Skywalking-07：OAL原理——解释器实现

OAL 解释器实现

OAL 解释器是基于 Antlr4 实现的，我们先来了解下 Antlr4

Antlr4 基本介绍

Antlr4 使用案例

参考Antlr4的使用简介这篇文章，我们实现了一个简单的案例：antlr案例：简单的计算器，下面来讲讲这个案例。

首先，装好ANTLR v4(IDEA插件)插件，这个之后验证语法树的时候会用到。

在 pom.xml 中配置 antlr4 的依赖和插件

  org.antlr
  antlr4-runtime
  4.7.1

  org.antlr
  antlr4-maven-plugin
  ${antlr.version}
  
    
      antlr
      
        antlr4
      
    
  

在 src/main/antlr4/com/switchvov/antlr/demo/calc 目录下添加一个 Calc.g4 文件

grammar Calc;   //名称需要和文件名一致

root : expr EOF;   //解决问题: no viable alternative at input ''

expr
    : expr '+' expr     #add   //标签会生成对应访问方法方便我们实现调用逻辑编写
    | expr '-' expr     #sub
    | INT               #int
    ;

INT : [0-9]+                   //定义整数
    ;

WS : [ \r\n\t]+ -> skip      //跳过空白类字符
   ;

执行一下： mvn compile -Dmaven.test.skip=true ，在 target/generated-sources/antlr4 会生成相应的 Java 代码。

使用方式默认是监听器模式，也可以配置成访问者模式。

监听器模式：主要借助了 ParseTreeWalker 这样一个类,相当于是一个 hook ，每经过一个树的节点,便会触发对应节点的方法。好处就算是比较方便,但是灵活性不够,不能够自主性的调用任意节点进行使用。

访问者模式：将每个数据的节点类型高度抽象出来够,根据你传入的上下文类型来判断你想要访问的是哪个节点,触发对应的方法

PS：结论，简单语法监听器模式就可以了，如果语法比较灵活可以考虑使用访问者模式。

antlr4
├── Calc.tokens
├── CalcLexer.tokens
└── com
    └── switchvov
        └── antlr
            └── demo
                └── calc
                    ├── Calc.interp
                    ├── CalcBaseListener.java # 监听模式下生成的监听器基类，实现类监听器接口，通过继承该类可以实现相应的功能
                    ├── CalcLexer.interp
                    ├── CalcLexer.java  # 词法解析器
                    ├── CalcListener.java # 监听模式下生成的监听器接口
                    └── CalcParser.java # 语法解析器

继承 com.switchvov.antlr.demo.calc.CalcBaseListener ，实现计算器相应功能

package com.switchvov.antlr.demo.calc;

import java.util.ArrayDeque;
import java.util.Deque;

/**
 * @author switch
 * @since 2021/6/30
 */
public class CalcExecuteListener extends CalcBaseListener {
    
    Deque queue = new ArrayDeque<>(16);

    @Override
    public void exitInt(CalcParser.IntContext ctx) {
        queue.add(Integer.parseInt(ctx.INT().getText()));
    }

    @Override
    public void exitAdd(CalcParser.AddContext ctx) {
        int r = queue.pop();
        int l = queue.pop();
        queue.add(l + r);
    }

    @Override
    public void exitSub(CalcParser.SubContext ctx) {
        int r = queue.pop();
        int l = queue.pop();
        queue.add(l - r);
    }

    public int result() {
        return queue.pop();
    }
}

测试一下

package com.switchvov.antlr.demo.calc;

import org.antlr.v4.runtime.CharStreams;
import org.antlr.v4.runtime.CodePointCharStream;
import org.antlr.v4.runtime.CommonTokenStream;
import org.antlr.v4.runtime.tree.ParseTree;
import org.antlr.v4.runtime.tree.ParseTreeWalker;
import org.junit.Test;


/**
 * @author switch
 * @since 2021/6/30
 */
public class CalcTest {

    public static int exec(String input) {
        // 读入字符串
        CodePointCharStream cs = CharStreams.fromString(input);
        // 词法解析
        CalcLexer lexer = new CalcLexer(cs);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        // 语法解析
        CalcParser parser = new CalcParser(tokens);

        // 监听器触发获取执行结果
        ParseTree tree = parser.expr();
        ParseTreeWalker walker = new ParseTreeWalker();
        CalcExecuteListener listener = new CalcExecuteListener();
        walker.walk(listener, tree);
        return listener.result();
    }

    @Test
    public void testCalc() {
        String input = "1+2";
        // 输出结果：3
        System.out.println(exec(input));
    }

}

Antlr4 IDEA 插件使用

在 Calc.g4 语法定义文件中，鼠标右击可以选择 Test Rule root ，然后在 ANTLR Preview 的输入框中填入 1 + 2 就可以校验语法文件是否 OK ，并且也可以看到相应的语法树

file

file

Antlr4 在 Skywalking 的应用

通过“ Antlr4 基本介绍”一节，基本上对 Antlr4 使用有了个大概的认识。下面来看看 Skywalking 中 Antlr4 是如何使用的。

词法定义

在 oap-server/oal-grammar/src/main/antlr4/org/apache/skywalking/oal/rt/grammar/OALLexer.g4 文件中，我们能看到 OAL 的词法定义

// Observability Analysis Language lexer
lexer grammar OALLexer;

@Header {package org.apache.skywalking.oal.rt.grammar;}

// Keywords

FROM: 'from';
FILTER: 'filter';
DISABLE: 'disable';
SRC_ALL: 'All';
SRC_SERVICE: 'Service';
SRC_SERVICE_INSTANCE: 'ServiceInstance';
SRC_ENDPOINT: 'Endpoint';
SRC_SERVICE_RELATION: 'ServiceRelation';
SRC_SERVICE_INSTANCE_RELATION: 'ServiceInstanceRelation';
SRC_ENDPOINT_RELATION: 'EndpointRelation';
SRC_SERVICE_INSTANCE_JVM_CPU: 'ServiceInstanceJVMCPU';
SRC_SERVICE_INSTANCE_JVM_MEMORY: 'ServiceInstanceJVMMemory';
SRC_SERVICE_INSTANCE_JVM_MEMORY_POOL: 'ServiceInstanceJVMMemoryPool';
SRC_SERVICE_INSTANCE_JVM_GC: 'ServiceInstanceJVMGC';
SRC_SERVICE_INSTANCE_JVM_THREAD: 'ServiceInstanceJVMThread';
SRC_SERVICE_INSTANCE_JVM_CLASS:'ServiceInstanceJVMClass';
SRC_DATABASE_ACCESS: 'DatabaseAccess';
SRC_SERVICE_INSTANCE_CLR_CPU: 'ServiceInstanceCLRCPU';
SRC_SERVICE_INSTANCE_CLR_GC: 'ServiceInstanceCLRGC';
SRC_SERVICE_INSTANCE_CLR_THREAD: 'ServiceInstanceCLRThread';
SRC_ENVOY_INSTANCE_METRIC: 'EnvoyInstanceMetric';

// Browser keywords
SRC_BROWSER_APP_PERF: 'BrowserAppPerf';
SRC_BROWSER_APP_PAGE_PERF: 'BrowserAppPagePerf';
SRC_BROWSER_APP_SINGLE_VERSION_PERF: 'BrowserAppSingleVersionPerf';
SRC_BROWSER_APP_TRAFFIC: 'BrowserAppTraffic';
SRC_BROWSER_APP_PAGE_TRAFFIC: 'BrowserAppPageTraffic';
SRC_BROWSER_APP_SINGLE_VERSION_TRAFFIC: 'BrowserAppSingleVersionTraffic';

// Constructors symbols

DOT:                                 '.';
LR_BRACKET:                          '(';
RR_BRACKET:                          ')';
LS_BRACKET:                          '[';
RS_BRACKET:                          ']';
COMMA:                               ',';
SEMI:                                ';';
EQUAL:                               '=';
DUALEQUALS:                          '==';
ALL:                                 '*';
GREATER:                             '>';
LESS:                                '<';
GREATER_EQUAL:                       '>=';
LESS_EQUAL:                          '<=';
NOT_EQUAL:                           '!=';
LIKE:                                'like';
IN:                                  'in';
CONTAIN:                            'contain';
NOT_CONTAIN:                        'not contain';

// Literals

BOOL_LITERAL:       'true'
            |       'false'
            ;

NUMBER_LITERAL :   Digits+;

CHAR_LITERAL:       '\'' (~['\\\r\n] | EscapeSequence) '\'';

STRING_LITERAL:     '"' (~["\\\r\n] | EscapeSequence)* '"';

DelimitedComment
    : '/*' ( DelimitedComment | . )*? '*/'
      -> channel(HIDDEN)
    ;

LineComment
    : '//' ~[\u000A\u000D]*
      -> channel(HIDDEN)
    ;

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

// Identifiers

IDENTIFIER:         Letter LetterOrDigit*;

// Fragment rules

fragment EscapeSequence
    : '\\' [btnfr"'\\]
    | '\\' ([0-3]? [0-7])? [0-7]
    | '\\' 'u'+ HexDigit HexDigit HexDigit HexDigit
    ;

fragment HexDigits
    : HexDigit ((HexDigit | '_')* HexDigit)?
    ;

fragment HexDigit
    : [0-9a-fA-F]
    ;

fragment Digits
    : [0-9] ([0-9_]* [0-9])?
    ;

fragment LetterOrDigit
    : Letter
    | [0-9]
    ;

fragment Letter
    : [a-zA-Z$_] // these are the "java letters" below 0x7F
    | ~[\u0000-\u007F\uD800-\uDBFF] // covers all characters above 0x7F which are not a surrogate
    | [\uD800-\uDBFF] [\uDC00-\uDFFF] // covers UTF-16 surrogate pairs encodings for U+10000 to U+10FFFF
    ;

语法定义

在 oap-server/oal-grammar/src/main/antlr4/org/apache/skywalking/oal/rt/grammar/OALParser.g4 文件中，我们能看到 OAL 的语法定义

parser grammar OALParser;

@Header {package org.apache.skywalking.oal.rt.grammar;}

options { tokenVocab=OALLexer; }


// Top Level Description

root
    : (aggregationStatement | disableStatement)*
    ;

aggregationStatement
    : variable (SPACE)? EQUAL (SPACE)? metricStatement DelimitedComment? LineComment? (SEMI|EOF)
    ;

disableStatement
    : DISABLE LR_BRACKET disableSource RR_BRACKET DelimitedComment? LineComment? (SEMI|EOF)
    ;

metricStatement
    : FROM LR_BRACKET source (sourceAttributeStmt+) RR_BRACKET (filterStatement+)? DOT aggregateFunction
    ;

filterStatement
    : DOT FILTER LR_BRACKET filterExpression RR_BRACKET
    ;

filterExpression
    : expression
    ;

source
    : SRC_ALL | SRC_SERVICE | SRC_DATABASE_ACCESS | SRC_SERVICE_INSTANCE | SRC_ENDPOINT |
      SRC_SERVICE_RELATION | SRC_SERVICE_INSTANCE_RELATION | SRC_ENDPOINT_RELATION |
      SRC_SERVICE_INSTANCE_JVM_CPU | SRC_SERVICE_INSTANCE_JVM_MEMORY | SRC_SERVICE_INSTANCE_JVM_MEMORY_POOL | SRC_SERVICE_INSTANCE_JVM_GC | SRC_SERVICE_INSTANCE_JVM_THREAD | SRC_SERVICE_INSTANCE_JVM_CLASS |// JVM source of service instance
      SRC_SERVICE_INSTANCE_CLR_CPU | SRC_SERVICE_INSTANCE_CLR_GC | SRC_SERVICE_INSTANCE_CLR_THREAD |
      SRC_ENVOY_INSTANCE_METRIC |
      SRC_BROWSER_APP_PERF | SRC_BROWSER_APP_PAGE_PERF | SRC_BROWSER_APP_SINGLE_VERSION_PERF |
      SRC_BROWSER_APP_TRAFFIC | SRC_BROWSER_APP_PAGE_TRAFFIC | SRC_BROWSER_APP_SINGLE_VERSION_TRAFFIC
    ;

disableSource
    : IDENTIFIER
    ;

sourceAttributeStmt
    : DOT sourceAttribute
    ;

sourceAttribute
    : IDENTIFIER | ALL
    ;

variable
    : IDENTIFIER
    ;

aggregateFunction
    : functionName LR_BRACKET ((funcParamExpression (COMMA funcParamExpression)?) | (literalExpression (COMMA literalExpression)?))? RR_BRACKET
    ;

functionName
    : IDENTIFIER
    ;

funcParamExpression
    : expression
    ;

literalExpression
    : BOOL_LITERAL | NUMBER_LITERAL | IDENTIFIER
    ;

expression
    : booleanMatch | stringMatch | greaterMatch | lessMatch | greaterEqualMatch | lessEqualMatch | notEqualMatch | booleanNotEqualMatch | likeMatch | inMatch | containMatch | notContainMatch
    ;

containMatch
    : conditionAttributeStmt CONTAIN stringConditionValue
    ;

notContainMatch
    : conditionAttributeStmt NOT_CONTAIN stringConditionValue
    ;

booleanMatch
    : conditionAttributeStmt DUALEQUALS booleanConditionValue
    ;

stringMatch
    :  conditionAttributeStmt DUALEQUALS (stringConditionValue | enumConditionValue)
    ;

greaterMatch
    :  conditionAttributeStmt GREATER numberConditionValue
    ;

lessMatch
    :  conditionAttributeStmt LESS numberConditionValue
    ;

greaterEqualMatch
    :  conditionAttributeStmt GREATER_EQUAL numberConditionValue
    ;

lessEqualMatch
    :  conditionAttributeStmt LESS_EQUAL numberConditionValue
    ;

booleanNotEqualMatch
    :  conditionAttributeStmt NOT_EQUAL booleanConditionValue
    ;

notEqualMatch
    :  conditionAttributeStmt NOT_EQUAL (numberConditionValue | stringConditionValue | enumConditionValue)
    ;

likeMatch
    :  conditionAttributeStmt LIKE stringConditionValue
    ;

inMatch
    :  conditionAttributeStmt IN multiConditionValue
    ;

multiConditionValue
    : LS_BRACKET (numberConditionValue ((COMMA numberConditionValue)*) | stringConditionValue ((COMMA stringConditionValue)*) | enumConditionValue ((COMMA enumConditionValue)*)) RS_BRACKET
    ;

conditionAttributeStmt
    : conditionAttribute ((DOT conditionAttribute)*)
    ;

conditionAttribute
    : IDENTIFIER
    ;

booleanConditionValue
    : BOOL_LITERAL
    ;

stringConditionValue
    : STRING_LITERAL
    ;

enumConditionValue
    : IDENTIFIER DOT IDENTIFIER
    ;

numberConditionValue
    : NUMBER_LITERAL
    ;

Antlr4 生成 Java 代码

在 oap-server/oal-grammar 下执行 mvn compile -Dmaven.test.skip=true 会在 oap-server/oal-grammar/target/generated-sources/antlr4 目录下生成相应的 Java 代码

.
├── OALLexer.tokens
├── OALParser.tokens
└── org
    └── apache
        └── skywalking
            └── oal
                └── rt
                    └── grammar
                        ├── OALLexer.interp
                        ├── OALLexer.java # 词法解析器
                        ├── OALParser.interp
                        ├── OALParser.java # 语法解析器
                        ├── OALParserBaseListener.java # 监听器
                        └── OALParserListener.java

在 Skywalking 的使用

通过“ Antlr4 使用案例”一节，可以知道 Antlr4 有两种功能实现方式：监听器或者访问器。

通过“ Antlr4 生成 Java 代码”一节，知道 Skywalking 使用的是监听器模式。

Skywalking 关于 OAL 的相应的代码都在 oap-server/oal-rt 模块中。

org.apache.skywalking.oal.rt.grammar.OALParserBaseListener 的继承类坐标是 org.apache.skywalking.oal.rt.parser.OALListener

package org.apache.skywalking.oal.rt.parser;

import java.util.Arrays;
import java.util.List;
import org.antlr.v4.runtime.misc.NotNull;
import org.apache.skywalking.oal.rt.grammar.OALParser;
import org.apache.skywalking.oal.rt.grammar.OALParserBaseListener;
import org.apache.skywalking.oap.server.core.source.DefaultScopeDefine;

public class OALListener extends OALParserBaseListener {
    private List results;
    private AnalysisResult current;
    private DisableCollection collection;

    private ConditionExpression conditionExpression;

    private final String sourcePackage;

    public OALListener(OALScripts scripts, String sourcePackage) {
        this.results = scripts.getMetricsStmts();
        this.collection = scripts.getDisableCollection();
        this.sourcePackage = sourcePackage;
    }

    @Override
    public void enterAggregationStatement(@NotNull OALParser.AggregationStatementContext ctx) {
        current = new AnalysisResult();
    }

    @Override
    public void exitAggregationStatement(@NotNull OALParser.AggregationStatementContext ctx) {
        DeepAnalysis deepAnalysis = new DeepAnalysis();
        results.add(deepAnalysis.analysis(current));
        current = null;
    }

    @Override
    public void enterSource(OALParser.SourceContext ctx) {
        current.setSourceName(ctx.getText());
        current.setSourceScopeId(DefaultScopeDefine.valueOf(metricsNameFormat(ctx.getText())));
    }

    @Override
    public void enterSourceAttribute(OALParser.SourceAttributeContext ctx) {
        current.getSourceAttribute().add(ctx.getText());
    }

    @Override
    public void enterVariable(OALParser.VariableContext ctx) {
    }

    @Override
    public void exitVariable(OALParser.VariableContext ctx) {
        current.setVarName(ctx.getText());
        current.setMetricsName(metricsNameFormat(ctx.getText()));
        current.setTableName(ctx.getText().toLowerCase());
    }

    @Override
    public void enterFunctionName(OALParser.FunctionNameContext ctx) {
        current.setAggregationFunctionName(ctx.getText());
    }

    @Override
    public void enterFilterStatement(OALParser.FilterStatementContext ctx) {
        conditionExpression = new ConditionExpression();
    }

    @Override
    public void exitFilterStatement(OALParser.FilterStatementContext ctx) {
        current.addFilterExpressionsParserResult(conditionExpression);
        conditionExpression = null;
    }

    @Override
    public void enterFuncParamExpression(OALParser.FuncParamExpressionContext ctx) {
        conditionExpression = new ConditionExpression();
    }

    @Override
    public void exitFuncParamExpression(OALParser.FuncParamExpressionContext ctx) {
        current.addFuncConditionExpression(conditionExpression);
        conditionExpression = null;
    }

    /////////////
    // Expression
    ////////////
    @Override
    public void enterConditionAttribute(OALParser.ConditionAttributeContext ctx) {
        conditionExpression.getAttributes().add(ctx.getText());
    }

    @Override
    public void enterBooleanMatch(OALParser.BooleanMatchContext ctx) {
        conditionExpression.setExpressionType("booleanMatch");
    }

    @Override
    public void enterStringMatch(OALParser.StringMatchContext ctx) {
        conditionExpression.setExpressionType("stringMatch");
    }

    @Override
    public void enterGreaterMatch(OALParser.GreaterMatchContext ctx) {
        conditionExpression.setExpressionType("greaterMatch");
    }

    @Override
    public void enterGreaterEqualMatch(OALParser.GreaterEqualMatchContext ctx) {
        conditionExpression.setExpressionType("greaterEqualMatch");
    }

    @Override
    public void enterLessMatch(OALParser.LessMatchContext ctx) {
        conditionExpression.setExpressionType("lessMatch");
    }

    @Override
    public void enterLessEqualMatch(OALParser.LessEqualMatchContext ctx) {
        conditionExpression.setExpressionType("lessEqualMatch");
    }

    @Override
    public void enterNotEqualMatch(final OALParser.NotEqualMatchContext ctx) {
        conditionExpression.setExpressionType("notEqualMatch");
    }

    @Override
    public void enterBooleanNotEqualMatch(final OALParser.BooleanNotEqualMatchContext ctx) {
        conditionExpression.setExpressionType("booleanNotEqualMatch");
    }

    @Override
    public void enterLikeMatch(final OALParser.LikeMatchContext ctx) {
        conditionExpression.setExpressionType("likeMatch");
    }

    @Override
    public void enterContainMatch(final OALParser.ContainMatchContext ctx) {
        conditionExpression.setExpressionType("containMatch");
    }

    @Override
    public void enterNotContainMatch(final OALParser.NotContainMatchContext ctx) {
        conditionExpression.setExpressionType("notContainMatch");
    }

    @Override
    public void enterInMatch(final OALParser.InMatchContext ctx) {
        conditionExpression.setExpressionType("inMatch");
    }

    @Override
    public void enterMultiConditionValue(final OALParser.MultiConditionValueContext ctx) {
        conditionExpression.enterMultiConditionValue();
    }

    @Override
    public void exitMultiConditionValue(final OALParser.MultiConditionValueContext ctx) {
        conditionExpression.exitMultiConditionValue();
    }

    @Override
    public void enterBooleanConditionValue(OALParser.BooleanConditionValueContext ctx) {
        enterConditionValue(ctx.getText());
    }

    @Override
    public void enterStringConditionValue(OALParser.StringConditionValueContext ctx) {
        enterConditionValue(ctx.getText());
    }

    @Override
    public void enterEnumConditionValue(OALParser.EnumConditionValueContext ctx) {
        enterConditionValue(ctx.getText());
    }

    @Override
    public void enterNumberConditionValue(OALParser.NumberConditionValueContext ctx) {
        conditionExpression.isNumber();
        enterConditionValue(ctx.getText());
    }

    private void enterConditionValue(String value) {
        if (value.split("\\.").length == 2 && !value.startsWith("\"")) {
            // Value is an enum.
            value = sourcePackage + value;
        }
        conditionExpression.addValue(value);
    }

    /////////////
    // Expression end.
    ////////////

    @Override
    public void enterLiteralExpression(OALParser.LiteralExpressionContext ctx) {
        if (ctx.IDENTIFIER() == null) {
            current.addFuncArg(new Argument(EntryMethod.LITERAL_TYPE, Arrays.asList(ctx.getText())));
            return;
        }
        current.addFuncArg(new Argument(EntryMethod.IDENTIFIER_TYPE, Arrays.asList(ctx.getText().split("\\."))));
    }

    private String metricsNameFormat(String source) {
        source = firstLetterUpper(source);
        int idx;
        while ((idx = source.indexOf("_")) > -1) {
            source = source.substring(0, idx) + firstLetterUpper(source.substring(idx + 1));
        }
        return source;
    }

    /**
     * Disable source
     */
    @Override
    public void enterDisableSource(OALParser.DisableSourceContext ctx) {
        collection.add(ctx.getText());
    }

    private String firstLetterUpper(String source) {
        return source.substring(0, 1).toUpperCase() + source.substring(1);
    }
}

简单来说，就是通过监听器封装了个 org.apache.skywalking.oal.rt.parser.OALScripts 对象

package org.apache.skywalking.oal.rt.parser;

import java.util.LinkedList;
import java.util.List;
import lombok.Getter;

@Getter
public class OALScripts {
    // 解析出来的分析结果集合
    private List metricsStmts;
    // 禁用表达式集合
    private DisableCollection disableCollection;

    public OALScripts() {
        metricsStmts = new LinkedList<>();
        disableCollection = new DisableCollection();
    }
}

org.apache.skywalking.oal.rt.parser.ScriptParser 类读取 oal 文件，使用 Antlr 生成的 Java 类进行解析

package org.apache.skywalking.oal.rt.parser;

import java.io.IOException;
import java.io.Reader;
import org.antlr.v4.runtime.CharStreams;
import org.antlr.v4.runtime.CommonTokenStream;
import org.antlr.v4.runtime.tree.ParseTree;
import org.antlr.v4.runtime.tree.ParseTreeWalker;
import org.apache.skywalking.oal.rt.grammar.OALLexer;
import org.apache.skywalking.oal.rt.grammar.OALParser;

/**
 * Script reader and parser.
 */
public class ScriptParser {
    private OALLexer lexer;

    private String sourcePackage;

    private ScriptParser() {

    }

    public static ScriptParser createFromFile(Reader scriptReader, String sourcePackage) throws IOException {
        ScriptParser parser = new ScriptParser();
        parser.lexer = new OALLexer(CharStreams.fromReader(scriptReader));
        parser.sourcePackage = sourcePackage;
        return parser;
    }

    public static ScriptParser createFromScriptText(String script, String sourcePackage) throws IOException {
        ScriptParser parser = new ScriptParser();
        parser.lexer = new OALLexer(CharStreams.fromString(script));
        parser.sourcePackage = sourcePackage;
        return parser;
    }

    public OALScripts parse() throws IOException {
        OALScripts scripts = new OALScripts();

        CommonTokenStream tokens = new CommonTokenStream(lexer);

        OALParser parser = new OALParser(tokens);

        ParseTree tree = parser.root();
        ParseTreeWalker walker = new ParseTreeWalker();

        walker.walk(new OALListener(scripts, sourcePackage), tree);

        return scripts;
    }

    public void close() {
    }
}

参考文档

• ANTLR官网
• antlr4 GitHub
• antlr4 语法案例
• Antlr4的使用简介
• antlr案例：简单的计算器
• 某小伙的Antlr4学习笔记
• ANTLR v4(IDEA插件)

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