java - 从子元素开始使用 JAXB 解码

Question

我正在尝试使用 JAXB 以以下格式解码 XML:

<return_data api_call="getuser">
    <users>
       <user>
          <username>test_me</username>
          <account_expiration_date>0000-00-00</account_expiration_date>        
       </user>
    </users>
</return_data>

我想从用户元素开始解码 XML。但是，当我尝试使用 '@XmlRootElement(name = "user")' 时，我收到此错误:

Exception in thread "main" java.lang.RuntimeException: Unable to parse XML:
    at com.example.findAccount(AccountService.java:30)
    at com.example.main(AccountService.java:50)
Caused by: javax.xml.bind.UnmarshalException: unexpected element (uri:"", local:"return_data"). Expected elements are <{}user>
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.handleEvent(UnmarshallingContext.java:726)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.Loader.reportError(Loader.java:247)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.Loader.reportError(Loader.java:242)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.Loader.reportUnexpectedChildElement(Loader.java:109)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext$DefaultRootLoader.childElement(UnmarshallingContext.java:1131)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext._startElement(UnmarshallingContext.java:556)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.startElement(UnmarshallingContext.java:538)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.SAXConnector.startElement(SAXConnector.java:153)
    at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:509)
    at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.scanStartElement(XMLNSDocumentScannerImpl.java:379)
    at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl$NSContentDriver.scanRootElementHook(XMLNSDocumentScannerImpl.java:605)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:3138)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(XMLDocumentScannerImpl.java:880)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:606)
    at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:117)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:510)
    at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:848)
    at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:777)
    at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:141)
    at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1213)
    at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:649)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:243)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:214)
    at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:157)
    at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:162)
    at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:171)
    at com.example.findAccount(AccountService.java:26)
    ... 1 more

这是我的帐户模型类:

package com.example;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement(name = "user")
public class Account {

    private String username;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

服务:

package com.example;

import java.net.MalformedURLException;
import java.net.URL;
import java.util.Optional;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;

import com.example.Account;

public class AccountService {

    public Optional<Account> findAccount(final String username) {

        try {
            URL url = new URL("https://example.com/foo/bar" + username);
            JAXBContext jaxbContext = JAXBContext.newInstance(Account.class);
            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
            Account account = (Account) jaxbUnmarshaller.unmarshal(url);

            return Optional.ofNullable(account);
        } catch (JAXBException | MalformedURLException e) {
            throw new RuntimeException("Unable to parse XML:", e);
        }
    }

    public static void main(String[] args) {
        // test it //
        new AccountService().findAccount("user");
    }

}

Answer 1

JAXB 解码完整的 XML，@XmlRootElement(name = "user") 不会过滤“您在 XML 中看到的第一个 user 元素”。

如果您想要这个(“您在 XML 中看到的第一个 user 元素”)，有几个选项。

• 最简单的方法可能是解码完整的 XML，然后从结果结构中检索 user 对象。
• 另一种选择是使用 StAX。请参阅this article by Blaise Doughan关于如何做到这一点。
• 您还可以预先过滤使用 SAX 解析的 XML，但这可能并不那么容易。
• 最后，您可以先将 XML 解析为 DOM，找到所需的 user 节点并解码该节点。

如果 user 元素周围没有太多 XML，我将只解析完整的 XML。否则我会选择 StAX 选项。这大概是这样的:

    XMLInputFactory xif = XMLInputFactory.newFactory();
    StreamSource xml = new StreamSource("https://example.com/foo/bar" + username);
    XMLStreamReader xsr = xif.createXMLStreamReader(xml);
    xsr.nextTag();
    while(!xsr.getLocalName().equals("user")) {
        xsr.nextTag();
    }

    JAXBContext jc = JAXBContext.newInstance(Account.class);
    Unmarshaller unmarshaller = jc.createUnmarshaller();
    JAXBElement<Account> jb = unmarshaller.unmarshal(xsr, Account.class);
    xsr.close();

    Account user = jb.getValue();

(从 this article 复制粘贴并进行一些调整，根本没有测试。)

感谢Blaise Doughan .