ANTLR 入门(未完待续)

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ANTLR是什么

ANTLR (ANother Tool for Language Recognition) is a powerful parser generator for reading, processing, executing, or translating structured text or binary files. It’s widely used to build languages, tools, and frameworks. From a grammar, ANTLR generates a parser that can build and walk parse trees.

简单的说就是一个语言识别器,什么时候会用到呢?当你想构建自己的DSL(Domain Specific Language)就会用到了.编译语言的前面的步骤就是词法分析,和语法分析.而ANTLR就能帮助我们简化这个过程,通过简单的配置就能生成词法分析器和语法分析器,极大提高了效率.

快速开始

安装和简单入门都很简单,官网有详细介绍

最重要的资源是他们的官方的例子,模仿是学习新东西很好的一个手段.除了这些我还推荐一个超级好用的插件,一个IntelliJ Idea的插件,貌似也是官方的,地址在这里

练习

我之前写过一个简单的四则运算的解析器,现在我们尝试用ANTLR实现一个最简单的加减运算:

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grammar Calculator;

// SKIP
SPACE:                               [ \t\r\n]+    -> channel(HIDDEN);

expr:   atom (op atom)*
    ;

atom:
        INT
   ;
op
    : ADD|SUB
   ;

ADD :   '+' ;
SUB :   '-' ;
INT :   [0-9]+ ;

完成后生成Java代码,然后实现一个CalculatorBaseListener,代码如下:

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package Calculator;

import org.antlr.v4.runtime.CharStream;
import org.antlr.v4.runtime.CharStreams;
import org.antlr.v4.runtime.CommonTokenStream;
import org.antlr.v4.runtime.tree.ParseTreeWalker;

/**
 * Created by charles on 2017-十二月-1.
 */
public class MyHandler extends CalculatorBaseListener {


    private Integer result = null;
    private String op = null;


    @Override
    public void exitAtom(CalculatorParser.AtomContext ctx) {
        Integer atom = Integer.valueOf(ctx.getText());
        if (null == result) {
            result = atom;
        } else if (op != null) {
            switch (op) {
                case "+":
                    result += atom;
                    break;
                case "-":
                    result -= atom;
                    break;
                default:
                    break;

            }
        }
    }

    @Override
    public void exitOp(CalculatorParser.OpContext ctx) {
        op = ctx.getText();
    }

    @Override
    public void exitExpr(CalculatorParser.ExprContext ctx) {
        System.out.println("result:" + result);
    }

    public static void main(String[] args) {
        String cal = "1+2+3-3-2+5+2+3";
        CharStream input = CharStreams.fromString(cal);
        CalculatorLexer lexer = new CalculatorLexer(input);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        CalculatorParser parser = new CalculatorParser(tokens);
        MyHandler handler = new MyHandler();
        ParseTreeWalker.DEFAULT.walk(handler, parser.expr());
    }


}

运行一下试试,虽然例子非常简单,但是还是把最核心的展示出来了.