如何在ANTLR中定义启动规则？

grammar MyGrammar;

startRule: expression EOF;

expression: INT '+' INT;

INT: [0-9]+;

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // 创建词法分析器
        MyGrammarLexer lexer = new MyGrammarLexer(CharStreams.fromString("1 + 2"));

        // 创建词法符号流
        CommonTokenStream tokens = new CommonTokenStream(lexer);

        // 创建语法分析器
        MyGrammarParser parser = new MyGrammarParser(tokens);

        // 解析启动规则
        ParseTree tree = parser.startRule();

        // 打印解析树
        System.out.println(tree.toStringTree(parser));
    }
}