没有 @XMLRootElement 的 JAXB 部分解组元素

Posted 2023-02-16

【中文标题】没有 @XMLRootElement 的 JAXB 部分解组元素

【英文标题】：JAXB partial-unmarshalling Elements without @XMLRootElement

【发布时间】：2011-09-23 00:04:41

【问题描述】：

我正在使用 JAXB 的 partial-unmarshalling 示例，但我无法解组不在根级别的 XML 元素（因为它们没有t 有一个 @XmlRootElement 标记）。在我的示例中，我尝试阅读 shipTo-Element 而不是 purchaseOrder-Element。

通常我会使用 JAXBElement unmarshal(Source source,Class declaredType) 但由于该示例使用 UnmarshallerHandler 和 XMLFilterImpl 我不知道在哪里告诉 Jaxb 它应该使用哪个类。

我的错误信息是：Caused by: javax.xml.bind.UnmarshalException: unexpected element (uri:"", local:"shipTo")。预期元素是 、、

我用谷歌搜索了很多，但还没有找到任何有用的东西。

这是来自 JaxB 网页的示例代码：

Main.java

public class Main 
public static void main( String[] args ) throws Exception 

    // create JAXBContext for the primer.xsd
    JAXBContext context = JAXBContext.newInstance("primer");

    // create a new XML parser
    SAXParserFactory factory = SAXParserFactory.newInstance();
    factory.setNamespaceAware(true);
    XMLReader reader = factory.newSAXParser().getXMLReader();

    // prepare a Splitter
    Splitter splitter = new Splitter(context);

    // connect two components
    reader.setContentHandler(splitter);

    for( int i=0; i<args.length; i++ ) 
        // parse all the documents specified via the command line.
        // note that XMLReader expects an URL, not a file name.
        // so we need conversion.
        reader.parse(new File(args[i]).toURL().toExternalForm());
    

Splitter.java

public class Splitter extends XMLFilterImpl 

public Splitter( JAXBContext context ) 
    this.context = context;


/**
 * We will create unmarshallers from this context.
 */
private final JAXBContext context;


public void startElement(String namespaceURI, String localName, String qName, Attributes atts)
    throws SAXException 

    if( depth!= 0 ) 
        // we are in the middle of forwarding events.
        // continue to do so.
        depth++;
        super.startElement(namespaceURI, localName, qName, atts);
        return;
    

    if( namespaceURI.equals("") && localName.equals("purchaseOrder") ) 
        // start a new unmarshaller
        Unmarshaller unmarshaller;
        try 
            unmarshaller = context.createUnmarshaller();
         catch( JAXBException e ) 
            // there's no way to recover from this error.
            // we will abort the processing.
            throw new SAXException(e);
        
        unmarshallerHandler = unmarshaller.getUnmarshallerHandler();

        // set it as the content handler so that it will receive
        // SAX events from now on.
        setContentHandler(unmarshallerHandler);

        // fire SAX events to emulate the start of a new document.
        unmarshallerHandler.startDocument();
        unmarshallerHandler.setDocumentLocator(locator);

        Enumeration e = namespaces.getPrefixes();
        while( e.hasMoreElements() ) 
            String prefix = (String)e.nextElement();
            String uri = namespaces.getURI(prefix);

            unmarshallerHandler.startPrefixMapping(prefix,uri);
        
        String defaultURI = namespaces.getURI("");
        if( defaultURI!=null )
            unmarshallerHandler.startPrefixMapping("",defaultURI);

        super.startElement(namespaceURI, localName, qName, atts);

        // count the depth of elements and we will know when to stop.
        depth=1;
    


public void endElement(String namespaceURI, String localName, String qName) throws SAXException 

    // forward this event
    super.endElement(namespaceURI, localName, qName);

    if( depth!=0 ) 
        depth--;
        if( depth==0 ) 
            // just finished sending one chunk.

            // emulate the end of a document.
            Enumeration e = namespaces.getPrefixes();
            while( e.hasMoreElements() ) 
                String prefix = (String)e.nextElement();
                unmarshallerHandler.endPrefixMapping(prefix);
            
            String defaultURI = namespaces.getURI("");
            if( defaultURI!=null )
                unmarshallerHandler.endPrefixMapping("");
            unmarshallerHandler.endDocument();

            // stop forwarding events by setting a dummy handler.
            // XMLFilter doesn't accept null, so we have to give it something,
            // hence a DefaultHandler, which does nothing.
            setContentHandler(new DefaultHandler());

            // then retrieve the fully unmarshalled object
            try 
                JAXBElement<PurchaseOrderType> result = 
        (JAXBElement<PurchaseOrderType>)unmarshallerHandler.getResult();

                // process this new purchase order
                process(result.getValue());
             catch( JAXBException je ) 
                // error was found during the unmarshalling.
                // you can either abort the processing by throwing a SAXException,
                // or you can continue processing by returning from this method.
                System.err.println("unable to process an order at line "+
                    locator.getLineNumber() );
                return;
            

            unmarshallerHandler = null;
        
    


public void process( PurchaseOrderType order ) 
    System.out.println("this order will be shipped to "
        + order.getShipTo().getName() );


/**
 * Remembers the depth of the elements as we forward
 * SAX events to a JAXB unmarshaller.
 */
private int depth;

/**
 * Reference to the unmarshaller which is unmarshalling
 * an object.
 */
private UnmarshallerHandler unmarshallerHandler;


/**
 * Keeps a reference to the locator object so that we can later
 * pass it to a JAXB unmarshaller.
 */
private Locator locator;
public void setDocumentLocator(Locator locator) 
    super.setDocumentLocator(locator);
    this.locator = locator;



/**
 * Used to keep track of in-scope namespace bindings.
 * 
 * For JAXB unmarshaller to correctly unmarshal documents, it needs
 * to know all the effective namespace declarations.
 */
private NamespaceSupport namespaces = new NamespaceSupport();

public void startPrefixMapping(String prefix, String uri) throws SAXException 
    namespaces.pushContext();
    namespaces.declarePrefix(prefix,uri);

    super.startPrefixMapping(prefix, uri);


public void endPrefixMapping(String prefix) throws SAXException 
    namespaces.popContext();

    super.endPrefixMapping(prefix);

Primer.xsd

    <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:annotation>
    <xsd:documentation xml:lang="en">
      Purchase order schema for Example.com.
      Copyright 2000 Example.com. All rights reserved.
    </xsd:documentation>
  </xsd:annotation>


  <xsd:element name="purchaseOrders">
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element ref="purchaseOrder" minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>


  <xsd:element name="purchaseOrder" type="PurchaseOrderType"/>

  <xsd:element name="comment" type="xsd:string"/>

  <xsd:complexType name="PurchaseOrderType">
    <xsd:sequence>
      <xsd:element name="shipTo" type="USAddress"/>
      <xsd:element name="billTo" type="USAddress"/>
      <xsd:element ref="comment" minOccurs="0"/>
      <xsd:element name="items" type="Items"/>
    </xsd:sequence>
    <xsd:attribute name="orderDate" type="xsd:date"/>
  </xsd:complexType>

  <xsd:complexType name="USAddress">
    <xsd:sequence>
      <xsd:element name="name" type="xsd:string"/>
      <xsd:element name="street" type="xsd:string"/>
      <xsd:element name="city" type="xsd:string"/>
      <xsd:element name="state" type="xsd:string"/>
      <xsd:element name="zip" type="xsd:decimal"/>
    </xsd:sequence>
    <xsd:attribute name="country" type="xsd:NMTOKEN"
                   fixed="US"/>
  </xsd:complexType>

  <xsd:complexType name="Items">
    <xsd:sequence>
      <xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="productName" type="xsd:string"/>
            <xsd:element name="quantity">
              <xsd:simpleType>
                <xsd:restriction base="xsd:positiveInteger">
                  <xsd:maxExclusive value="100"/>
                </xsd:restriction>
              </xsd:simpleType>
            </xsd:element>
            <xsd:element name="USPrice" type="xsd:decimal"/>
            <xsd:element ref="comment" minOccurs="0"/>
            <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
          </xsd:sequence>
          <xsd:attribute name="partNum" type="SKU" use="required"/>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <!-- Stock Keeping Unit, a code for identifying products -->
  <xsd:simpleType name="SKU">
    <xsd:restriction base="xsd:string">
      <xsd:pattern value="\d3-[A-Z]2"/>
    </xsd:restriction>
  </xsd:simpleType>

</xsd:schema>

test.xml

<purchaseOrders>
      <!-- 1st -->
      <purchaseOrder orderDate="1999-10-20">
        <shipTo country="US">
          <name>Alice Smith</name>
          <street>123 Maple Street</street>
          <city>Cambridge</city>
          <state>MA</state>
          <zip>12345</zip>
        </shipTo>
        <billTo country="US">
          <name>Robert Smith</name>
          <street>8 Oak Avenue</street>
          <city>Cambridge</city>
          <state>MA</state>
          <zip>12345</zip>
        </billTo>
        <items/>
      </purchaseOrder>
    </purchaseOrders>

【问题讨论】：

当我运行你的例子时，我得到“这个订单将被运送给 Alice Smith”。

是的，正确！该代码有效，但是如果您将其更改为应该读取“shipTo”而不是“purchaseOrder”，则会出现错误

【参考方案1】：

我有这个确切的问题；尝试使用 jaxb 参考实现中的部分解组示例。

我确定的解决方案是将自定义 com.sun.xml.bind.api.ClassResolver 添加到上面 startElement 方法中创建的解组器中。见：

 try 
        unmarshaller = context.createUnmarshaller();
        unmarshaller.setProperty(ClassResolver.class.getName(), myClassResolver);
  catch( JAXBException e ) 
...

这是一个模型解析器...

new ClassResolver()
    
        @Override
        public Class<?> resolveElementName(String nsUri, String localName) throws Exception
        
            if(MY_NAMESPACE.equals(nsUri) && MY_BAR.equals(localName))
                return BarType.class;
            else
                return null;
        
    

【参考方案2】：

您的示例过于复杂（> 300 行）。拜托，你能试着让它适合 30 行代码吗？

实际上，JAXB 可以用 2 行代码解组流（假设您的类已正确注释）：

private <T> T parse(URL url, Class<T> clazz) throws JAXBException 
  Unmarshaller unmarshaller = JAXBContext.newInstance(clazz).createUnmarshaller();
  return clazz.cast(unmarshaller.unmarshal(url));

请参阅this complete sample（带有测试）了解更多信息。

this article 了解更多关于该主题的信息。

【参考方案3】：

您可以利用SAXSource 来获得您正在寻找的行为：

InputSource inputSource = new InputSource(new File(args[i]).toURL().toExternalForm());
SAXSource saxSource = new SAXSource(reader, inputSource);
unmarshaller.unmarshal(saxSource, TargetClass.class);

完整示例：

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Unmarshaller;
import javax.xml.parsers.SAXParserFactory;
import javax.xml.transform.sax.SAXSource;

import org.xml.sax.InputSource;
import org.xml.sax.XMLReader;

public class Main 
    public static void main( String[] args ) throws Exception 

        // create JAXBContext for the primer.xsd
        JAXBContext context = JAXBContext.newInstance("primer");

        // create a new XML parser
        SAXParserFactory factory = SAXParserFactory.newInstance();
        factory.setNamespaceAware(true);
        XMLReader reader = factory.newSAXParser().getXMLReader();

        // prepare a Splitter
        Splitter splitter = new Splitter(context);

        // connect two components
        reader.setContentHandler(splitter);

        Unmarshaller unmarshaller = context.createUnmarshaller();

        for( int i=0; i<args.length; i++ ) 
            // parse all the documents specified via the command line.
            // note that XMLReader expects an URL, not a file name.
            // so we need conversion.
            InputSource inputSource = new InputSource(new File(args[i]).toURL().toExternalForm());
            SAXSource saxSource = new SAXSource(reader, inputSource);
            unmarshaller.unmarshal(saxSource, TargetClass.class);
        
    

【参考方案4】：

当您尝试将子元素类作为参数传递给 jaxb 实例时，通常会引发此错误。尝试将根类作为参数传递并检查它是否有效

【参考方案5】：

这是我在这种情况下使用的功能。

/**
 * Loads an xml with a non <code>XmlRootElement</code> annotated element
 *
 * @param <T> Class to be unserialized
 * @param xmlStream @link InputStream The input stream of the xml file
 * @param modelClass <code>.class</code> of the model to read. This class may or not have the <code>XmlRootElement</code> annotation
 * @return @link AppModel Instance of the model
 * @throws JAXBException Error while reading the xml from the input stream
 */
public static <T> T readNonRootDataModelFromXml(InputStream xmlStream, Class<T> modelClass) throws JAXBException

    JAXBContext jaxbContext = JAXBContext.newInstance(modelClass);
    Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
    JAXBElement<T> xmlRootElement = unmarshaller.unmarshal(new StreamSource(xmlStream), modelClass);
    return (T) xmlRootElement.getValue();

你可以把它放在你的静态 Utils 类中。