我有books.xml个文件，其中包含作者姓名和书名。我使用以下代码段来查询books.xml。
XPathFactory factory = XPathFactory.newInstance();
XPath xpath = factory.newXPath();
XPathExpression expr
= xpath.compile("//book[author= 'Larry Niven']/title/text()");
现在，如果我想在程序作为String变量运行时传递它而不是直接将名称放在查询中，该怎么做。只是把字符串变量名称不起作用！
答案 0 :(得分：6)
这里的问题是当你有一个臭名昭着的拉里“Basher”O'Niven 这样的作家时。
在这种情况下，您需要转义变量，就像在这个天真的实现中一样：
public static String escape(String s) {
Matcher matcher = Pattern.compile("['\"]")
.matcher(s);
StringBuilder buffer = new StringBuilder("concat(");
int start = 0;
while (matcher.find()) {
buffer.append("'")
.append(s.substring(start, matcher.start()))
.append("',");
buffer.append("'".equals(matcher.group()) ? "\"'\"," : "'\"',");
start = matcher.end();
}
if (start == 0) {
return "'" + s + "'";
}
return buffer.append("'")
.append(s.substring(start))
.append("'")
.append(")")
.toString();
}
这可以通过以下代码证明：
String xml =
"<xml><foo bar=\"Larry "Basher" O'Niven\">Ringworm</foo></xml>";
String query =
String.format("//foo[@bar=%s]", escape("Larry \"Basher\" O'Niven"));
System.out.println(query);
String book = XPathFactory.newInstance()
.newXPath()
.evaluate(query, new InputSource(new StringReader(xml)));
System.out.println(query + " > " + book);
答案 1 :(得分：4)
事实上，您可以在XPath中同时使用自定义函数和变量 - 但快速入侵可能会在许多用途中提高效率。
以下是我为学生开发的一些学习工具代码。它可以让你这样做：
// create some variable we want to use in the xpath
xPathVariableAndFunctionResolver.newVariable("myNamespace", "id", "xs:string", "l2"); // myNamespace is declared in the namespace context with prefix 'my'
// create an XPath expression
String expression = "//did:Component[@id=$my:id]"; // variable $namespace:name
XPathExpression findComponents = xPathFunctionAndVariableOperator.compile(expression);
// execute the XPath expression against the document
NodeList statements = (NodeList)findComponents.evaluate(document, XPathConstants.NODESET);
与XPath函数大致相同。代码，首先是正常XPath评估的包装器：
public class XPathOperator {
protected XPath xPath;
protected XPathFactory xPathFactory;
private Hashtable<String, XPathExpression> compiled = new Hashtable<String, XPathExpression>();
protected void initFactory() throws XPathFactoryConfigurationException {
xPathFactory = XPathFactory.newInstance(XPathConstants.DOM_OBJECT_MODEL);
}
protected void initXPath(NamespaceContext context) {
xPath = xPathFactory.newXPath();
xPath.setNamespaceContext(context);
}
public XPathOperator(NamespaceContext context) throws XPathFactoryConfigurationException {
initFactory();
initXPath(context);
}
public Object evaluate(Document document, String expression, QName value) throws XPathExpressionException {
// create an XPath expression - http://www.zvon.org/xxl/XPathTutorial/General/examples.html
XPathExpression findStatements = compile(expression);
// execute the XPath expression against the document
return (NodeList)findStatements.evaluate(document, value);
}
public XPathExpression compile(String expression) throws XPathExpressionException {
if(compiled.containsKey(expression)) {
return (XPathExpression) compiled.get(expression);
}
XPathExpression xpath = xPath.compile(expression);
System.out.println("Compiled XPath " + expression);
compiled.put(expression, xpath);
return xpath;
}
}
然后我们添加自定义变量和函数的概念，当然还有命名空间：
public class XPathFunctionAndVariableOperator extends XPathOperator {
public XPathFunctionAndVariableOperator(NamespaceContext context, XPathVariableResolver xPathVariableResolver, XPathFunctionResolver xPathFunctionResolver) throws XPathFactoryConfigurationException {
super(context);
xPath.setXPathVariableResolver(xPathVariableResolver);
xPath.setXPathFunctionResolver(xPathFunctionResolver);
}
}
没有变量和函数解析器，这将不会很有趣：
public class XPathVariableAndFunctionResolver implements XPathVariableResolver, XPathFunctionResolver {
private Hashtable functions = new Hashtable();
private Hashtable variables = new Hashtable();
private SchemaDVFactory factory = SchemaDVFactory.getInstance();
public XPathFunction resolveFunction(QName functionName, int arity) {
Hashtable table = (Hashtable)functions.get(functionName.getNamespaceURI());
if(table != null) {
XPathFunction function = (XPathFunction)table.get(functionName.getLocalPart());
if(function == null) {
throw new RuntimeException("Function " + functionName.getLocalPart() + " does not exist in namespace " + functionName.getNamespaceURI() + "!");
}
System.out.println("Resolved function " + functionName + " with " + arity + " argument(s)");
return function;
}
throw new RuntimeException("Function namespace " + functionName.getNamespaceURI() + " does not exist!");
}
/**
*
* Adds a variable using namespace and name, primitive type and default value
*
* @param namespace
* @param name
* @param datatype one of the built-in XML datatypes
* @param value
* @throws InvalidDatatypeValueException if value is not of correct datatype
*/
@SuppressWarnings("unchecked")
public void newVariable(String namespace, String name, String datatype, String value) throws InvalidDatatypeValueException {
int index = datatype.indexOf(":");
if(index != -1) {
datatype = datatype.substring(index+1);
}
XSSimpleType builtInType = factory.getBuiltInType(datatype);
if(builtInType == null) {
throw new RuntimeException("Null type for " + datatype);
}
ValidationState validationState = new ValidationState();
ValidatedInfo validatedInfo = new ValidatedInfo();
builtInType.validate(value, validationState, validatedInfo);
System.out.println("Defined variable " + name + " as " + datatype + " with value " + value);
Hashtable table;
if(!variables.containsKey(namespace)) {
table = new Hashtable();
variables.put(namespace, table);
} else {
table = (Hashtable)variables.get(namespace);
}
table.put(name, new Object[]{validatedInfo, builtInType});
}
public void newVariableValue(String namespace, String name, String value) throws InvalidDatatypeValueException {
ValidationState validationState = new ValidationState();
Hashtable table;
if(!variables.containsKey(namespace)) {
throw new RuntimeException("Unknown variable namespace " + namespace);
} else {
table = (Hashtable)variables.get(namespace);
}
Object[] bundle = (Object[])table.get(name);
ValidatedInfo validatedInfo = (ValidatedInfo)bundle[0];
XSSimpleType builtInType = (XSSimpleType)bundle[1];
builtInType.validate(value, validationState, validatedInfo); // direct reference transfer of value
System.out.println("Assigned value " + validatedInfo.normalizedValue + " to variable " + name);
}
public Object resolveVariable(QName variableName) {
Hashtable table;
if(!variables.containsKey(variableName.getNamespaceURI())) {
throw new RuntimeException("Unknown variable namespace " + variableName.getNamespaceURI());
} else {
table = (Hashtable)variables.get(variableName.getNamespaceURI());
}
Object[] bundle = (Object[])table.get(variableName.getLocalPart());
if(bundle != null) {
ValidatedInfo var = (ValidatedInfo)bundle[0];
if(var != null) {
switch(var.actualValueType) { // some types omitted, customize your own
case XSConstants.INTEGER_DT:
case XSConstants.DECIMAL_DT:
case XSConstants.INT_DT:
case XSConstants.LONG_DT:
case XSConstants.SHORT_DT:
case XSConstants.BYTE_DT:
case XSConstants.UNSIGNEDBYTE_DT:
case XSConstants.UNSIGNEDINT_DT:
case XSConstants.UNSIGNEDLONG_DT:
case XSConstants.UNSIGNEDSHORT_DT:
return new Integer(var.normalizedValue);
case XSConstants.DATE_DT:
case XSConstants.DATETIME_DT:
case XSConstants.GDAY_DT:
case XSConstants.GMONTH_DT:
case XSConstants.GMONTHDAY_DT:
case XSConstants.GYEAR_DT:
case XSConstants.GYEARMONTH_DT:
case XSConstants.DURATION_DT:
case XSConstants.TIME_DT:
return new Date(var.normalizedValue);
case XSConstants.FLOAT_DT:
return new Float(Float.parseFloat(var.normalizedValue));
case XSConstants.DOUBLE_DT:
return new Double(Double.parseDouble(var.normalizedValue));
case XSConstants.STRING_DT:
case XSConstants.QNAME_DT:
return var.normalizedValue;
default:
throw new RuntimeException("Unknown datatype " + var.actualValueType + " for variable " + variableName + " in namespace " + variableName.getNamespaceURI());
}
}
}
throw new RuntimeException("Could not resolve value " + variableName + " in namespace " + variableName.getNamespaceURI());
}
public void addFunction(String namespace, String name, XPathFunction function) {
Hashtable table;
if(!functions.containsKey(namespace)) {
table = new Hashtable();
functions.put(namespace, table);
} else {
table = (Hashtable)functions.get(namespace);
}
table.put(name, function);
}
}
这些功能显然不能包含在上面，因为通常运行自定义代码（即整个点是您编写自己的类），所以请使用类似
的内容public abstract class XPathFunctionImpl implements XPathFunction {
/**
* This function is called by the XPath expression as it implements the interface XPathFunction
*/
protected int numberArguments;
public Object evaluate(List args) throws XPathFunctionException {
if(args.size() == numberArguments) {
return evaluateImpl(args);
}
throw new RuntimeException("Illegal number of arguments for " + this);
}
public abstract Object evaluateImpl(List args) throws XPathFunctionException;
}
然后实现/子类在evaluateImpl（..）中以某种方式使用你自己的逻辑。
这肯定会使String追加看起来很有吸引力;）注意：这段代码已有几年的历史了，可能有更好的方法来完成这一切。
答案 2 :(得分：0)
如果你想要一个现成的实现，你可以使用支持变量声明的公共JXPath： http://commons.apache.org/jxpath/users-guide.html#Variables
答案 3 :(得分：-2)
String rawXPath = "//book[author= '" + larrysName + "']/title/text()";
或
String rawXPath = String.format("//book[author= '%s']/title/text()", larrysName);
其中larrysName是来自某个地方的String类型的变量。