简单框架xml反序列化 - 查找属性的存在

Question

Is it possible to look up for existence of specified attributes in the Simple framework for java? Let's say I have the following xml:

<packet id="250">
  <msg id="1" uniqueAttribute1="some value" x="7" />
  <msg id="2" someStuff="123" />
  <msg id="3" someOtherStuff="abc" />
</packet>

All the msgs above should be deserialized into objects of different classes perhaps inherited from a common base class. Which msg should instantiate an object of which class should depend on name of a attribute. Meaning if there in the msg exists attribute named 'uniqueAttribute' then an object of class Foo should be created, if there is 'someStuff' an object of class Bar and so on.

The only way which comes to my mind is to try to deserialize to objects of the classes and catching exceptions and retrying which sounds bad.

If it is not possible with the 'Simple' framework maybe you could suggest some other method?

Answer 1

I'm not seeing the need for a framework with such a basic requirement but then again I may not understand your needs totally. I would initially start with something as simple as a SAX parse handler. Totally hand-coded and untested but for a basic idea see here:

import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

class XmlHandler extends DefaultHandler {
    Map msgClasses = new HashMap(){{ put("uniqueAttribute1", "Foo"); put("someStuff", "Bar"); }};

    public MessageBase messageFromAttributes(final org.xml.sax.Attributes attributes) {
        for(int i=0; i<attributes.getLength(); i++) {
            if(msgClasses.contains(attributes.getValue(i))) {
                try{
                    Class msgClass = Class.forName(attributes.getValue(i));
                    return msgClass.newInstance();
                } catch(ClassNotFoundException e) {
                    return null;
                }
            }
        }
    }

    @Override
    public final void startElement(String uri, String localName, String name,
            final org.xml.sax.Attributes attributes) throws SAXException {
        if (localName.equals("msg")) {
            MessageBase message = messageFromAttributes(attributes);
        }
    }

    @Override
    public final void endElement(String uri, String localName, String name) throws SAXException {

        if (localName.equals("msg")) {
        }
    }

}

The idea of my above example is to map class names to specific attributes, then while handling the SAX parse events instantiate find any of the given attributes in the mapping which should point to the correct class name. At this point you can instantiate the class using reflection and carry on. You could optionally populate the class using the remaining attibute values if necessary.

Answer 2

There are a number of ways to do this. Typically a Visitor or Strategy can be used. See the following for how to use a Visitor. This uses a namespace to determine the type, but you could change this to use an attribute or a number of them.

https://simple.svn.sourceforge.net/svnroot/simple/trunk/download/stream/src/test/java/org/simpleframework/xml/strategy/ClassToNamespaceVisitor.java

Also, you could use a Converter to do this. Take a look at the tutorial.