Question

I am trying to use JAXB to serialize a HashTable<String, String> to XML. I am very new to Java (came from C#), so I am kinda perplexed by this task.

I have seen the following code:

public static <T> String ObjectToXml(T object, Class<T> classType) throws JAXBException
{
  JAXBContext jaxbContext = JAXBContext.newInstance(classType);
  StringWriter writerTo = new StringWriter();
  Marshaller marshaller = jaxbContext.createMarshaller();
  marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true); 
  marshaller.marshal(object, writerTo); //create xml string from the input object
  return writerTo.toString();
}

Which is invoked like so: ObjectToXml(o, ClassOfO.class), but HashTable<String, String>.class is wrong (that I already know).

Can Java gurus out there show me how to invoke this code? Proposing a simpler implementation (along with an invocation example, of course) is most welcome as well.

Thanks.

Answer 1

You will need to create a wrapper class to hold onto the Hashtable:

package forum7534500;

import java.util.Hashtable;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Wrapper {

    private Hashtable<String, String> hashtable;

    public Hashtable<String, String> getHashtable() {
        return hashtable;
    }

    public void setHashtable(Hashtable<String, String> hashtable) {
        this.hashtable = hashtable;
    }

}

Then you can do the following:

package forum7534500;

import java.io.StringWriter;
import java.util.Hashtable;

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

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Wrapper.class);
        Wrapper wrapper = new Wrapper();
        Hashtable<String, String> hashtable = new Hashtable<String,String>();
        hashtable.put("foo", "A");
        hashtable.put("bar", "B");
        wrapper.setHashtable(hashtable);
        System.out.println(objectToXml(jc, wrapper));
    }

    public static String objectToXml(JAXBContext jaxbContext, Object object) throws JAXBException
    {
      StringWriter writerTo = new StringWriter();
      Marshaller marshaller = jaxbContext.createMarshaller();
      marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true); 
      marshaller.marshal(object, writerTo); //create xml string from the input object
      return writerTo.toString();
    }

}

This will produce the following output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<wrapper>
    <hashtable>
        <entry>
            <key>bar</key>
            <value>B</value>
        </entry>
        <entry>
            <key>foo</key>
            <value>A</value>
        </entry>
    </hashtable>
</wrapper>

Things to Note

• JAXBContext is a thread-safe object and should be created once and reused.
• Hashtable is synchronized, if you do not need this then using HashMap is the common replacement.
• The convention is to start Java method names with a lower case letter.

Customizing the Mapping

You can use an XmlAdapter in JAXB to customize the mapping of any class. Below is an link to a post on my blog where I demonstrate how to do just that:

• http://blog.bdoughan.com/2010/07/xmladapter-jaxbs-secret-weapon.html