使用 SAXON 评估 XPATH 2.0 表达式

Question

我努力寻找一个有效的 SAXON 示例来执行一个简单的任务，例如评估以下 XPATH 2.0 表达式：

abs(-a)

正在评估类似

的表达式

3 + 4

工作得很好，但我没能为变量 a.

提供一个值给 SAXON

我试过类似的东西：

    final Processor proc = new Processor(false);
    final XPathCompiler xpath = proc.newXPathCompiler();
    QName a = new QName("a");
    xpath.declareVariable(a);

    System.out.println(xpath.evaluate("abs(-a)", null));

但显然这不起作用，因为我没有为 a.

提供值

如何声明 a 的值？

非常感谢任何帮助！

编辑：这也不起作用：

final Processor proc = new Processor(false);
final XPathCompiler xpath = proc.newXPathCompiler();

XPathSelector exp = xpath.compile("abs(-$a)").load();

exp.setVariable(new QName("a"), new XdmAtomicValue(17));
XdmItem result = exp.evaluateSingle();

这会引发异常：

net.sf.saxon.s9api.SaxonApiException: Undeclared variable in XPath expression: $a at net.sf.saxon.s9api.XPathCompiler.internalCompile(XPathCompiler.java:500) at net.sf.saxon.s9api.XPathCompiler.compile(XPathCompiler.java:472) at xpathtest.SaxonTest.test(SaxonTest.java:26) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:483) at org.junit.runners.model.FrameworkMethod.runReflectiveCall(FrameworkMethod.java:47) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50) at org.junit.runners.ParentRunner.run(ParentRunner.java:238) at org.junit.runners.ParentRunner.schedule(ParentRunner.java:63) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236) at org.junit.runners.ParentRunner.access[=14=]0(ParentRunner.java:53) at org.junit.runners.ParentRunner.evaluate(ParentRunner.java:229) at org.junit.runners.ParentRunner.run(ParentRunner.java:309) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192) Caused by: net.sf.saxon.trans.XPathException: Undeclared variable in XPath expression: $a at net.sf.saxon.sxpath.IndependentContext.bindVariable(IndependentContext.java:458) at net.sf.saxon.expr.parser.XPathParser.parseVariableReference(XPathParser.java:1905) at net.sf.saxon.expr.parser.XPathParser.parseBasicStep(XPathParser.java:1703) at net.sf.saxon.expr.parser.XPathParser.parseStepExpression(XPathParser.java:1638) at net.sf.saxon.expr.parser.XPathParser.parseRelativePath(XPathParser.java:1564) at net.sf.saxon.expr.parser.XPathParser.parsePathExpression(XPathParser.java:1529) at net.sf.saxon.expr.parser.XPathParser.parseSimpleMappingExpression(XPathParser.java:1541) at net.sf.saxon.expr.parser.XPathParser.parseUnaryExpression(XPathParser.java:1419) at net.sf.saxon.expr.parser.XPathParser.parseUnaryExpression(XPathParser.java:1390) at net.sf.saxon.expr.parser.XPathParser.parseExprSingle(XPathParser.java:511) at net.sf.saxon.expr.parser.XPathParser.parseFunctionArgument(XPathParser.java:2629) at net.sf.saxon.expr.parser.XPathParser.parseFunctionCall(XPathParser.java:2371) at net.sf.saxon.expr.parser.XPathParser.parseBasicStep(XPathParser.java:1723) at net.sf.saxon.expr.parser.XPathParser.parseStepExpression(XPathParser.java:1638) at net.sf.saxon.expr.parser.XPathParser.parseRelativePath(XPathParser.java:1564) at net.sf.saxon.expr.parser.XPathParser.parsePathExpression(XPathParser.java:1529) at net.sf.saxon.expr.parser.XPathParser.parseSimpleMappingExpression(XPathParser.java:1541) at net.sf.saxon.expr.parser.XPathParser.parseUnaryExpression(XPathParser.java:1419) at net.sf.saxon.expr.parser.XPathParser.parseExprSingle(XPathParser.java:511) at net.sf.saxon.expr.parser.XPathParser.parseExpression(XPathParser.java:450) at net.sf.saxon.expr.parser.XPathParser.parse(XPathParser.java:386) at net.sf.saxon.expr.parser.ExpressionTool.make(ExpressionTool.java:102) at net.sf.saxon.sxpath.XPathEvaluator.createExpression(XPathEvaluator.java:153) at net.sf.saxon.s9api.XPathCompiler.internalCompile(XPathCompiler.java:497) ... 25 more

Answer 1

首先应该是abs(-$a)。注意 $.

要提供参数，您不能使用 "short-cut" 方法 XPathCompiler.evaluate()。你必须做很长的路要走：

XPathSelector exp = xpath.compile("abs(-$a)").load();
exp.setVariable(new QName("a"), new XdmAtomicValue(17));
XdmItem result = exp.evaluateSingle();

Answer 2

我知道这个帖子很旧，但我最近运行进入了这个问题，并且花了将近 2 个小时来寻找答案。最后，我在 Saxon-HE 9.4 上使用了它。尽管在我还调用之前声明了外部变量，但我一直从编译中收到错误：

    xpath.setAllowUndeclaredVariables(true);

使用 SAXON 评估 XPATH 2.0 表达式

Evaluate XPATH 2.0 expression with SAXON

saxon

xpath-2.0