ANTLR v4、JavaLexer和JavaParser返回null作为解析树

我使用antlr v4提取java程序的解析树以用于其他目的。我从这个示例开始: ANTLR v4 visitor sample

我已经测试了给定链接上的步骤,以检查它是否有效,以及一切是否正常:

java Run
a = 1+2
b = a^2
c = a+b*(a-1)
a+b+c
^Z
Result: 33.0

|_Java.g4                                                               
|_Java.tokens                                                           
|_JavaBaseVisitor.java                                                  
|_JavaLexer.java                                                        
|_JavaLexer.tokens                                                      
|_JavaParser.java                                                       
|_JavaTreeExtractorVisitor.java                                         
|_JavaVisitor.java           
|_Run.java 

还有跑步。java如下:

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

public class Run {
    public static void main(String[] args) throws Exception {
        CharStream input = CharStreams.fromFileName("F:\\Projects\\Java\\Netbeans\\ASTProj\\JavaTreeExtractor\\prog.java");
        JavaLexer lexer = new JavaLexer(input);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        JavaParser parser = new JavaParser(tokens);
        ParseTree tree = parser.getContext();

        JavaTreeExtractorVisitor calcVisitor = new JavaTreeExtractorVisitor();
        String result = calcVisitor.visit(tree);
        System.out.println("Result: " + result);
    }
}

ParseTree tree = parser.getContext(); 树对象为null。

(如果需要更多信息,请通知我)。