Visitors

Introduction#

What is the difference between a listener and a visitor? The difference between listener and visitor mechanisms is listener methods are called by the ANTLR-provided walker object, whereas visitor methods must walk their children with explicit visit calls. Forgetting to invoke visit() on a node’s children means those subtrees don’t get visited. In visitor we have the ability to tree walking while in listener you are only reacting to the tree walker.

Example

Grammar Example (Expr.g4)

grammar Expr; 
prog:    (expr NEWLINE)* ;
expr:    expr ('*'|'/') expr

    |    expr ('+'|'-') expr
    |    INT
    |    '(' expr ')'
    ;
NEWLINE : [\r\n]+ ;
INT     : [0-9]+ ;

Generating the visitor

To generate a Visitor, or to disable a visitor for your grammar you use the following flags:

 -visitor            generate parse tree visitor
 -no-visitor         don't generate parse tree visitor (default)

The commandline/terminal command to build your grammar with a visitor will be formatted as shown below, with respect to flag chosen and possible aliases:

java - jar antlr-4.5.3-complete.jar Expr.g4 -visitor
java - jar antlr-4.5.3-complete.jar Expr.g4 -no-visitor

The output will be a parser/lexer with a visitor or no visitor respectively.

Output The output will be ExprBaseVisitor.java and ExprVisitor.java for this example. These are the relevant java files for you to implement visitor functionality. It is often ideal to create a new class and extend the ExprBaseVisitor to implement new visitor functionality for each method.

// Generated from Expr.g4 by ANTLR 4.5.3
import org.antlr.v4.runtime.tree.AbstractParseTreeVisitor;

/**
 * This class provides an empty implementation of {@link ExprVisitor},
 * which can be extended to create a visitor which only needs to handle a subset
 * of the available methods.
 *
 * @param <T> The return type of the visit operation. Use {@link Void} for
 * operations with no return type.
 */
public class ExprBaseVisitor<T> extends AbstractParseTreeVisitor<T> implements ExprVisitor<T> {
    /**
     * {@inheritDoc}
     *
     * <p>The default implementation returns the result of calling
     * {@link #visitChildren} on {@code ctx}.</p>
     */
    @Override public T visitProg(ExprParser.ProgContext ctx) { return visitChildren(ctx); }
    /**
     * {@inheritDoc}
     *
     * <p>The default implementation returns the result of calling
     * {@link #visitChildren} on {@code ctx}.</p>
     */
    @Override public T visitExpr(ExprParser.ExprContext ctx) { return visitChildren(ctx); }
}