ANTLR4: How to inject tokens - lexer

Question

I'm trying to implement a preprocessor for a DSL, modeled after the CPP example in code/extras. However, I'm not using token factory. Is one required? Calling emit(token) does not inject the tokens into the tokens stream as expected.
Here's the lexer:
// string-delimited path
SPATH : '"' (~[\n\r])*? '"'
{
emit(); // inject the current token
// launch another lexer on the include file, get tokens,
// emit them all at once here
List<CommonToken> tokens = Preprocessor.include(getText());
if (null != tokens) {
for (CommonToken tok : tokens) {
emit(tok);
}
}
}
;
Here's the include method:
#SuppressWarnings("unchecked")
public static List<CommonToken> include(String filename) {
List<CommonToken> tokens = null;
try (FileReader fr = openFile(filename.substring(1, filename.length() - 1));
BufferedReader br = new BufferedReader(fr)) {
ANTLRInputStream input = new ANTLRInputStream(br);
PreprocessorLexer lexer = new PreprocessorLexer(input);
tokens = (List<CommonToken>) lexer.getAllTokens();
} catch (IOException ioe) {
log.error("Can't load ~{}~", ioe.getLocalizedMessage());
}
return tokens;
}

Answer 1

You need to override Lexer.nextToken to provide this feature. In your lexer, keep a Deque<Token> of injected tokens that have not yet been returned by nextToken. When the queue is empty, your implementation of nextToken should return the next token according to the superclass implementation.
Here's some sample code. I have not tried to compile or run it so it might not be perfect.
private final Deque<Token> pendingTokens = new ArrayDeque<>();
#Override
public Token nextToken() {
Token pending = pendingTokens.pollFirst();
if (pending != null) {
return pending;
}
Token next = super.nextToken();
pending = pendingTokens.pollFirst();
if (pending != null) {
pendingTokens.addLast(next);
return pending;
}
return next;
}