Catching (and keeping) all comments with ANTLR

Question

I'm writing a grammar in ANTLR that parses Java source files into ASTs for later analysis. Unlike other parsers (like JavaDoc) I'm trying to keep all of the comments. This is difficult comments can be used literally anywhere in the code. If a comment is somewhere in the source code that doesn't match the grammar, ANTLR can't finish parsing the file.

Is there a way to make ANTLR automatically add any comments it finds to the AST? I know the lexer can simply ignore all of the comments using either {skip();} or by sending the text to the hidden channel. With either of those options set, ANTLR parses the file without any problems at all.

Any ideas are welcome.

Answer 1

Section 12.1 in "The Definitive Antlr 4 Reference" shows how to get access to comments without having to sprinkle the comments rules throughout the grammar. In short you add this to the grammar file:

grammar Java;

@lexer::members {
    public static final int WHITESPACE = 1;
    public static final int COMMENTS = 2;
}

Then for your comments rules do this:

COMMENT
    : '/*' .*? '*/' -> channel(COMMENTS)
    ;

LINE_COMMENT
    : '//' ~[\r\n]* -> channel(COMMENTS)
    ;

Then in your code ask for the tokens through the getHiddenTokensToLeft/getHiddenTokensToRight and look at the 12.1 section in the book and you will see how to do this.

Answer 2

first: direct all comments to a certain channel (only comments)

COMMENT
    : '/*' .*? '*/' -> channel(2)
    ;

LINE_COMMENT
    : '//' ~[\r\n]* -> channel(2)
    ;

second: print out all comments

      CommonTokenStream tokens = new CommonTokenStream(lexer);
      tokens.fill();
      for (int index = 0; index < tokens.size(); index++)
      {
         Token token = tokens.get(index);
         // substitute whatever parser you have
         if (token.getType() != Parser.WS) 
         {
            String out = "";
            // Comments will be printed as channel 2 (configured in .g4 grammar file)
            out += "Channel: " + token.getChannel();
            out += " Type: " + token.getType();
            out += " Hidden: ";
            List<Token> hiddenTokensToLeft = tokens.getHiddenTokensToLeft(index);
            for (int i = 0; hiddenTokensToLeft != null && i < hiddenTokensToLeft.size(); i++)
            {
               if (hiddenTokensToLeft.get(i).getType() != IDLParser.WS)
               {
                  out += "\n\t" + i + ":";
                  out += "\n\tChannel: " + hiddenTokensToLeft.get(i).getChannel() + "  Type: " + hiddenTokensToLeft.get(i).getType();
                  out += hiddenTokensToLeft.get(i).getText().replaceAll("\\s", "");
               }
            }
            out += token.getText().replaceAll("\\s", "");
            System.out.println(out);
         }
      }

Answer 3

Is there a way to make ANTLR automatically add any comments it finds to the AST?

No, you'll have to sprinkle your entire grammar with extra comments rules to account for all the valid places comments can occur:

...

if_stat
 : 'if' comments '(' comments expr comments ')' comments ...
 ;

...

comments
 : (SingleLineComment | MultiLineComment)*
 ;

SingleLineComment
 : '//' ~('\r' | '\n')*
 ;

MultiLineComment
 : '/*' .* '*/'
 ;

Answer 4

The feature "island grammars" can also be used. See the the following section in the ANTLR4 book:

Island Grammars: Dealing with Different Formats in the Same File