ANTLR3 Tree Grammar: loop did not match anything at input 'EOF' - antlr

Question

I am trying to create my first ANTLR3 tree grammar, but I keep hitting the same problem. The output of the parser is:
$ echo 'foo, bar' | ./run.sh
foo bar
TreeGrammar.g: node from line 0:0 required (...)+ loop did not match anything at input 'EOF'
Exception in thread "main" java.lang.NullPointerException
at Driver.main(Driver.java:29)
The output clearly shows that the stage-1 parser results in the right tokens ('foo' and 'bar'). Somehow the stage-2 tree-parser refuses to parse the results from stage-1. Since the code is very basic, it must be some simple, dumb oversight at my part ;-)
Here's my simple test code:
Grammar.g:
grammar Grammar;
options {
output = AST;
}
statement: word (','! word)* EOF!;
word: ID;
ID: ('a'..'z'|'A'..'Z')+;
WS: (' ' | '\t' | '\n' | '\r')+ { $channel = HIDDEN; } ;
TreeGrammar.g:
tree grammar TreeGrammar;
options {
tokenVocab = Grammar;
ASTLabelType = CommonTree;
output = template;
}
statement: word+;
word: ID;
Driver.java:
import java.io.*;
import org.antlr.runtime.*;
import org.antlr.runtime.tree.*;
import org.antlr.stringtemplate.*;
public class Driver {
public static void main(String[] args) throws Exception {
FileReader groupFileR = new FileReader("Template.stg" );
StringTemplateGroup templates = new StringTemplateGroup(groupFileR);
groupFileR.close();
ANTLRInputStream input = new ANTLRInputStream(System.in);
GrammarLexer lexer = new GrammarLexer(input);
CommonTokenStream tokens = new CommonTokenStream(lexer);
GrammarParser parser = new GrammarParser(tokens);
GrammarParser.statement_return result = parser.statement();
CommonTree t = (CommonTree)result.getTree();
System.out.println(t.toStringTree());
CommonTreeNodeStream nodes = new CommonTreeNodeStream(t);
nodes.setTokenStream(tokens);
TreeGrammar walker = new TreeGrammar(nodes);
walker.setTemplateLib(templates);
walker.statement();
TreeGrammar.statement_return r2 = walker.statement();
StringTemplate output = (StringTemplate) r2.getTemplate();
System.out.println(output.toString());
}
}

Answer 1

Assuming your Stringtemplate groups is properly formed, your problem is most probably the fatc you walk your AST twice:
walker.statement();
TreeGrammar.statement_return r2 = walker.statement();
E.g., you call walker.statement() twice. This is what the (first) error is telling you:
TreeGrammar.g: node from line 0:0 required (...)+ loop did not match anything at input 'EOF'
You consume the input once with walker.statement() resulting the node stream is at the end (EOF), and then you call walker.statement() again and it expects tow walk word+ again, yet there's only a EOF left.