如何建立访问者访问和打印与antlr 4？

Question

我用eclipse的antlr 4为我的语法（表达式）构建了ast， 我想建立访问者访问ast并打印ast ...  我如何建立访客？

Hello.g4：

/**
 * Define a grammar called Hello
 */
grammar Hello;

@parser::header{

    import java.util.Map;
    import java.util.HashMap;
    import java.util.List;
    import java.util.ArrayList;
    import ast.*;
}
@parser::members{
    Map<String,Object> symbolTable=new HashMap<String,Object>();
}
r  : PROGRAM ID BRACKET_OPEN 
    {
      List<ASTNode> body= new ArrayList<ASTNode>();
    }
  (sentence{body.add($sentence.node);})*
 BRACKET_CLOSE 
 {
    for(ASTNode n: body )
    {
        n.excute();
    }
 }

 ;         // match keyword hello followed by an identifier


 sentence returns [ASTNode node]:println{$node=$println.node;};

 println returns [ASTNode node]:PRINTLN  expression SEMICOLON {$node=new Println($expression.node);};
 expression returns [ASTNode node]:
  t1=factor {$node=$t1.node;}
  (PLUS t2=factor{$node= new Addition($node,$t2.node);}
    |MINUS t2=factor{$node= new Sub($node,$t2.node);}
  )*
  ;

  factor returns [ASTNode node]: t1=term {$node=$t1.node;}
  (MULT t2=term{$node= new Multiplication($node,$t2.node);}
    | DIV t2=term{$node= new Div($node,$t2.node);}
  )*
  ;

 term returns [ASTNode node]:
  NUMBER {$node=new Constant(Integer.parseInt($NUMBER.text));}
   | ID {}
   | PAR_OPEN expression {$node=$expression.node;} PAR_CLOSE
   ;


PROGRAM:'program';
VAR:'var';
PRINTLN:'println';
INT :'int';
DOUBLE :'double';
STRING :'string';
MULT :'*';
PLUS :'+';

MINUS:'-';
DIV:'/' ;
ASSIGN:'=';
BRACKET_OPEN : '{';
BRACKET_CLOSE : '}';

PAR_OPEN :'(';
PAR_CLOSE :')';

SEMICOLON:';';

ID : [a-zA-Z_][a-zA-Z0-9_]* ;             // match lower-case identifiers
NUMBER :[0-9]+;

WS : [ \t\r\n]+ -> skip ; // skip spaces, tabs, newlines

ASTNode接口：

package ast;
public interface ASTNode {
    public Object excute();

}

例如附加类：

package ast;
public class Addition implements ASTNode {
    private ASTNode operand1;
    private ASTNode operand2;
    public Addition(ASTNode operand1, ASTNode operand2) {
        super();
        this.operand1 = operand1;
        this.operand2 = operand2;
    }

    @Override
    public Object excute() {
        // TODO Auto-generated method stub
        return (int)operand1.excute() + (int)operand2.excute();
    }

}

主要课程：

import java.io.IOException;
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;

import java.io.*;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class MyServlet
 */
public class MyServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public MyServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request,response);
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    try
    {
     String comment = request.getParameter("preview-form-comment") ;
     request.setAttribute("data", comment);
     ANTLRInputStream input = new ANTLRInputStream( comment);
     HelloLexer lexer = new HelloLexer(input);
     CommonTokenStream tokens = new CommonTokenStream(lexer);
     HelloParser parser = new HelloParser(tokens);
     lexer.removeErrorListener(ConsoleErrorListener.INSTANCE);
     parser.removeErrorListener(ConsoleErrorListener.INSTANCE);
     ParseTree tree = parser.r(); // begin parsing at rule 'r'  http://localhost:8080/MyParser/
    // HelloVisitor visitor= new HelloVisitor();

     response.setContentType("text/html");
     PrintWriter writer =  response.getWriter();
     RequestDispatcher rd=request.getRequestDispatcher("/index.jsp");  
     rd.include(request, response);
    // writer.println("<h4>Your Comment Is :</h4>");
     //ANTLRInputStream input=new ANTLRInputStream(System.in);
    // writer.println("<br><font color=black>"+comment+"</font>");
     writer.println("<br><font color=black>"+tree.toStringTree(parser)+"</font>");
     writer.close();


    }
    catch(Exception exception)
    {
        exception.printStackTrace();    
    }
    }

}