使用XmlEncoder创建增量/差异xml

Question

The idea is the following: The XmlEncoder records the operations needed to reproduce the serialized bean starting from nothing. But what if we do not start from nothing?

Can XmlEncoder be used to record the operations needed to transform an object to an other? And can XmlDecoder be used to make this transformation?

Or is there any other alternative to create an xml describing the difference between two object and transform an object using this xml data?

Answer 1

By itself XMLEncoder and XMLDecoder cannot be used to do what you require: mapping the XML difference between two objects and using this to transform one object into another.

But, if the 2 objects are of the same type, you can work some magic to make it work as you would like to.

What I would do:

A) Creates an object that holds the propertyName/propertyValues pairs. This class will be used to hold the differences between the objects. Even a simple Map(String, Object) is enough. I call this ABDifference for example.

B) Makes up an utility method which by introspection analyses object A and object B and write a ABDifference class holding the property names and values that differs in A and B, holding B values. Use Introspector.getBeanInfo to get the propertyDescriptor and use those to invoke the getter and check the property values using equals.

Example:

OBJECT A (name: "pippo", description: "version A of pippo", age: 25)

OBJECT B (name: "pippo", description: "version B of pippo", age: 27)

ABDIFFERENCE (Map: description--->"version B of pippo", age--->27)

C) Encodes this object of this class, which holds the 'differences' between the two original objects using XMLEncoder. This way you will get a simple XML encoding that just holds the property values that differs from A to B for example, and not the whole object.

D) Do an utility method that transforms A into B by checking ABDifference. It will be implemented easily, by looping on the Key/Value pairs of the map and using reflection to invoke the setter of that property (use Introspector.getBeanInfo to easily get the java beans properties) to transform A into B.

This way you have 2 utility methods that maps the difference between 2 objects of the same class in a special class which holds just the differences. You XMLEncode and transfer this and XMLDecode the other way, then use the object to transform A into B.

ADDITIONAL IMPROVEMENTS: you could even add special treatmeants for common types, like pattern matching for string properties to avoid writing the WHOLE string in the difference, but just the real difference between the 2 strings.

One easy simple way to do this would be find the longest matching "start" and "end" of the string and just encode the difference between them in a special way.

For example you could use this format: $charnumber$, so the encoded string would be $n$central part of the string$m$, where n is the char where the central part to substitute starts and m is the char where the central part to substitute ends. Of course you should escape then the special char chosen ($ in this case) with a double special char and transform it back after.

OBJECT A (name: "pippo", description: "version A of pippo", age: 25)

OBJECT B (name: "pippo", description: "version B of my pippo", age: 27)

ABDIFFERENCE (Map: description--->"$8$B of my$14$", age--->27)

So reading the string difference you would know you have to compose the string like this:

"version " (1-8 chars from A value, like the $8$ tells us) + "B of my" (the difference in the diff string between the $8$ $14$ + "pippo" (chars 14+ from A value)

Answer 2

I believe if we generate a xml based on the javabeans.dtd , XmlDecoder can transform the object from the xml. The javabenas.dtd is XmlEncoder schema. I have enclosed the URL that explains about the how XmlEncoder encodes the object to xml.

I hope it helps. :)

tiger.

Answer 3

XMLDecoder与XMLEncoder完全相反，您从对象开始并将其解码为xml，然后可以像以前一样使用XMLEncoder再次将其转换为对象。