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An overview of Qt's XQuery support.
The QtXmlPatterns module is part of the Qt Desktop Edition, and the Qt Open Source Edition.Introduction
The query opens the file library.xml, and for each book element that is a child of the top element bib, and whose attribute by name year is larger than 1991 and has Addison-Wesley as a publisher, it constructs a book element and attaches it to the parent element called bibliography.Why use XQuery?
XQuery is made for selecting and aggregating information in safe and efficient ways. Hence, if an application selects and navigates data, XQuery could be used to perform the selection and navigation tasks quickly and bug-free. With QAbstractXmlNodeModel, these advantages are not constrained to operating only on XML files, but can be applied to other data as well.
The strengths of XQuery can be summarized as follows:
#include <QtXmlPatterns>The application is linked with the QtXmlPatterns module by adding the following line to the qmake .pro file:
QT += xmlpatternsNote that if you build Qt yourself, QtXmlPatterns will not be built if exceptions are disabled, or if you compile Qt with a compiler that doesn't support member templates, e.g., MSVC 6.
See the QXmlQuery documentation for the QtXmlPatterns C++ API.Command line utility
xmlpatterns is a command line utility for running XQueries. It takes as its single argument the name of a file containing the text of the XQuery to be evaluated.
xmlpatterns myQuery.xqThe XQuery in myQuery.xq will be evaluated and its output written to stdout.
Passing the -help switch on the command line tells xmlpatterns to print brief descriptions of the other flags it accepts.
xmlpatterns can be used for scripting, but the descriptions and messages it outputs are not designed to be parsed, and they may be changed in future releases of Qt.XQuery Language Tutorial
See A Short Path to XQuery for a brief introduction to the XQuery language.The Qt and XQuery Data Models
XQuery and Qt don't represent data the same way. XQuery represents data as a sequence of items, where an item is either an atomic value or a node. Atomic values are the primitives specified in the W3C XML Schema. Nodes are normally XML elements or attributes, but nodes can also represent non-XML data items, when non-XML data is modeled with a custom subclass of QAbstractXmlNodeModel.Mapping XQuery Atomic Value Types to QVariant Types
When XQuery Atomic Values are returned as XQuery result items via the QtXmlPatterns API, they are represented as instances of the QVariant class (or in one case, QXmlName). The mapping from XQuery Atomic Value types to QVariant types (or to QXmlName) is as follows.
xs:QName | QXmlName (see Using QXmlNames below) |
xs:integer | QVariant::LongLong |
xs:string | QVariant::String |
xs:string* | QVariant::StringList |
xs:double | QVariant::Double |
xs:float | QVariant::Double |
xs:boolean | QVariant::Bool |
xs:decimal | QVariant::Double |
xs:hexBinary | QVariant::ByteArray |
xs:base64Binary | QVariant::ByteArray |
xs:gYear | QVariant::DateTime |
xs:gYearMonth | QVariant::DateTime |
xs:gMonthDay | QVariant::DateTime |
xs:gDay | QVariant::DateTime |
xs:gMonth | QVariant::DateTime |
xs:anyURI | QVariant::Url |
xs:untypedAtomic | QVariant::String |
xs:ENTITY | QVariant::String |
xs:date | QVariant::DateTime |
xs:dateTime | QVariant::DateTime |
xs:time | (see No mapping for xs:time below) |
QVariant::LongLong | xs:integer |
QVariant::Int | xs:integer |
QVariant::UInt | xs:nonNegativeInteger |
QVariant::ULongLong | xs:unsignedLong |
QVariant::String | xs:string |
QVariant::Double | xs:double |
QVariant::Bool | xs:boolean |
QVariant::Double | xs:decimal |
QVariant::ByteArray | xs:base64Binary |
QVariant::StringList | xs:string* |
QVariant::Url | xs:string |
QVariant::Date | xs:date. |
QVariant::DateTime | xs:dateTime |
QVariant::Time. | xs:time. (see Binding To QVariant::Time below) |
QVariantList | (see Binding To QVariantList below) |
When QtXmlPatterns loads and queries XML files and produces XML output, it can always load the XML data into its default XML node model, where it can be traversed efficiently. The XQuery below traverses the product orders found in the XML file myOrders.xml to find all the skin care product orders and output them ordered by shipping date. QtXmlPatterns can be used out of the box to perform this query, provided myOrders.xml actually contains valid XML. It can be loaded directly into the default XML node model and traversed. But suppose we want QtXmlPatterns to perform queries on the hierarchical structure of the local file system. The default XML node model in QtXmlPatterns is not suitable for navigating the file system, because there is no XML file to load that contains a description of it. Such an XML file, if it existed, might look something like this: There is no such file to load into the default XML node model, but one can write a subclass of QAbstractXmlNodeModel to represent the file system. This custom XML node model, once populated with all the directory and file descriptors obtained directly from the system, presents the complete file system hierarchy to the query engine via the same API used by the default XML node model to present the contents of an XML file. In other words, once the custom XML node model is populated, it presents the file system to the query engine as if a description of it had been loaded into the default XML node model from an XML file like the one shown above.
Now we can write an XQuery to find all the XML files and parse them to find the ones that don't contain valid XML.snippets/patternist/introNavigateFS.xq Without QtXmlPatterns, there is no simple way to solve this kind of problem. You might do it by writing a C++ program to traverse the file system, sniff out all the XML files, and submit each one to an XML parser to test that it contains valid XML. The C++ code required to write that program will probably be more complex than the C++ code required to subclass QAbstractXmlNodeModel, but even if the two are comparable, your custom C++ program can be used only for that one task, while your custom XML node model can be used by any XQuery that must navigate the file system.
The general approach to using XQuery to perform queries on non-XML data has been a three step process. In the first step, the data is loaded into a non-XML data model. In the second step, the non-XML data model is serialized as XML and output to XML (text) files. In the final step, an XML tool loads the XML files into a second, XML data model, where the XQueries can be performed. The development cost of implementing this process is often high, and the three step system that results is inefficient because the two data models must be built and maintained separately.
With QtXmlPatterns, subclassing QAbstractXmlNodeModel eliminates the transformation required to convert the non-XML data model to the XML data model, because there is only ever one data model required. The non-XML data model presents the non-XML data to the query engine via the XML data model API. Also, since the query engine uses the API to access the QAbstractXmlNodeModel, the data model subclass can construct the elements, attributes and other data on demand, responding to the query's specific requests. This can greatly improve efficiency, because it means the entire model might not have to be built. For example, in the file system model above, it is not necessary to parse the contents of an XML file until the contents of that file are actually required.
Examples of other places where XQuery could be used in QtXmlPatterns to query non-XML data:
Consider a word processor application that must import and export data in several different formats. Rather than writing a lot of C++ code to convert each input format to an intermediate form, and a lot more C++ code to convert the intermediate form back to each output format, one can implement a solution based on QtXmlPatterns that uses simple XQueries to transform each XML or non-XML format (e.g. MathFormula.xml below) to the intermediate form (e.g. the DocumentRepresentation node model class below), and more simple XQueries to transform the intermediate form back to each XML or non-XML format.
The articles Avoid the dangers of XPath injection, Robi Sen and Blind XPath Injection, Amit Klein discuss the XQuery code injection problem in more detail.Denial of Service Attacks
Applications using QtXmlPatterns will be subject to the same software limits as any other system. Generally, these can not be checked. This means QtXmlPatterns does not prevent rogue queries from consuming too many resources. For example, a query could take too much time to execute, or could attempt to transfer too much data. Or a query could cause an unreasonable amount of recursion, which could crash the system. XQueries can do these things accidentally, but they can also be meant as deliberate, denial of service attacks.Features and Conformance
Conformance
QtXmlPatterns is a conformant XQuery processor because it meets the requirements for Minimal Coformance. Additionally, QtXmlPatterns supports the Serialization Feature and the Full Axis Feature. QtXmlPatterns passes 97% of the tests in the XML Query Test Suite, and it is expected this will improve over time. Areas where conformance may be questionable and where behavior may be changed in future releases are:
Since XPath 2.0 is a subset of XQuery 1.0, it is supported.
The specifications discusses conformance further: XQuery 1.0: An XML Query Language. W3C's XQuery testing effort can be of interest as well, XML Query Test Suite.
Currently fn:collection() does not access any data set, and there is no API for providing data through the collection. As a result, evaluating fn:collection() returns the empty sequence. We intend to provide functionality for this in a future release of Qt.
Processing of XML files supports xml:id. In practice, this allows elements that have an attribute named xml:id to be looked up efficiently with the fn:id() function. See xml:id Version 1.0 for details.
Only queries encoded in UTF-8 are supported.Resource Loading
When QtXmlPatterns loads an XML resource, e.g., using fn:doc() function, the following schemes are supported:
file | Local files. |
data | The bytes are encoded in the URI itself. For instance, data:application/xml,%3Ce%2F%3E is <e/>. |
ftp | Resources retrieved via FTP. |
http | Resources retrieved via HTTP. |
https | Resources retrieved via HTTPS. This will succeed if no SSL errors are encountered. |
qrc | Qt Resource files. Expressing it as an empty scheme, :/..., is not supported. |
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