Introduction

The objective of the Axis2Databinding framework is to provide a lightweight and simple schema compiler/ Java bean generator for Axis2. By no means it's to be a fully functional schema compiler like XMLBeans. This document aims to provide the architectural overview of the Axis2 Databinding Framework (referred to as ADB from here onwards) and be a guide to anyone who wants to use and modify ADB. Note that ADB is written in a fashion that allows it to be used as a standalone schema compiler and also to be extended to generate code for other languages.

Architectural Overview

ADB is built on a modular architecture that allows it to utilize a pre configured writer depending on the configuration.The 'big block diagram' for the code generator architecture is depicted below

ADB utilizes the WS-commons XmlSchema library for reading the Schema. The object model for the schema comes in the form of an XmlSchema object. The schema compiler keeps an instance of the writer (in the default case it's the JavaBeanWriter) which actually writes the classes. The writers may use whatever technique they prefer, in the case of the JavaBeanWriter, it uses an XSLT template. The SchemaCompiler also uses a typemapper object that tells it what classnames to be used for the QNames that it encounters. This type mapper is also part of the configuration and the users can override the default type mapper by overriding the property setting.

Code and Dependencies

As explained in the previous section, the schema compiler depends on the WS-Commons XmlSchema library. The XSLT transformations are dependent on the JVM's DOMimplementation (either crimson or xerces) which means that the underlying JVM should be 1.4 or higher. Apart from that ADB has no special dependencies on any other special jar files. The code for the schema compiler is completely in the org.apache.axis2.schema.* package. This package resides in the codegen module of the Axis2 source tree.

Following are the important classes and files

1. SchemaCompiler - The work horse that really compiles the schema into classes.
2. BeanWriter - BeanWriter represents the kind of interface the SchemaCompiler accepts as a writer. The writer needs to handle the actual writing of the clasess
3. JavaBeanWriter - The default implementation of the BeanWriter interface.
4. TypeMap - represents the interface that the schema compiler looks towards to find classes
5. JavaTypeMap - the default implementation of the TypeMap
6. BeanTemplate.xsl - the XSLtemplate the JavaBeanWriter uses.
7. Schema-compile.properties - The property file for the schema compiler

The easiest way to obtain the ADB binaries is to run the maven build for the codegen module. This will generate the codegen-{$version}.jar inside the target folder which is directly usable when the ADB schema compiler is required.

Invoking the ADB Code Generator

As a standalone schema compiler

ADB comes with a mainclass XSD2Java that allows the schemas to be compiled just by giving the schema file reference.This main class is pretty much primitive and does not provide much control over the code generation process. This is bound to improve in the near future.

Code generator accepts the following parameters

1. The Schema file name - This should be a complete file name pointing to the local file system
2. The output folder name - This should be the name of a folder within the local file system

Since the main class has no validations built in, the compiler is likely to cough up an unexpected error message if these parameters are not supplied properly.

Through the API

This is the only way to harness the full potential of the schema compiler. The current Axis2 integration of ADB happens through this API. The most important classes and methods of the Schema compiler are as follows.

• SchemaCompiler - Constructor

  The constructor of the schema compiler expects a CompilerOptions object. This compilerOptions object is more of a holder for the parameters that are passed to the SchemaCompiler. The only mandatory parameter in the CompilerOptions is the output directory

• SchemaCompiler - Compile(XMLSchema schema)

  The compile method to call for a single schema. The expected object is a XMLSchema which is part of the XmlSchema library.

• SchemaCompiler - Compile(List schemaList)

  Similar to the previous method but accepts a list of schemas instead of one.

For a comprehensive code sample in invoking the schema compiler through the API, the following classes would be helpful. One would also need an understanding of the generation modes of the ADB schema compiler when using it through the API.

• org.apache.axis2.schema.XSD2Java
• org.apache.axis2.wsdl.codegen.extension.SimpleDBExtension

Generation Modes

ADB extension provides several Generation modes for the data bound classes.

1. Integrated mode

   In this mode the classes are generated as inner classes of the stub, message receiver or the interface. The ADB framework refrains from writing the classes but provides a map of DOM document objects that contains the model for the databinding class. The implementers are free to use the models for their needs. The Axis2 codegen engine parses these documents with the an XSLT parser to create the necessary code.

   Integrated mode is supposed to be used by the tool builders.

2. Wrapped mode

   In the wrapped mode, the ADB databinder generates one class that contains all the databound classes. This is convenient when the number of classes need to be limited.

3. Expanded mode

   This is the usual mode where the codegenerator generates a class for each of the outer elements and the named complex types. The command line tool (XSD2Java) always generates code in the expanded mode.

The rules for generating code (described in the next section) applies regardless of the mode. Switching these modes can be done by passing the correct options via the CompilerOptions object. The following is table lists the options and the consequences of using them.

Deep into the Generated Code

When the schema compiler is invoked (oneway or another) it generates code depending on the following rules

1. All named complex types become bean classes. Any attribute or element encapsulated in this complex type will become a field in the generated class. Note that the support for constructs other than the sequence and all is not yet implemented.
2. All top level elements become classes. This is a rather questioning feature since unless classes are generated for the top level elements the handling of elements become difficult and messy!
3. SimpleType restrictions are handled by replacing the relevant type with the basetype

Once the code is generated according to the rules it looks like the following. Consider the following piece of schema

<schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:tns="http://soapinterop.org/types" targetNamespace="http://soapinterop.org/types">
<import namespace="http://schemas.xmlsoap.org/soap/encoding/"/>
 <complexType name="SOAPStruct">
  <sequence>
   <element name="varString" type="xsd:string"/>
   <element name="varInt" type="xsd:int"/>
   <element name="varFloat" type="xsd:float"/>
  </sequence>
 </complexType>
<element name="myElement" type="tns:SOAPStruct"/>
</schema>

For comprehension let us consider the expanded mode for the code generator. Unless specifically mentioned, the rest of this document assumes that the expanded mode of the code generation is used. This particular schema generates the following two classes in the designated package. This package is derived from the target namespace of the schema.

1. myElement.java
2. SOAPStruct.java

As explained earlier, SOAPStruct refers to the complexType. MyElement is the class that refers to the element. Just as expected, the SOAPStruct bean has getters and Setters for varString, varInt and varFloat which are String, int and float respectively. myElement on the other hand has a single field representing the SOAPStruct object that it encapsulates.

The most important aspect of the generated code is that it encapsulates two methods for creation and serializing the beans. Note that to make this work, the generated beans implement the org.apache.axis2.databinding.ADBBean interface

The creator and reader methods look like the following

• public javax.xml.stream.XMLStreamReader
      getPullParser(javax.xml.namespace.QName qName)

  This method returns a pull parser that throws the right events for this particular object. However there is a subtle difference between element based classes and complexType based classes

  1. An element based bean class (like myElement.java in the example) will ignore the passed in QName. Instead of using the passed in QName it'll utilize it's own QName which is embedded in the class under the constant MY_QNAME, during the code generation.
  2. A ComplexType based bean class(like SOAPStruct.java in the example) will use the passed in QName to return an instance of the ADBpullparser. This will effectively wrap the elements inside with an element having the passed QName

•  public static [Object] 
               parse(javax.xml.stream.XMLStreamReader reader) 
               throws java.lang.Exception 

  This method returns a populated instance of the class in question. Note that

  [Object]

  will be replaced by the actual class that contains this method. Say for SOAPStruct the method looks like

  public static SOAPStruct 
                  parse(javax.xml.stream.XMLStreamReader reader) 
                  throws java.lang.Exception 

An Example!

Consider the following XML fragment

<myElement>
  <varInt>5</varInt>
  <varString>Hello</varString>
  <varFloat>3.3</varFloat>
</myElement>

Enthusiastic readers might already have figured out that this piece of XML complies to the Schema mentioned above. The following piece of code shows how to build a populated instance of myElement with this fragment of XML

XMLStreamReader reader = XMLInputFactory.newInstance().
                                createXMLStreamReader(
                                        new ByteArrayInputStream(xmlString.getBytes()));
myElement elt = myElement.parse(reader);

Although this example takes on the tedious effort of creating a reader out of the String, inside the Axis2 environment an XMLStreamReader can be direclty asked from the OMElement! Hence the parse method becomes a huge advantage for hassle free object creation.

Similarly the reader obtained from the object can also be utilized as needed. The following code fragment shows how to utilize the getPullParser method to create an OMElement

XMLStreamReader reader = elt.getPullParser(null);
OMElement omElt =  new StAXOMBuilder(reader).getDocumentElement();

That's all to it! If you are interested in learning more on ADB the following documents may also be helpful

• Tweaking the ADB code generator
• ADB and Axis2 Integration

Known Limitations

ADB is meant to be a 'Simple' databinding framework and was not meant to compile all types of schemas. The following limitations are the most highlighted.

1. Complex Extensions and Restrictions are not supported.
2. Choice (Particle) is not supported
3. SimpleType Unions are not supported