我需要创建一个(非常)简单的 XPath 表达式解析器。我正在尝试使用 JavaCC 来实现此目的。我对 JavaCC 完全陌生(尽管我们在学校学习了 Flex 和 Bison),因此我尝试通过一次添加一小部分功能来逐步构建 JJ 脚本。
到目前为止,我掌握了以下语法:
XPATHEXPRESSION ::= ("/" <STEP>)+
STEP ::= <AXIS_NAME> ":" <NODE_TEST> ( "[" <EXPRESSION> "]" )*
EXPRESSION ::= <XPATHEXPRESSION> "=" """ <IDENTIFIER> """
相关的 JJ 文件如下所示:
options {
STATIC = false ;
}
PARSER_BEGIN(XPathParser)
package cz.me.generator.parser;
import cz.me.generator.expression.*;
import java.io.Reader;
import java.io.StringReader;
public class XPathParser
{
public static XPathExpr parse(String exprLiteral)
throws TokenMgrError, ParseException
{
Reader in = new StringReader(exprLiteral);
XPathParser parser = new XPathParser(in);
return parser.XPathExpr();
}
}
PARSER_END(XPathParser)
SKIP : { " " }
TOKEN : { < SLASH : "/" > }
TOKEN : { < COLON : ":" > }
TOKEN : { < OPEN_PAR : "[" > }
TOKEN : { < CLOSE_PAR : "]" > }
TOKEN : { < QUOTE : "\"" > }
TOKEN : { < EQ : "=" > }
TOKEN : { < GT : ">" > }
TOKEN : { < LT : "<" > }
TOKEN : { < IDENTIFIER : (["a"-"z","A"-"Z","0"-"9"])+ > }
TOKEN : { < NUMBER : (["0"-"9"])+ > }
Expression Expression() :
{
Token t;
XPathExpr xPathExpr;
String value;
}
{
xPathExpr = XPathExpr()
<EQ>
<QUOTE>
t = <IDENTIFIER>
{ value = t.image; }
<QUOTE>
{ return new EqExpr(xPathExpr, new StringLiteral(value)); }
}
XPathExpr XPathExpr() :
{
XPathExpr xPathExpr;
Step step;
}
{
{ xPathExpr = new XPathExpr(); }
(
<SLASH>
step = Step()
{ xPathExpr.addStep(step); }
)+
<EOF>
{ return xPathExpr; }
}
Step Step() :
{
Token t;
Step step;
Axis axis;
NodeTest nodeTest;
Expression predicate;
}
{
t = <IDENTIFIER>
{ axis = Axis.valueOf(t.image); }
<COLON>
t = <IDENTIFIER>
{ nodeTest = new NodeNameTest(t.image); }
{ step = new Step(axis, nodeTest); }
(
<OPEN_PAR>
predicate = Expression()
{ step.addPredicate(predicate); }
<CLOSE_PAR>
)*
{ return step; }
}
但是,这不起作用。
以下代码:
XPathExpr expr = XPathParser.parse("/self:house/child:window[/child:material = \"glass\"]");
System.out.println(expr);
产生以下输出(在 Debug模式下运行 JavaCC 时):
Call: XPathExpr
Consumed token: <"/" at line 1 column 1>
Call: Step
Consumed token: <<IDENTIFIER>: "self" at line 1 column 2>
Consumed token: <":" at line 1 column 6>
Consumed token: <<IDENTIFIER>: "house" at line 1 column 7>
Return: Step
Consumed token: <"/" at line 1 column 12>
Call: Step
Consumed token: <<IDENTIFIER>: "child" at line 1 column 13>
Consumed token: <":" at line 1 column 18>
Consumed token: <<IDENTIFIER>: "window" at line 1 column 19>
Consumed token: <"[" at line 1 column 25>
Call: Expression
Call: XPathExpr
Consumed token: <"/" at line 1 column 26>
Call: Step
Consumed token: <<IDENTIFIER>: "child" at line 1 column 27>
Consumed token: <":" at line 1 column 32>
Consumed token: <<IDENTIFIER>: "material" at line 1 column 33>
Return: Step
Return: XPathExpr
Return: Expression
Return: Step
Return: XPathExpr
Exception in thread "main" cz.me.generator.parser.ParseException: Encountered " "=" "= "" at line 1, column 42.
Was expecting one of:
<EOF>
"/" ...
"[" ...
at cz.me.generator.parser.XPathParser.generateParseException(XPathParser.java:270)
at cz.me.generator.parser.XPathParser.jj_consume_token(XPathParser.java:207)
at cz.me.generator.parser.XPathParser.XPathExpr(XPathParser.java:65)
at cz.me.generator.parser.XPathParser.Expression(XPathParser.java:32)
at cz.me.generator.parser.XPathParser.Step(XPathParser.java:100)
at cz.me.generator.parser.XPathParser.XPathExpr(XPathParser.java:54)
at cz.me.generator.parser.XPathParser.parse(XPathParser.java:22)
at cz.me.generator.Main.main(Main.java:17)
我尝试了一些变体来查看 JavaCC 是否没有递归问题,但问题似乎出在其他地方。
出了什么问题?
最佳答案
问题是<EOF>
在 XPathExpr 中。把那个拿出来。添加生产
void Start() :
{ }
{
XPathExpr()
< EOF >
}
并重写parse
使用“开始”。
关于JavaCC - XPath 解析器,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/16587787/