Antlr4程序中测试代码的enterProg方法重复执行，如何修复？

我为自定义语法编写了测试代码，发现simple_grailTagsImpl类的enterProg方法内的打印语句被执行了两次，重复输出Starting program size=0。相关代码如下：

语法定义 #

grammar simple_grail;

prog: tag_block ;

tag_block: tag*;

/* root definition of any tag in Grail */
tag:
    p | div | canvas | rect | button | txt | textlit;

p: '<' 'p' CLASS_NAME? '>' tag_block '<' '/' 'p' '>' ;
div: '<' 'div' CLASS_NAME? '>' tag_block '<' '/' 'div' '>' ;
canvas: '<' 'canvas' CLASS_NAME? '>' tag_block '<' '/' 'canvas' '>' ;
rect:
     '<' 'rect' CLASS_NAME?
     'x' ASSIGN_COORD 'y' ASSIGN_COORD 'w' ASSIGN_COORD 'h' ASSIGN_COORD
     '/>' ;
button:
    '<' 'button' CLASS_NAME? (NAME)? 'text' ASSIGN_TEXT '/>' ;
txt:
    '<' 'text' 'x' ASSIGN_COORD 'y' ASSIGN_COORD '/>' ;

textlit:
    TEXT
    ;

CLASS_NAME: [a-zA-Z_] [a-zA-Z_0-9]* ;

ASSIGN_COORD: '=' [0-9]+ ;

name: 'name' '=' NAME ;

NAME: [A-Za-z_][A-Za-z_0-9]* ;

ASSIGN_TEXT: '=' ('"' [^"]* '"' | '\'' [^'\']* '\'');

TEXT: ~[<]+ ;

监听器实现代码 #

public class simple_grailTagsImpl {
    @Override public void enterProg(simple_grailParser.ProgContext ctx) {
        numInstructions = 0;
        numInts = 0;
        numFloats = 0;
        numStrings = 0;
        System.out.println("Starting program size=0");
    }
}

主程序代码 #

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
public class SimpleGrailParser {
    public static void main(String[] args) {
        String text = "<p>text is here</p>";
        simple_grailLexer lexer = new simple_grailLexer(CharStreams.fromString(text));
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        simple_grailParser parser = new simple_grailParser(tokens);
        simple_grailTagsImpl listener= new simple_grailTagsImpl();
        parser.addParseListener(listener);
        ParseTree tree = parser.prog();
        ParseTreeWalker.DEFAULT.walk(listener, tree);
    }
}

问题的核心是两次触发了监听器逻辑：

1. 调用parser.prog()时，ANTLR解析器会自动触发已添加的监听器，执行enterProg等方法
2. 后续调用ParseTreeWalker.DEFAULT.walk(listener, tree)时，又手动遍历语法树，再次触发监听器的所有进入/退出方法

只需二选一修改即可解决：

方法一：移除手动遍历代码 #

删除ParseTreeWalker.DEFAULT.walk(listener, tree);这一行，仅保留解析器触发的监听器逻辑：

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
public class SimpleGrailParser {
    public static void main(String[] args) {
        String text = "<p>text is here</p>";
        simple_grailLexer lexer = new simple_grailLexer(CharStreams.fromString(text));
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        simple_grailParser parser = new simple_grailParser(tokens);
        simple_grailTagsImpl listener= new simple_grailTagsImpl();
        parser.addParseListener(listener);
        ParseTree tree = parser.prog();
        // 移除手动遍历语句
    }
}

方法二：不向解析器添加监听器，仅手动遍历 #

删除parser.addParseListener(listener);这一行，只通过树遍历触发监听器：

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
public class SimpleGrailParser {
    public static void main(String[] args) {
        String text = "<p>text is here</p>";
        simple_grailLexer lexer = new simple_grailLexer(CharStreams.fromString(text));
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        simple_grailParser parser = new simple_grailParser(tokens);
        simple_grailTagsImpl listener= new simple_grailTagsImpl();
        // 移除添加监听器的语句
        ParseTree tree = parser.prog();
        ParseTreeWalker.DEFAULT.walk(listener, tree);
    }
}

内容的提问来源于stack exchange，提问作者Dov