如何在java中解析无效XML时的XMP？

Question

我正在使用javax.imageio从PNG图像中提取元数据。这可以很好地工作。但是用于获取实际元数据的getAsTree方法返回的XML是无效的。所以我不知道如何解析这个XML来获得特定的元数据：

run:
Format name: javax_imageio_png_1.0
<javax_imageio_png_1.0>
    <IHDR width="256" height="256" bitDepth="8" colorType="RGBAlpha" compressionMethod="deflate" filterMethod="adaptive" interlaceMethod="none"/>
    <cHRM whitePointX="31269" whitePointY="32899" redX="63999" redY="33001" greenX="30000" greenY="60000" blueX="15000" blueY="5999"/>
    <gAMA value="45454"/>
    <iTXt>
        <iTXtEntry keyword="XML:com.adobe.xmp" compressionFlag="FALSE" compressionMethod="0" languageTag="" translatedKeyword="" text="<?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.0-c061 64.140949, 2010/12/07-10:57:01        ">
 <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <rdf:Description rdf:about=""
    xmlns:xmp="http://ns.adobe.com/xap/1.0/"
    xmlns:xmpMM="http://ns.adobe.com/xap/1.0/mm/"
    xmlns:stEvt="http://ns.adobe.com/xap/1.0/sType/ResourceEvent#"
    xmlns:lr="http://ns.adobe.com/lightroom/1.0/"
    xmlns:dc="http://purl.org/dc/elements/1.1/"
   xmp:MetadataDate="2012-12-05T21:36:19+01:00"
   xmpMM:InstanceID="xmp.iid:EF7F11740720681192B08F682498C71D"
   xmpMM:DocumentID="xmp.did:FC7F11740720681192B0AE5890E66CAE"
   xmpMM:OriginalDocumentID="xmp.did:FC7F11740720681192B0AE5890E66CAE">
   <xmpMM:History>
    <rdf:Seq>
     <rdf:li
      stEvt:action="saved"
      stEvt:instanceID="xmp.iid:FC7F11740720681192B0AE5890E66CAE"
      stEvt:when="2012-12-04T00:23:34+01:00"
      stEvt:changed="/metadata"/>
     <rdf:li
      stEvt:action="saved"
      stEvt:instanceID="xmp.iid:EF7F11740720681192B08F682498C71D"
      stEvt:when="2012-12-05T21:36:19+01:00"
      stEvt:changed="/metadata"/>
    </rdf:Seq>
   </xmpMM:History>
   <lr:hierarchicalSubject>
    <rdf:Bag>
     <rdf:li>Component|Software</rdf:li>
     <rdf:li>Places|Paris</rdf:li>
     <rdf:li>Product|Christensen</rdf:li>
     <rdf:li>Product|Simba</rdf:li>
    </rdf:Bag>
   </lr:hierarchicalSubject>
   <dc:subject>
    <rdf:Bag>
     <rdf:li>Christensen</rdf:li>
     <rdf:li>Paris</rdf:li>
     <rdf:li>Simba</rdf:li>
     <rdf:li>Software</rdf:li>
    </rdf:Bag>
   </dc:subject>
  </rdf:Description>
 </rdf:RDF>
</x:xmpmeta>
<?xpacket end="r"?>"/>
    </iTXt>
    <pHYs pixelsPerUnitXAxis="2835" pixelsPerUnitYAxis="2835" unitSpecifier="meter"/>
</javax_imageio_png_1.0>
Format name: javax_imageio_1.0
<javax_imageio_1.0>
    <Chroma>
        <ColorSpaceType name="RGB"/>
        <NumChannels value="4"/>
        <Gamma value="0.45453998"/>
        <BlackIsZero value="TRUE"/>
    </Chroma>
    <Compression>
        <CompressionTypeName value="deflate"/>
        <Lossless value="TRUE"/>
        <NumProgressiveScans value="1"/>
    </Compression>
    <Data>
        <PlanarConfiguration value="PixelInterleaved"/>
        <SampleFormat value="UnsignedIntegral"/>
        <BitsPerSample value="8 8 8 8"/>
    </Data>
    <Dimension>
        <PixelAspectRatio value="1.0"/>
        <ImageOrientation value="Normal"/>
        <HorizontalPixelSize value="0.35273367"/>
        <VerticalPixelSize value="0.35273367"/>
    </Dimension>
    <Text>
        <TextEntry keyword="XML:com.adobe.xmp" value="<?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.0-c061 64.140949, 2010/12/07-10:57:01        ">
 <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <rdf:Description rdf:about=""
    xmlns:xmp="http://ns.adobe.com/xap/1.0/"
    xmlns:xmpMM="http://ns.adobe.com/xap/1.0/mm/"
    xmlns:stEvt="http://ns.adobe.com/xap/1.0/sType/ResourceEvent#"
    xmlns:lr="http://ns.adobe.com/lightroom/1.0/"
    xmlns:dc="http://purl.org/dc/elements/1.1/"
   xmp:MetadataDate="2012-12-05T21:36:19+01:00"
   xmpMM:InstanceID="xmp.iid:EF7F11740720681192B08F682498C71D"
   xmpMM:DocumentID="xmp.did:FC7F11740720681192B0AE5890E66CAE"
   xmpMM:OriginalDocumentID="xmp.did:FC7F11740720681192B0AE5890E66CAE">
   <xmpMM:History>
    <rdf:Seq>
     <rdf:li
      stEvt:action="saved"
      stEvt:instanceID="xmp.iid:FC7F11740720681192B0AE5890E66CAE"
      stEvt:when="2012-12-04T00:23:34+01:00"
      stEvt:changed="/metadata"/>
     <rdf:li
      stEvt:action="saved"
      stEvt:instanceID="xmp.iid:EF7F11740720681192B08F682498C71D"
      stEvt:when="2012-12-05T21:36:19+01:00"
      stEvt:changed="/metadata"/>
    </rdf:Seq>
   </xmpMM:History>
   <lr:hierarchicalSubject>
    <rdf:Bag>
     <rdf:li>Component|Software</rdf:li>
     <rdf:li>Places|Paris</rdf:li>
     <rdf:li>Product|Christensen</rdf:li>
     <rdf:li>Product|Simba</rdf:li>
    </rdf:Bag>
   </lr:hierarchicalSubject>
   <dc:subject>
    <rdf:Bag>
     <rdf:li>Christensen</rdf:li>
     <rdf:li>Paris</rdf:li>
     <rdf:li>Simba</rdf:li>
     <rdf:li>Software</rdf:li>
    </rdf:Bag>
   </dc:subject>
  </rdf:Description>
 </rdf:RDF>
</x:xmpmeta>
<?xpacket end="r"?>" language="" compression="none"/>
    </Text>
    <Transparency>
        <Alpha value="nonpremultipled"/>
    </Transparency>
</javax_imageio_1.0>
BUILD SUCCESSFUL (total time: 3 seconds)

无效的XML从iTXtEntry元素开始，该元素具有xpact位并包含子元素，尽管它具有自结束标记格式，而不是结束标记。因此，当我尝试使用DOM文档和xpath解析它时，我得到一个错误，指出该元素的内容中不能包含">“。

我已经在DocumentBuilderFactory上禁用了DTD验证。这没什么用。我觉得我只能使用正则表达式了，但这似乎不太对。为什么我首先从imageio中的getAsTree方法获得无效的XML，我能做些什么呢？

Answer 1

您的问题是无意义的，因为IIOMetaData.getAsTree()返回一个DOM Node对象，该对象是节点树的根。这是XML在内存中的表示。它不是从任何地方解析的，所以它不可能是无效的。xml文档字符串可能是无效的，但是这里没有要解析的字符串。getAsTree方法直接在内存中创建XML。

问题出在您的输出生成了无效的XML。无论是什么从getAsTree()序列化你的节点，都是不正确的。也就是说，它没有正确地转义text属性的值，该属性本身就是一个XML文档字符串。

下面是一个完整的示例，演示了如何获取图像元数据并将其序列化为(有效) XML字符串。

import java.io.*;
import java.util.*;

// for imageio metadata
import javax.imageio.*;
import javax.imageio.stream.*;
import javax.imageio.metadata.*;

// for xml handling
import org.w3c.dom.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.*;
import javax.xml.transform.stream.*;

public class imgmeta {
    // Very lazy exception handling
    // This is just a quick example
    public static void main(String[] args) throws Exception {
        String filename = args[0];

        File file = new File(filename);
        ImageInputStream imagestream = ImageIO.createImageInputStream(file);

        // get a reader which is able to read this file
        Iterator<ImageReader> readers = ImageIO.getImageReaders(imagestream);
        ImageReader reader = readers.next();

        // feed image to reader
        reader.setInput(imagestream, true);

        // get metadata of first image
        IIOMetadata metadata = reader.getImageMetadata(0);

        // get any metadata format name
        // (you should prefer the native one, but not all images have one)
        // String mdataname = metadata.getNativeMetadataFormatName(); // might be null
        String[] mdatanames = metadata.getMetadataFormatNames();

        String mdataname = mdatanames[0];

        Node metadatadom = metadata.getAsTree(mdataname);

        // metadatadom is now a DOM Node root of a DOM tree
        // representing metadata in the image
        // Since it's in-memory, it can't be "invalid"
        // because it's already been parsed


        // now let's serialize to an XML string
        // javax.xml.transform.Transformer takes xml sources
        // in one representation and transforms them to xml
        // in another representation
        // Representations include: DOM, JAXB, SAX, stream, etc
        DOMSource source = new DOMSource(metadatadom);

        StringWriter writer = new StringWriter();
        StreamResult result = new StreamResult(writer);

        Transformer transformer = TransformerFactory.newInstance().newTransformer();
        transformer.transform(source, result);

        // THIS is what you want:
        String metadata_in_xml = writer.toString();

        // now print it:
        System.out.print(metadata_in_xml);
    }
}

这是使用我周围的图像运行的测试输出：

$ java imgtest testimage.png | xmllint --format -
<?xml version="1.0" encoding="UTF-8"?>
<javax_imageio_png_1.0>
  <IHDR width="149" height="237" bitDepth="8" colorType="RGBAlpha" compressionMethod="deflate" filterMethod="adaptive" interlaceMethod="none"/>
  <iTXt>
    <iTXtEntry keyword="XML:com.adobe.xmp" compressionFlag="0" compressionMethod="0" languageTag="" translatedKeyword="" text="&lt;?xpacket begin=&quot;?&quot; id=&quot;W5M0MpCehiHzreSzNTczkc9d&quot;?&gt; &lt;x:xmpmeta xmlns:x=&quot;adobe:ns:meta/&quot; x:xmptk=&quot;Adobe XMP Core 5.0-c061 64.140949, 2010/12/07-10:57:01        &quot;&gt; &lt;rdf:RDF xmlns:rdf=&quot;http://www.w3.org/1999/02/22-rdf-syntax-ns#&quot;&gt; &lt;rdf:Description rdf:about=&quot;&quot; xmlns:xmp=&quot;http://ns.adobe.com/xap/1.0/&quot; xmlns:xmpMM=&quot;http://ns.adobe.com/xap/1.0/mm/&quot; xmlns:stRef=&quot;http://ns.adobe.com/xap/1.0/sType/ResourceRef#&quot; xmp:CreatorTool=&quot;Adobe Photoshop CS5.1 Macintosh&quot; xmpMM:InstanceID=&quot;xmp.iid:D281E43D34DC11E2BFE69DA1E5D17E5F&quot; xmpMM:DocumentID=&quot;xmp.did:D281E43E34DC11E2BFE69DA1E5D17E5F&quot;&gt; &lt;xmpMM:DerivedFrom stRef:instanceID=&quot;xmp.iid:D281E43B34DC11E2BFE69DA1E5D17E5F&quot; stRef:documentID=&quot;xmp.did:D281E43C34DC11E2BFE69DA1E5D17E5F&quot;/&gt; &lt;/rdf:Description&gt; &lt;/rdf:RDF&gt; &lt;/x:xmpmeta&gt; &lt;?xpacket end=&quot;r&quot;?&gt;"/>
  </iTXt>
  <tEXt>
    <tEXtEntry keyword="Software" value="Adobe ImageReady"/>
  </tEXt>
</javax_imageio_png_1.0>

生成的XML是有效的：

$ java imgmeta testimage.png | xmllint --noout -
$

(无输出表示有效。)

注意iTXtEntry的text属性值是如何转义的。如果想要检索该属性中的数据，则需要检索字符串，然后将其解析为自己的keyword="XML:com.adobe.xmp"文档，以获得另一个DOM (或其他任何)树。

更新:解析XMP数据

下面是一些示例代码，演示了如何提取属性值，并在XML和DOM树之间进行解析。

public class XMPExample {
public static String transformXML(Node xml) throws Exception {
    StringWriter writer = new StringWriter();

    Transformer transformer = TransformerFactory.newInstance().newTransformer();
    transformer.transform(new DOMSource(xml), new StreamResult(writer));

    return writer.toString();
}

public static Document transformXML(String xml) throws Exception {
    StringReader reader = new StringReader(xml);
    Document doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument();
    Transformer transformer = TransformerFactory.newInstance().newTransformer();

    transformer.transform(new StreamSource(reader), new DOMResult(doc));
    return doc;
}

public static String getXMP(Element metadata_dom) throws Exception {
            // (Element) type because getElementsByTagName() method is required

    // There are many more robust ways of selecting nodes
    // (e.g. javax.xml.xpath), but this is for a simple example
    // that only uses the native DOM methods

    // This is very brittle because we're making assumptions about
    // the metadata_dom structure. There are two sources of brittleness:

    // 1. The metadata format from `metadata.getMetadataFormatNames()`.
    //    You should probably settle on a standard one you know will
    //    exist, like 'javax_imageio_1.0'
    // 2. How the image stores the metadata. Usually XMP data will
    //    be in a text field with keyword 'XML:com.adobe.xmp', but
    //    I don't know that this is *always* the case.

    // the code below assumes "javax_imageio_png_1.0" format
    NodeList iTXtEntries = metadata_dom.getElementsByTagName("iTXtEntry");
    Element iTXtEntry = null;
    Element entry = null;
    for (int i = 0; i < iTXtEntries.getLength(); i++) {
        entry = (Element) iTXtEntries.item(i);
        if (entry.getAttribute("keyword").equals("XML:com.adobe.xmp")) {
            iTXtEntry = entry;
            break;
        }
    }
    if (iTXtEntry == null) {
        return null;
    }

    String xmp_xml_doc = iTXtEntry.getAttribute("text");

    return xmp_xml_doc;

}
}

// Use like so:
Node metadatanode = metadata.getAsTree(metadataname);

String xmp_xml = XMPExample.getXMP((Element) metadatanode);

// xmp_xml is now an xml document STRING
System.out.print(xmp_xml);

// If you want to parse it as an XML document, use an XML parser.
Document xmp_dom = XMPExample.transformXML(xmp_xml);

// ...and you can serialize it again when you are done.
String xmp_xml_roundtripped = XMPExample.transformXML(xmp_dom);