JAXB - 解析XML列表順序（一個接一個）

Question

我要解析XML這根元素是一個列表：

<SomeObjectsCollection> 
    <SomeObject><!-- content1 --></SomeObject> 
    <SomeObject><!-- content2 --></SomeObject> 
    <SomeObject><!-- content3 --></SomeObject> 
    <!-- and hundreds more --> 
</SomeObjectsCollection> 

我不想整個XML解析到內存性能的原因。我寧願寫一些像Iterable<SomeObjetType>這樣的東西，它不會強制用戶在內存中保留整個非編組列表，只是逐個處理它。

到目前爲止，我寫了這樣的類，它實現Iterable<SomeObject>和我自己的Iterator內：

public class SomeObjectsIterableParser implements Iterable<SomeObjectType> { 

    private final Unmarshaller jaxbUnmarshaller; 
    private final XMLStreamReader xmlReader; 

    public SomeObjectsIterableParser(Schema schema, java.io.Reader xmlStringReader) throws ExtractorException { 
    try { 
     jaxbUnmarshaller = JAXBContext.newInstance(SomeObjectType.class).createUnmarshaller(); 
     xmlReader = XMLInputFactory.newFactory().createXMLStreamReader(xmlStringReader); 
    } catch (JAXBException | XMLStreamException e) { 
     throw new ExtractorException("Could not create jaxbUnmarshaller", e); 
    } 
    jaxbUnmarshaller.setSchema(schema); //turns on schema validation 

    //Move reader to first occurence of SomeObject - really necessary? 
    try { 
     while (xmlReader.hasNext()) { 
     if (!xmlReader.isStartElement() || !xmlReader.getLocalName().equals("SomeObject")) 
      xmlReader.next(); 
     else break; 
     } 
    } catch (XMLStreamException e) { 
     e.printStackTrace(); 
    } 

    } 

    @Override 
    public Iterator<SomeObjectType> iterator() { 
    return new MyIterator(); 
    } 

    class MyIterator implements Iterator<SomeObjectType> { 

    @Override 
    public boolean hasNext() { 
     try { 
     return xmlReader.hasNext(); 
     } catch (XMLStreamException e) { 
     throw new RuntimeException(e); 
     } 
    } 

    @Override 
    public SomeObjectType next() { 
     try { 
     return (SomeObjectType) jaxbUnmarshaller.unmarshal(xmlReader); 
     } catch (JAXBException | XMLStreamException e) { 
     throw new RuntimeException(e); 
     } 
    } 

    @Override 
    public void remove() { 
     throw new UnsupportedOperationException("Not supported yet"); 
    } 
    } 
} 

我收到例外next()法消息：org.xml.sax.SAXParseException; lineNumber: 2; columnNumber: 17; cvc-elt.1: Cannot find the declaration of element 'SomeObject'.

我在做什麼錯？

Answer 1

我發現the article，它描述了我想要做的事情。因此，我寫了這樣的代碼：

public class SomeObjectsIterableParser implements Iterable<SomeObjectType> { 

    private final Unmarshaller jaxbUnmarshaller; 
    private final XMLStreamReader xmlReader; 

    public SomeObjectsIterableParser(Schema schema, Reader SomeObjectResponse) throws ExtractorException { 
    try { 
     jaxbUnmarshaller = JAXBContext.newInstance(SomeObjectType.class).createUnmarshaller(); 
     xmlReader = XMLInputFactory.newFactory().createXMLStreamReader(SomeObjectResponse); 
    } catch (JAXBException | XMLStreamException e) { 
     throw new ExtractorException("Could not create jaxbUnmarshaller", e); 
    } 
    //jaxbUnmarshaller.setSchema(schema); //schema can handle only root element 
    advanceReaderToFirstProfile(); 
    } 

    private void advanceReaderToFirstProfile() { 
    try { 
     xmlReader.nextTag(); 
     while(!xmlReader.getLocalName().equals("SomeObject")) { 
     xmlReader.nextTag(); 
     } 
    } catch (XMLStreamException e) { 
     e.printStackTrace(); 
    } 
    } 

    @Override 
    public Iterator<SomeObjectType> iterator() { 
    return new MyIterator(); 
    } 

    class MyIterator implements Iterator<SomeObjectType> { 

    @Override 
    public boolean hasNext() { 
     try { 
     if (xmlReader.isWhiteSpace() && xmlReader.hasNext()) { 
      //ommit witespaces 
      xmlReader.nextTag(); 
     } 
     } catch (XMLStreamException e) { 
     throw new RuntimeException(e); 
     } 
     return xmlReader.isStartElement() 
      && xmlReader.getLocalName().equals("SomeObject"); 
    } 

    @Override 
    public SomeObjectType next() { 
     try { 
     JAXBElement<SomeObjectType> element = jaxbUnmarshaller.unmarshal(xmlReader, SomeObjectType.class); 
     return element.getValue(); 
     } catch (JAXBException | XMLStreamException e) { 
     throw new RuntimeException(e); 
     } 
    } 

    @Override 
    public void remove() { 
     throw new UnsupportedOperationException("Not supported yet"); 
    } 
    } 
} 

注意的3認爲：

1. 架構驗證，不能適用於非根元素。有相關的問題here。
2. 使用語法：
   JAXBElement<SomeObjectType> element = jaxbUnmarshaller.unmarshal(xmlReader, SomeObjectType.class);
   而不是
   (SomeObjectType) jaxbUnmarshaller.unmarshal(xmlReader);。
   否則，您會收到一條異常：

   了java.lang.RuntimeException：javax.xml.bind.UnmarshalException

   • 與鏈接的異常：
     [com.sun.istack.internal.SAXParseException2; lineNumber：2; columnNumber：22;意外元素（uri：「」，local：「SomeObject」）。預期元素是（無）]

3. 比例子中更仔細地處理異常。 Iterable和Iterator接口不允許您拋出非運行時異常。