java - 如何使用JAVA实现XQL Join？

Question

我对 XQL 很陌生，最近在这里研究了 XQL 连接(连接 XML 文档):- http://www.ibiblio.org/xql/xql-proposal.html#Joins 。我只是想知道，如果我想在 JAVA 中实现 XQL Join，我该如何使用 JAVA 来实现呢？

加入是 XQL 的一项新功能。我尝试在 JAVA 中的 XQuery 和 XPath API 中搜索 XQL Join 示例，但尚未找到任何(截至目前)支持 XQL Join 的内容。

XQL 连接的示例如下:-

假设我们有书籍来源和评论来源:-

<book>
  <isbn> 84-7169-020-9 </isbn>
  <title> Tales of the Alhambra </title>
  <author> Washington Irving </author>
</book>

<review>
  <isbn> 84-7169-020-9 </isbn>
  <title> Tales of the Alhambra </title>
  <reviewer> Ricardo Sanchez </reviewer>
  <comments>
    A romantic and humorous account of the time that
    the author of "The Legend of Sleepy Hollow" lived
    in an Arabian palace in Spain.
  </comments>
</review>

我们可能希望将这些结合起来创建一个包含评论中评论的图书 View :

<book>
  <isbn> 84-7169-020-9 </isbn>
  <title> Tales of the Alhambra </title>
  <author> Washington Irving </author>

  <review>
     <reviewer> Ricardo Sanchez </reviewer>
     <comments>
          A romantic and humorous account of the time that
          the author of "The Legend of Sleepy Hollow" lived
          in an Arabian palace in Spain.
     </comments>
  </review>
</book>

为了实现这一点，我们可以编写以下 XQL 查询:-

/book[isbn][$i:=isbn] {
    $i  | title  | author
    | //review[isbn=$i] { reviewer | comments }
}

虽然我在 JAVA 中的 XPath 和 XQuery API 中没有找到任何此类支持，但仍然为了尝试，我尝试使用 XPath JAVA API 进行相同的操作。详情如下:-

XML 保存在测试文件 (books.txt) 中，其中包含以下数据:-

<?xml version="1.0"?>

<catalog>

  <book>
    <isbn>0470192747</isbn>
    <author>Kay, Michael</author>
    <title>XSLT 2.0 and XPath 2.0 (4th Edition)</title>
    <genre>Computer</genre>
    <price>33.99</price>
    <publish_date>2008-06-03</publish_date>
    <description>This book is primarily a practical reference
    book for professional XSLT developers.</description>
  </book>

  <book>
    <isbn>0596006349</isbn>
    <author>Walmsley, Priscilla</author>
    <title>XQuery</title>
    <genre>Computer</genre>
    <price>38.50</price>
    <publish_date>2007-03-30</publish_date>
    <description>This in-depth tutorial not only walks you through
    the XQuery specification, but also teaches you how to program with
    this widely anticipated query language.</description>
  </book>

  <book>
    <isbn>059652112X</isbn>
    <author>Kalin, Martin</author>
    <title>Java Web Services: Up and Running</title>
    <genre>Computer</genre>
    <price>26.99</price>
    <publish_date>2009-02-23</publish_date>
    <description>With this example-driven book, you get a quick, practical,
    and thorough introduction to Java's API for XML Web Services (JAX-WS)
    and the Java API for RESTful Web Services (JAX-RS).</description>
  </book>

  <book>
    <isbn>0321356683</isbn>
    <author>Bloch, Joshua</author>
    <title>Effective Java (2nd Edition)</title>
    <genre>Computer</genre>
    <price>35.99</price>
    <publish_date>2008-05-22</publish_date>
    <description>Presents the most practical, authoritative guidelines
    available for writing efficient,well-designed programs.</description>
  </book>

  <book>
    <isbn>0141014865</isbn>
    <author>de Botton, Alain</author>
    <title>Status Anxiety</title>
    <genre>Philosophy</genre>
    <price>9.99</price>
    <publish_date>2005-01-13</publish_date>
    <description>The author presents a universal condition of which
    many of us suffer from called Status Anxiety, investigates it's
    origins and possible solutions.</description>
  </book>

  <book>
    <isbn>0201771861</isbn>
    <author>Rusty Harold, Elliotte</author>
    <title>Processing XML with Java (SAX, DOM, JDOM, JAXP &amp; TrAX)</title>
    <genre>Computer</genre>
    <price>37.99</price>
    <publish_date>2002-11-14</publish_date>
    <description>Handing and processing XML in
    the Java programming language.</description>
  </book>

  <book>
    <isbn>1887521143</isbn>
    <author>Poomsan Becker, Benjawan</author>
    <title>Thai-English and English-Thai Dictionary</title>
    <genre>Dictionary</genre>
    <price>14.95</price>
    <publish_date>2005-04-30</publish_date>
    <description>With Transliteration for Non-Thai Speakers -
    Complete with Thai Alphabet Guide</description>
  </book>

  <book>
    <isbn>0415071771</isbn>
    <author>Jung, Carl Gustav</author>
    <title>Psychological Types</title>
    <genre>Psychology</genre>
    <price>19.99</price>
    <publish_date>1992-01-02</publish_date>
    <description>Essential reading for anyone requiring a proper
    understanding of Jung's psychology, this was the work in which Jung
    set out his theory of psychological types as a means of understanding
    ourselves and the world around us.</description>
  </book>

  <book>
    <isbn>0596003552</isbn>
    <author>Pawson, Dave</author>
    <title>XSL-FO: Making XML Look Good in Print</title>
    <genre>Computer</genre>
    <price>26.99</price>
    <publish_date>2002-08-19</publish_date>
    <description>Outlines XSL FO's strengths and weaknesses, provides
    a tutorial and reference guide.</description>
  </book>

  <book>
    <isbn>0321392795</isbn>
    <author>Gray, Simon</author>
    <title>Data Structures in Java</title>
    <genre>Computer</genre>
    <price>53.99</price>
    <publish_date>2006-11-13</publish_date>
    <description>From Abstract Data Types to the
    Java Collections Framework.</description>
  </book>

  <review>
      <isbn>0321392795</isbn>
      <title> Tales of the Alhambra </title>
      <reviewer> Ricardo Sanchez </reviewer>
      <comments>
        A romantic and humorous account of the time that
        the author of "The Legend of Sleepy Hollow" lived
        in an Arabian palace in Spain.
      </comments>
  </review>

</catalog>

实现的JAVA代码如下:-

import java.io.File;
import java.io.IOException;

import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;

import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
import org.w3c.dom.Node;
import org.w3c.dom.Element;
import org.xml.sax.SAXException;

public class XQLJoin {

   public static void main(String[] args) {

      try {
         File inputFile = new File("books.txt");
         DocumentBuilderFactory dbFactory 
            = DocumentBuilderFactory.newInstance();
         DocumentBuilder dBuilder;

         dBuilder = dbFactory.newDocumentBuilder();

         Document doc = dBuilder.parse(inputFile);
         doc.getDocumentElement().normalize();

         XPath xPath =  XPathFactory.newInstance().newXPath();

         String expression = "/catalog/book[isbn][$i:=isbn] {  $i  | title  | author | //review[isbn=$i] { reviewer | comments }}";         
         NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(doc, XPathConstants.NODESET);
         for (int i = 0; i < nodeList.getLength(); i++) {
            Node nNode = nodeList.item(i);
            System.out.println("\nCurrent Element :" 
               + nNode.getNodeName());
            if (nNode.getNodeType() == Node.ELEMENT_NODE) {
               Element eElement = (Element) nNode;
              System.out.println("Title : " 
                  + eElement
                     .getElementsByTagName("title")
                     .item(0)
                     .getTextContent());
               System.out.println("Reviewer : " 
                  + eElement
                     .getElementsByTagName("reviewer")
                     .item(0)
                     .getTextContent());
            }
         }
      } catch (ParserConfigurationException e) {
         e.printStackTrace();
      } catch (SAXException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      } catch (XPathExpressionException e) {
         e.printStackTrace();
      }
   }
}

上面的代码抛出了一个异常，堆栈跟踪如下:-

javax.xml.transform.TransformerException: Expected ], but found: isbn
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.error(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.consumeExpected(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.Predicate(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.Step(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.RelativeLocationPath(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.LocationPath(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.PathExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnionExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnaryExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.MultiplicativeExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.AdditiveExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.RelationalExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.EqualityExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.AndExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.OrExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.Expr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.initXPath(Unknown Source)
    at com.sun.org.apache.xpath.internal.XPath.<init>(Unknown Source)
    at com.sun.org.apache.xpath.internal.XPath.<init>(Unknown Source)
    at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.compile(Unknown Source)
    at XQLJoin.main(XQLJoin.java:36)
--------------- linked to ------------------
javax.xml.xpath.XPathExpressionException: javax.xml.transform.TransformerException: Expected ], but found: isbn
    at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.compile(Unknown Source)
    at XQLJoin.main(XQLJoin.java:36)
Caused by: javax.xml.transform.TransformerException: Expected ], but found: isbn
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.error(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.consumeExpected(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.Predicate(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.Step(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.RelativeLocationPath(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.LocationPath(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.PathExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnionExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnaryExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.MultiplicativeExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.AdditiveExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.RelationalExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.EqualityExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.AndExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.OrExpr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.Expr(Unknown Source)
    at com.sun.org.apache.xpath.internal.compiler.XPathParser.initXPath(Unknown Source)
    at com.sun.org.apache.xpath.internal.XPath.<init>(Unknown Source)
    at com.sun.org.apache.xpath.internal.XPath.<init>(Unknown Source)
    ... 2 more

请建议如何使用 Java 实现此功能。有什么线索吗？

Answer 1

首先让我们从 XQL 开始; XQL 已过时，据我所知当前没有实现，我相信 XQL 是 XQuery 的前身，所以您应该从 XQuery 3.1 开始而不是担心 XQL。

话虽如此，XQuery 缺乏 XQL 中描述的显式连接。 XQuery 确实具有创建连接的功能，但没有隐式的结果合并。您可以使用立即想到的 3 种方法之一来显式合并结果:

1. XQuery Update获取第一个查询的结果并插入连接的结果。
2. 安 identity transform在第一个查询的结果上，注入(inject)连接的结果。
3. 使用元素构造函数输出所需的 XML 形状，并将查询放入该结构中(如下所示)。

上面代码的问题是您的 XPath 无效，这正是您收到的错误告诉您的内容，即 [$i:=isbn] 是最初的问题，但是那么 {{ 部分也不是有效的 XPath 或 XQuery。

对于 XQuery 表达式来执行您正在寻找的操作，我将为初学者提供可能更简单的方法(即上面的选项 3):

<catalog>
{
    for $book in /catalog/book
    let $reviews := /catalog/review[isbn eq $book/isbn]
    return
        <book>
        {
            $book/*,
            for $review in $reviews
            return
                <review>
                {
                    $review/(reviewer | comments)
                }
                </review>
        }
        </book>
}
</catalog>

这将产生以下输出(来自您在问题中描述的源文档):

<?xml version="1.0" encoding="UTF-8"?>
<catalog>
   <book>
      <isbn>0470192747</isbn>
      <author>Kay, Michael</author>
      <title>XSLT 2.0 and XPath 2.0 (4th Edition)</title>
      <genre>Computer</genre>
      <price>33.99</price>
      <publish_date>2008-06-03</publish_date>
      <description>This book is primarily a practical reference
            book for professional XSLT developers.</description>
   </book>
   <book>
      <isbn>0596006349</isbn>
      <author>Walmsley, Priscilla</author>
      <title>XQuery</title>
      <genre>Computer</genre>
      <price>38.50</price>
      <publish_date>2007-03-30</publish_date>
      <description>This in-depth tutorial not only walks you through
            the XQuery specification, but also teaches you how to program with
            this widely anticipated query language.</description>
   </book>
   <book>
      <isbn>059652112X</isbn>
      <author>Kalin, Martin</author>
      <title>Java Web Services: Up and Running</title>
      <genre>Computer</genre>
      <price>26.99</price>
      <publish_date>2009-02-23</publish_date>
      <description>With this example-driven book, you get a quick, practical,
            and thorough introduction to Java's API for XML Web Services (JAX-WS)
            and the Java API for RESTful Web Services (JAX-RS).</description>
   </book>
   <book>
      <isbn>0321356683</isbn>
      <author>Bloch, Joshua</author>
      <title>Effective Java (2nd Edition)</title>
      <genre>Computer</genre>
      <price>35.99</price>
      <publish_date>2008-05-22</publish_date>
      <description>Presents the most practical, authoritative guidelines
            available for writing efficient,well-designed programs.</description>
   </book>
   <book>
      <isbn>0141014865</isbn>
      <author>de Botton, Alain</author>
      <title>Status Anxiety</title>
      <genre>Philosophy</genre>
      <price>9.99</price>
      <publish_date>2005-01-13</publish_date>
      <description>The author presents a universal condition of which
            many of us suffer from called Status Anxiety, investigates it's
            origins and possible solutions.</description>
   </book>
   <book>
      <isbn>0201771861</isbn>
      <author>Rusty Harold, Elliotte</author>
      <title>Processing XML with Java (SAX, DOM, JDOM, JAXP &amp; TrAX)</title>
      <genre>Computer</genre>
      <price>37.99</price>
      <publish_date>2002-11-14</publish_date>
      <description>Handing and processing XML in
            the Java programming language.</description>
   </book>
   <book>
      <isbn>1887521143</isbn>
      <author>Poomsan Becker, Benjawan</author>
      <title>Thai-English and English-Thai Dictionary</title>
      <genre>Dictionary</genre>
      <price>14.95</price>
      <publish_date>2005-04-30</publish_date>
      <description>With Transliteration for Non-Thai Speakers -
            Complete with Thai Alphabet Guide</description>
   </book>
   <book>
      <isbn>0415071771</isbn>
      <author>Jung, Carl Gustav</author>
      <title>Psychological Types</title>
      <genre>Psychology</genre>
      <price>19.99</price>
      <publish_date>1992-01-02</publish_date>
      <description>Essential reading for anyone requiring a proper
            understanding of Jung's psychology, this was the work in which Jung
            set out his theory of psychological types as a means of understanding
            ourselves and the world around us.</description>
   </book>
   <book>
      <isbn>0596003552</isbn>
      <author>Pawson, Dave</author>
      <title>XSL-FO: Making XML Look Good in Print</title>
      <genre>Computer</genre>
      <price>26.99</price>
      <publish_date>2002-08-19</publish_date>
      <description>Outlines XSL FO's strengths and weaknesses, provides
            a tutorial and reference guide.</description>
   </book>
   <book>
      <isbn>0321392795</isbn>
      <author>Gray, Simon</author>
      <title>Data Structures in Java</title>
      <genre>Computer</genre>
      <price>53.99</price>
      <publish_date>2006-11-13</publish_date>
      <description>From Abstract Data Types to the
            Java Collections Framework.</description>
      <review>
         <reviewer> Ricardo Sanchez </reviewer>
         <comments>
            A romantic and humorous account of the time that
            the author of "The Legend of Sleepy Hollow" lived
            in an Arabian palace in Spain.
        </comments>
      </review>
   </book>
</catalog>

如果您使用它作为上面的表达式，那么您应该可以开始了:-)