Jaxb如何优雅的处理CData

前言

  Jaxb确实是xml和java对象映射互转的一大利器. 但是在处理CData内容块的时候, 还是有些小坑. 结合网上搜索的资料, 本文提供了一种解决的思路, 看看能否优雅地解决CData产出的问题.

常规做法

  网上最常见的做法是借助XmlAdapter和CharacterEscapeHandler(sun的api)组合来实现.
  首先定义CDataAdapter类, 用于对象类型转换.

public class CDataAdapter extends XmlAdapter<String, String> {

    @Override
    public String unmarshal(String v) throws Exception {
        return v;
    }

    @Override
    public String marshal(String v) throws Exception {
        return new StringBuilder("<![CDATA[").append(v).append("]]>").toString();
    }

}

  其借助注解XmlJavaTypeAdapter作用于属性变量上, 如下面的类对象上:

@XmlRootElement(name="root")
public static class TNode {

     @XmlJavaTypeAdapter(value=CDataAdapter.class)
     @XmlElement(name="text", required = true)
     private String text;

}

  使用Marshaller转为xml文本的时候, 结果却是如下:

<root>
    <text>&lt;![CDATA[李雷爱韩梅梅]]&gt;</text>
</root>

  这和我们预期的其实有差异, 我们其实想要的是如下的:

<root>
    <text><![CDATA[李雷爱韩梅梅]]></text>
</root>

  本质的原因是Jaxb默认会把字符'<', '>'进行转义, 为了解决这个问题, CharacterEscapeHandler就华丽登场了.

import com.sun.xml.internal.bind.marshaller.CharacterEscapeHandler;

marshaller.setProperty(
    "com.sun.xml.internal.bind.marshaller.CharacterEscapeHandler",
    new CharacterEscapeHandler() {
        @Override
        public void escape(char[] ch, int start, int length, boolean isAttVal, Writer writer)
                throws IOException {
            writer.write(ch, start, length);
        }
    }
);

  测试结果, 完美地解决问题. 然后随之而来的问题, 稍有些尴尬, 使用maven进行编译打包的时候, 会遇到如下错误:

[ERROR] Compilation failure
[ERROR] 程序包com.sun.xml.internal.bind.marshaller不存在

  Java工程开发, 一般不建议直接调用内部的api(以com.sun开头).

改进方案:

  参考了不少网友的博文, 大致思路都是一样的, 就是借助重载XMLStreamWriter类实现. 更确实的做法是重载writeCharacters方法, 在遇到CData标记(<![CDATA[]]>)包围的文本时, 选择调用writeCData函数, 可用以下代码来大致说明:

public class CDataXMLStreamWriter implements XMLStreamWriter {

    // *) 重载writeCharacters, 遇CDATA标记, 则转而调用writeCData方法
    @Override
    public void writeCharacters(String text) throws XMLStreamException {
        if ( text.startsWith("<![CDATA[") && text.endsWith("]]>") ) {
            writeCData(text.substring(9, text.length() - 3));
        } else {
            writeCharacters(text);
        }
    }
    // *) 演示使用
}

  真实的做法, 不会采用完整的去实现XmlStreamWriter接口的方案, 而是采用代理模式.这边采用动态代理的方法.

private static class CDataHandler implements InvocationHandler {
    // *) 单独拦截 writeCharacters(String)方法
    private static Method gWriteCharactersMethod = null;
    static {
        try {
            gWriteCharactersMethod = XMLStreamWriter.class
                    .getDeclaredMethod("writeCharacters", String.class);
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }

    private XMLStreamWriter writer;

    public CDataHandler(XMLStreamWriter writer) {
        this.writer = writer;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if ( gWriteCharactersMethod.equals(method) ) {
            String text = (String)args[0];
            // *) 遇到CDATA标记时, 则转而调用writeCData方法
            if ( text != null && text.startsWith("<![CDATA[") && text.endsWith("]]>") ) {
                writer.writeCData(text.substring(9, text.length() - 3));
                return null;
            }
        }
        return method.invoke(writer, args);
    }

}

  具体的Marshaller代码片段如下所示:

public static <T> String mapToXmlWithCData(T obj) {

    try {

        StringWriter writer = new StringWriter();
        XMLStreamWriter streamWriter = XMLOutputFactory.newInstance()
                .createXMLStreamWriter(writer);
        // *) 使用动态代理模式, 对streamWriter功能进行干涉调整
        XMLStreamWriter cdataStreamWriter = (XMLStreamWriter) Proxy.newProxyInstance(
                streamWriter.getClass().getClassLoader(),
                streamWriter.getClass().getInterfaces(),
                new CDataHandler(streamWriter)
        );

        JAXBContext jc = JAXBContext.newInstance(obj.getClass());
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.setProperty(Marshaller.JAXB_ENCODING, "UTF-8");

        marshaller.marshal(obj, cdataStreamWriter);
        return writer.toString();

    } catch (JAXBException e) {
        e.printStackTrace();
    } catch (XMLStreamException e) {
        e.printStackTrace();
    }
    return null;

}

  测试的结果, 完美地解决了CData的问题(功能实现+绕过sun api), 不过这里面还有点小瑕疵, 就是对齐问题, 这段代码没法控制对齐.

对齐改进

  这边需要借助Transformer类实现, 思路是对最终的xml文本进行格式化处理.

// *) 对xml文本进行格式化转化
public static String indentFormat(String xml) {
    try {
        TransformerFactory factory = TransformerFactory.newInstance();
        Transformer transformer = factory.newTransformer();
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4");

        StringWriter formattedStringWriter = new StringWriter();
        transformer.transform(new StreamSource(new StringReader(xml)),
                new StreamResult(formattedStringWriter));
        return formattedStringWriter.toString();
    } catch (TransformerException e) {
    }
    return null;
}

　　

完整的解决方案

  这边把上述所有的代码完整的贴一遍:

import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamWriter;
import javax.xml.transform.OutputKeys;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.stream.StreamSource;

// *) XmlAdapter类, 修饰类字段, 达到自动添加CDATA标记的目标
public static class CDataAdapter extends XmlAdapter<String, String> {
    @Override
    public String unmarshal(String v) throws Exception {
        return v;
    }

    @Override
    public String marshal(String v) throws Exception {
        return new StringBuilder("<![CDATA[").append(v).append("]]>")
                .toString();
    }
}

// *) 动态代理
private static class CDataHandler implements InvocationHandler {

    private static Method gWriteCharactersMethod = null;
    static {
        try {
            gWriteCharactersMethod = XMLStreamWriter.class
                    .getDeclaredMethod("writeCharacters", String.class);
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }

    private XMLStreamWriter writer;

    public CDataHandler(XMLStreamWriter writer) {
        this.writer = writer;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if ( gWriteCharactersMethod.equals(method) ) {
            String text = (String)args[0];
            if ( text != null && text.startsWith("<![CDATA[") && text.endsWith("]]>") ) {
                writer.writeCData(text.substring(9, text.length() - 3));
                return null;
            }
        }
        return method.invoke(writer, args);
    }

}

// *) 生成xml
public static <T> String mapToXmlWithCData(T obj, boolean formatted) {

    try {

        StringWriter writer = new StringWriter();
        XMLStreamWriter streamWriter = XMLOutputFactory.newInstance()
                .createXMLStreamWriter(writer);
        // *) 使用动态代理模式, 对streamWriter功能进行干涉调整
        XMLStreamWriter cdataStreamWriter = (XMLStreamWriter) Proxy.newProxyInstance(
                streamWriter.getClass().getClassLoader(),
                streamWriter.getClass().getInterfaces(),
                new CDataHandler(streamWriter)
        );

        JAXBContext jc = JAXBContext.newInstance(obj.getClass());
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_ENCODING, "UTF-8");

        marshaller.marshal(obj, cdataStreamWriter);
        // *) 对齐差异处理
        if ( formatted ) {
            return indentFormat(writer.toString());
        } else {
            return writer.toString();
        }

    } catch (JAXBException e) {
        e.printStackTrace();
    } catch (XMLStreamException e) {
        e.printStackTrace();
    }
    return null;

}

// *) xml文本对齐
public static String indentFormat(String xml) {
    try {
        TransformerFactory factory = TransformerFactory.newInstance();
        Transformer transformer = factory.newTransformer();
        // *) 打开对齐开关
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        // *) 忽略掉xml声明头信息
        transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
        transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
        transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4");

        StringWriter formattedStringWriter = new StringWriter();
        transformer.transform(new StreamSource(new StringReader(xml)),
                new StreamResult(formattedStringWriter));

        return "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"
                + formattedStringWriter.toString();
    } catch (TransformerException e) {
    }
    return null;
}

  编写具体的测试案例:

@NoArgsConstructor
@AllArgsConstructor
@XmlRootElement(name="root")
public static class TNode {
    @XmlElement(name="key", required = true)
    private String key;

    @XmlJavaTypeAdapter(value=CDataAdapter.class)
    @XmlElement(name="text", required = true)
    private String text;
}

public static void main(String[] args) {
    TNode node = new TNode("key", "李雷爱韩梅梅");
    String xml = mapToXmlWithCData(node, true);
    System.out.println(xml);
}

  测试输出的结果如下:

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <key>key</key>
    <text><![CDATA[李雷爱韩梅梅]]></text>
</root>

　

总结

  总的来说, 改进的方案规避了sun api的编译限制. 同时能满足之前的功能需求, 值得小小鼓励一下, ^_^.