parser))

So the final code is:

import antlr4
from antlr4 import *
from java.antlr_unit2 import Java8Parser, Java8Lexer

def main():
    code = open('test.txt', 'r').read()
    lexer = Java8Lexer.Java8Lexer(antlr4.InputStream(code))
    stream = antlr4.CommonTokenStream(lexer)
    parser = Java8Parser.Java8Parser(stream)
    tree = parser.compilationUnit()
    print(tree.toStringTree(recog=parser))

if __name__ == '__main__':
    main()

Pavel Smirnov :

Your text file contains a compilationUnit, not an expression you try to parse with

tree = parser.expression()

Look carefully through the parser rules, the rule you need is

compilationUnit
    : packageDeclaration? importDeclaration* typeDeclaration* EOF
    ;

which has to be called as

tree = parser.compilationUnit()