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Support XSD versioning with JAXB

I am currently working on an application that performs the task of importing or exporting some entities. The file format being used for the same is XML. JAXB is being used for XML binding.
The problem is present XSD that defines the structure of entities has no provision for versioning. How do I get started with defining versioned XSD and subsequently XML instance documents provided JAXB lies as the underlying binding framework ?
I have read that there are three possible ways of introducing versions in XSD.
1) Change the internal schema version attribute
2) Create a attribute like schemaVersion on the root element
3) Change the schema's target namespace.
Which one best suits the usecase mentioned below?
Use case: The changes made to the XSD in the next version may invalidate the existing elements. Although the schema itself may not be backward compatible but the application needs to provides support for handling all versions of schema.

XML is designed to facilitate change and flexibility in document structures. Unfortunately, JAXB isn't. The very act of compiling knowledge of document structure into your Java source code makes change to the document structure a lot more difficult.
If structural change is part of your agenda, I think you should seriously consider not using JAXB: technologies like XQuery and XSLT are much better suited to this scenario.

Programatic generation of xml from a xsd that uses other xsds

I have a xsd that in turn uses/imports a set of xsds.
I would like to programtically generate sample xml from the xsd. The xml must contain all elements and attributes populated with example data based on data type.
How can I do this using eclipse api classes?
Also are there any other tools that accomplish this task and can be evoked in a java program or a batch file?
Any pointers to examples/documentation/api is highly appreciated.
Thanks in advance.

if I am reading your question correctly, I believe what you are trying to do is programmatically generate (i.e. using Java) XML documents based on an XML Schema Document (which may in turn import other supporting XSD's).
You may wish to have a look at Oracle/Sun's JAXB (Java Architecture for Xml Binding) which you can find more info about here:
http://jaxb.java.net/
JAXB works with J2SE-SDK and/or IDEs - such as Netbeans or Eclipse, and permits you to unmarshall (read XML documents into mapped Java Objects) or marshall (write Java objects as XML documents) as required. Standard mappings (known as binding declarations) are provided based on valid XML Schema provided to JAXB. You can also provide binding declarations through custom annotations directly within your XML Schema files or by using external JAXB declarations.
Another alternative (similar to JAXB) is Apache's XML-Beans.
Hope this helps!

I need some jar for Java and xml

classical way to handle XML in java is really lengthy and scary.
For this purpose i made my own class which can return me result without giving me more detail like,
myXML mx=new myXML("filename");
:
mx.getAll("node name");
mx.getFirst("node name");
:
I had completed it 80%. But unfortunately, i had lost it in PC crash.
is there any jar under GPL or apache license which provides facility to read & write XML in simplest way?

JDOM is simple API for parsing, creating, manipulating, and serializing XML documents in Java. API's you mentioned in your question are supported by JDOM (Other than many more useful API's).
Checkout JDOM documentation/book chapter here for more reading:
http://www.jdom.org/downloads/docs.html
http://www.cafeconleche.org/books/xmljava/chapters/ch14.html
Following are lines from http://www.jdom.org/docs/oracle/jdom-part1.pdf
So what’s the point of JDOM (Java
Document Object Model), and why do
developers need it? JDOM is an open
source library for Java-optimized XML
data manipulations. Although it’s
similar to the World Wide Web
Consortium’s (W3C) DOM, it’s an
alternative document object model that
was not built on DOM or modeled after
DOM. The main difference is that while
DOM was created to be language-neutral
and initially used for JavaScript
manipulation of HTML pages, JDOM was
created to be Java-specific and
thereby take advantage of Java’s
features, including method
overloading, collections, reflection,
and familiar programming idioms. For
Java programmers, JDOM tends to feel
more natural and “right.”

Try Apache Digester.Using digester will really simplify your XML parsing.You can refer this link for an example.

For your use case you may be interested in the javax.xml.xpath APIs available in the JDK. For an example see one of my answers to another question (below):
Remove XML Node using java parser
You may also prefer Service Data Objects (SDO). It is a generic data structure for representing XML data. For more information see:
http://www.eclipse.org/eclipselink/sdo.php
http://bdoughan.blogspot.com/2010/09/processing-atom-feeds-with-sdo.html
When parsing XML I recommend using the standard technologies: StAX, SAX, DOM, and JAXB. An implementation of each is included in the. JDK and alternate open source implementations are available offering improved performance and extended features, such as MOXy JAXB's XPath based mapping:
http://bdoughan.blogspot.com/2010/09/xpath-based-mapping-geocode-example.html
http://bdoughan.blogspot.com/2010/10/how-does-jaxb-compare-to-xstream.html
The advantage of the standard libraries is that they all work together:
StAX, SAX, and DOM are all valid inputs/outputs for JAXB
StAX, SAX, DOM, and JAXB are all compatible with javax.xml.transform libraries
StAX, SAX, DOM, and JAXB are all compatible with javax.xml.xpath libraries
StAX, SAX, DOM, and JAXB are all compatible with javax.xml.validation libraries
JAXB is the binding layer for two Web Service standards: JAX-WS and JAX-RS

Sending large xml data through a socket

I'm newbie to XML using Java. I've to write a method to send a large XML data having lots of nodes through a socket to client application.
What is the suitable method to generate XML?
What is the best method to send large XML through sockets?

Since you are using sockets you just need to deal with Java InputStream/OutputStream. This gives you alot of flexibility in your XML handling as almost all XML technologies handle streams as input/output.
You could represent your data as plain old Java objects (POJOs), and then bind them to XML using JAXB. An implementation of JAXB is included in Java SE 6. There are other implementations such as MOXy (I'm the tech lead) and JaxMe.
For an example see:
http://wiki.eclipse.org/EclipseLink/Examples/MOXy/GettingStarted

To generate XML you use DOM implementation provided by any XML DOM parser and generator.
Here is a nice tutorial. But for only generation try to use some small and light-weight parcers e.g. [tinyxml][2] or [qdparcer][3], because the xerces and others are going to be heavy weight for that. But if the parcing is also involved libxml or xerces will be of good choice because they provide nice SAX implementation for parsing, but you need to have schema defined for your data. Again try to serialize the data before sending so you can get rid of other problems.

Java equivalent of .NET's System.Xml.Schema classes?

The .NET Framework exposes a rather useful set of classes as part of the System.Xml.Schema namespace.
Among other things, classes such as XmlSchema and XmlSchemaElement provide a useful API for defining/writing and parsing/reading/traversing XML schema files.
Does an equivalent library exist for Java?

The tools to accomplish this are there, but I don't think there's anything specific like that in Java. What I would do is run JAXB on the XML Schema schema (http://www.w3.org/2001/XMLSchema), which will generate Java classes corresponding to the XSD constructs. Then you can use those to read/write XML schemas, and validate against the XML Schema schema.