SaxStore: an aspect oriented persistence library for Java based on SAX events

1
SaxStore an aspect orientedpersistence library
for Javabased on SAX events

• Riccardo Solmi
• University of BolognaMay 2001

2
Summary

• SaxStore features and usage scenarios
• Choice of a perspective
• XML representation
• Accessing XML data
• Accessing application objects
• SaxStore persistence management

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Features

• SaxStore provides a mechanism to map arbitrary
  Java objects into a stream of SAX events and
  backward.
• SAX events can be serialized to an XML stream and
  saved to disk or transmitted across a network.
• XSLT transformations (translets) can be used to
  support versioning, stylesheet application and
  conformance to a given XML Schema.

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Long-term persistence scenario
Runtime Application objects
Stylesheet 1.0 to 1.1
Stylesheet 1.1 to 1.2
Stylesheet 1.2 to 1.1
Stylesheet 1.1 to 1.0
SaxStore
XSLT compiler
XSLT compiler
XML serializer
XSLT compiler
XSLT compiler
XML parser
XML

• Compression
• Integrity signature
• Given XML Schema

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Representation scenario
Runtime Application objects
Stylesheet for XSLFO representation
XSLFO representation
SaxStore
XSLT compiler

• Dynamic form filling
• XSLFO, XHTML, SVG,

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Data exchange scenario
Runtime Application objects
Runtime Application objects
Stylesheet transformation to a given XML Schema
Stylesheet transformation from a given XML Schema
Schema instance
SaxStore
XSLT compiler
XSLT compiler
SaxStore

• Given XML Schema
• Sun XMLB is better (may be)

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Persistence

• To preserve something of the current execution
  state between successive program runnings.
• To store a (runtime) graph of objects to a non
  volatile memory and to reconstruct an equivalent
  graph of objects.

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Requirements

• Exchangable and neutral data format(i.e. XML)
• Almost automatic but customizable
• Large graphs of objects
• Versioning
• Performance

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Goals

• Powerful and easy to use as Java Serialization
  API
• Support short-term long-term persistence
• Data exchange between running applications
• Document format for successive executions
• Decouple producers and consumers
• Support distributed applications

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Persistence and Java

• Java has not yet a standard persistence library
• Java Serialization API provides only short term
  persistence and was introduced to support RMI.
• SUN is working only on projects that doesnt
  satisfy my requirements.

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Runtime and persistent representations

• Both runtime and persistent representations
  should be user defined.
• Runtime. To implement efficiently application
  algorithms
• Persistent. To support interoperability of
  applications and loosly coupled distributed
  systems

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Choice of a perspective

• An (almost) automatic transformation between two
  unconstrained and unrelated forms is not
  possible!
• Two alternative points of view are suitable
• Java side
• XML side

My classes
XML
My XML
custumizable
manual
My classes
Java classes
My XML
auto
manual
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Data Binding focus on XML side

• Data Binding (JSR-031) Bind Java objects to an
  XML document. Compiles an XML Schema to Java
  classes with specific accessor and mutator
  methods that affect the underlying XML document.
• Writing the XML Schema from application classes
  (or even from scratch) is expansive.
• Writing the code, in application classes, that
  store/retrive data to/from the graph of generated
  classes is not trivial.
• Generated classes requires additional memory

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SaxStore focus on Java side

• In many applications the format of XML
  representation doesnt matter.
• Versioning support is the major requirement.
• An easy to understand XML representation is
  enough.
• Customization is required only for a very small
  number of classes.

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SaxStore XML representation

• Is an XML-Schema instance.
• Objects are mapped to elements
• Objects have an id attribute with a unique value
  and a xsitype attribute with the runtime type.
• Fields are mapped to nested elements with the
  same name (because attributes are not
  referenceable)
• Objects already mapped are represented by an
  element with an idref attribute.

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Accessing XML data

• Callbacks (SAX) Is an event-driven model the
  parsing of an XML document trigger callback
  methods.
• Trees (DOM) The parsing of an XML document
  create a tree structure manageable with a set of
  standard methods.
• SaxStore is based on SAX2 events.

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Accessing application objects

• Reflection
• State exposition
• Field level accessor and mutator methods
• Object level persistence methods
• Aspect

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Persistence vs encapsulation

• Reflection violates encapsulation and performs
  poorly.
• Field level state exposition make encapsulation
  useless.
• Object level state exposition alone is a weak
  solution and requires excessive work.
• Aspects preserve encapsulation while retaining
  instance methods performance.

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SaxStore persistence management

• User defined
• XmlSerializable interface (source required)
• Custom aspect or single classes
• XmlSerializer interface or adapter class
• Singletons
• Part objects
• Automatic
• AspectWriter preprocessor (source required)
• Reflective XmlSerializer

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Future works

• Produce an XML-Schema
• Support XML-Schema constraints
• Generation of persistence code based on reverse
  engineering of classes that uses Java
  serialization API
• Integration with distributed API