Antlr用 Visitor 模式实现对 AST 的访问

RuleContext

+ attribute1:type = defaultValue+ attribute2:type- attribute3:type

+ operation1(params):returnType- operation2(params)- operation3()

ParseTree

ASTEvaluator.visitLiteral(LiteralContext)

1

ASTEvaluator

+ visitAdditiveExpression(AdditiveExpressionContext ctx):Integer+ visitMultiplicativeExpression(MultiplicativeExpressionContext ctx):Integer+ visitPrimaryExpression(PrimaryExpressionContext ctx):Integer+ visitLiteral(LiteralContext ctx) :Integer

abstract class Parser

8

@Override public T visit(ParseTree tree) {     return tree.accept(this); }

tree.accept(ASTEvaluator )

RuleNode

7

SyntaxTree

PlayScriptVisitor<T>visitLiteral(PlayScriptParser.LiteralContext ctx):TvisitPrimitiveType(PlayScriptParser.PrimitiveTypeContext ctx):TvisitStatement(PlayScriptParser.StatementContext ctx):TvisitExpressionStatement(PlayScriptParser.ExpressionStatementContext ctx):TvisitDeclaration(PlayScriptParser.DeclarationContext ctx):TvisitInitializer(PlayScriptParser.InitializerContext ctx):TvisitAdditiveExpression(PlayScriptParser.AdditiveExpressionContext ctx):TvisitMultiplicativeExpression(PlayScriptParser.MultiplicativeExpressionContext ctx):TvisitExpression(PlayScriptParser.ExpressionContext ctx):T

extends

TerminalNode

4

@Overridepublic <T> T accept(ParseTreeVisitor<? extends T> visitor) {    if (visitor instanceof PlayScriptVisitor)        return ((PlayScriptVisitor<? extends T>) visitor).visitExpression(this);    else return visitor.visitChildren(this);}

PlayScriptParser

implements

ASTEvaluator.visitExpression(ExpressionContext)

ParserRuleContext

2

@Overridepublic T visitExpression(PlayScriptParser.ExpressionContext ctx) {    return visitChildren(ctx);}

PlayScriptParser parser = new PlayScriptParser(tokens);ParseTree tree = parser.expression();//解释执行ASTEvaluator visitor = new ASTEvaluator();Integer result = visitor.visit(tree);

Inner LiteralContext

+ accept(ParseTreeVisitor<? extends T> visitor):T

Inner ExpressionContext

extends PlayScriptBaseVisitor

5

6

我们在纯手工编写的脚本语言解释器里，用了一个 evaluate() 方法自上而下地遍历了整棵树。随着要处理的语法越来越多，这个方法的代码量会越来越大，不便于维护。而 Visitor 设计模式针对每一种 AST 节点，都会有一个单独的方法来负责处理，能够让代码更清晰，也更便于维护。

abstract class AbstractParseTreeVisitor<T>

+ visit(ParseTree tree):T+ visitChildren(RuleNode node):T+ visitTerminal(TerminalNode node):T+ visitErrorNode(ErrorNode node):T

ParseTreeVisitor<T>visit(ParseTree tree):TvisitChildren(RuleNode node):TvisitTerminal(TerminalNode node):TvisitErrorNode(ErrorNode node):T

@Overridepublic <T> T accept(ParseTreeVisitor<? extends T> visitor) {    if (visitor instanceof PlayScriptVisitor)        return ((PlayScriptVisitor<? extends T>) visitor).visitLiteral(this);    else return visitor.visitChildren(this);}

@Overridepublic Integer visitLiteral(LiteralContext ctx) {    if (ctx.IntegerLiteral() !=null){        return Integer.valueOf(ctx.IntegerLiteral().getText());    }    return 0;}

ExpressionContext.accept(ASTEvaluator ){    ((PlayScriptVisitor<? extends T>) visitor).visitExpression(this)}

3

9

PlayScriptBaseVisitor<T>

+ visitLiteral(PlayScriptParser.LiteralContext ctx):T+ visitPrimitiveType(PlayScriptParser.PrimitiveTypeContext ctx):T+ visitStatement(PlayScriptParser.StatementContext ctx):T+ visitExpressionStatement(PlayScriptParser.ExpressionStatementContext ctx):T+ visitDeclaration(PlayScriptParser.DeclarationContext ctx):T+ visitAdditiveExpression(PlayScriptParser.AdditiveExpressionContext ctx):T+ visitMultiplicativeExpression(PlayScriptParser.MultiplicativeExpressionContext ctx):T+ visitExpression(PlayScriptParser.ExpressionContext ctx):T...