[antlr-interest] NullPointerException getting text from a rule

[Curtis Clauson]

In a tree parser rule action, to use the ".text" attribute with a 
matched element reference, you must provide the original TokenStream 
from which the tree was made. If you do not, then input.getTokenStream() 
will return null and you get that NullPointerException.

In your main() method, add this line
     nodes.setTokenStream(tokens);
after you create the NodeStream.

This grammar will then work correctly with the "from fred" source text. 
(tested AntLR v3.0.1)

-- Curtis


Travis Jensen wrote:
> I have the following action associated with my language:
> 
> selectStatement   
>     :    'from' type (localeClause)? (whereClause)?    { 
> contentType=$type.text; }
>     ;
> 
> If I use "from fred" as input, the rule is matched appropriately, but I 
> get an NPE when the action is run.  Breaking down the location of the 
> NPE, I find that my token stream (referenced at "contentType= 
> input.getTokenStream().toStream(...)") is null.
> 
> The very basic driver that I'm using looks like this, which is taken 
> pretty much verbatim from the ANTLR reference calculator example:
> 
> public class Test {
>     public static void main(String[] args) throws Exception {
>         // Create an input character stream from standard in
>         ANTLRInputStream input = new ANTLRInputStream( System.in 
> <http://System.in>);
>         // Create an ExprLexer that feeds from that stream
>         cqlLexer lexer = new cqlLexer(input);
>         // Create a stream of tokens fed by the lexer
>         CommonTokenStream tokens = new CommonTokenStream(lexer);
>         // Create a parser that feeds off the token stream
>         cqlParser parser = new cqlParser(tokens);
>         // Begin parsing at rule prog, get return value structure
>         cqlParser.query_return r= parser.query();
> 
>         // Walk the resulting tree
> 
>         // Get the tree from the parser
>         CommonTree t = (CommonTree)r.getTree();
>         // Create a tree node stream from the resulting tree
>         CommonTreeNodeStream nodes = new CommonTreeNodeStream(t);
>         // Create a tree walker
>         cqlVcm walker = new cqlVcm(nodes);
>         // launch at the start rule
>         walker.query();
>     }
> }
> 
> Just for completeness sake, my entire tree parsing grammar is:
> 
> tree grammar cqlVcm;
> 
> options {
>     tokenVocab=cql;
>     ASTLabelType=CommonTree;
> }
> 
> @header {
> import java.util.HashMap;
> import java.util.Map;
> }
> 
> @members {
> Map attributes = new HashMap();
> String contentType = null;
> }
> 
> query
>     :    (expr)+
>         {
>         StringBuilder qry=new StringBuilder();
>         qry.append("select * from ");
>         qry.append(contentType);
>         if (attributes.size() > 0)
>             qry.append(" where");
>         for (Object key:attributes.keySet())
>             qry.append(key).append('=').append(attributes.get(key));
>         System.out.println(qry);
>         }
>     ;
>    
> expr
>     :    selectStatement
>     ;
>    
> selectStatement   
>     :    'from' type (localeClause)? (whereClause)?    { 
> contentType=$type.text; }
>     ;
> 
> localeClause
>     :    ^('locale' QUOTED_STRING)            { attributes.put("locale", 
> $QUOTED_STRING.text); }
>     ;
>    
> whereClause
>     :    ^('where' whereCompare)
>     ;
>    
> whereCompare
>     :    ^('=' column QUOTED_STRING)            
> {attributes.put($column.text, $QUOTED_STRING.text); }
>     ;
>    
> type
>     :    ID
>     ;
> 
> column
>     :    ID
>     ;
> 
> Help appreciated. :)
> 
> tj
> -- 
> Travis Jensen
> travis.jensen at gmail.com 
> <mailto:travis.jensen at gmail.com>
> http://cmssphere.blogspot.com/
> Software Maven * Philosopher-in-Training * Avenged Nerd