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From "Craig R. McClanahan" <craig...@apache.org>
Subject [Design Discussion] DynaBean - JavaBeans with dynamic properties
Date Sat, 15 Dec 2001 21:26:05 GMT
There's been a lot of interest, both here (COMMONS-DEV), on the Struts
mailing lists, and elsewhere in the idea of a JavaBean-like thing where
the set of properties can be dynamically defined.  If support for this was
integrated into the BeanUtils package in Commons, we could also make the
methods in BeanUtils and PropertyUtils treat the properties of these beans
like they do for regular JavaBeans, and thus be able to leverage them in
environments based on BeanUtils (such as Struts).

The purpose of this email is to summarize a set of design requirements for
discussion purposes, so we can coalesce on a design and then get it
implemented.  For extra fun, I've thrown in a very-raw idea of what the
APIs might look like -- but I'd like to have the group's agreement before
turning that into code.

Before embarking on the actual development of dynamic beans support, we
should also do a release of BeanUtils with the current enhancements, to
provide a stable starting point.

Please let me know what you think!


BACKGROUND:

The purpose of the DynaBean design (I'm not firmly attached to the name,
but kinda like it :-) is to support application programming designs based
on JavaBeans design patterns, but where the static nature of JavaBeans
themselves (i.e. the fact that the set of properties is fixed at compile
time) is too restrictive.  Some typical use cases for such a capability:

* A bean object that represents a Row from a JDBC ResultSet
  (the names and types of the column properties cannot be known
  in advance because they are based on the SELECT statement).

* A way to construct and use "value objects" that extract the
  properties of an EJB into a structure that can be utilized by a
  presentation tier technology such as JSP pages, without having
  to hand code the value object.

* A way to capture *all* of the request parameters from a servlet
  request as a single object.

* A way to dynamically represnt XML input data as a tree of Java objects.


DESIGN REQUIREMENTS:

* Support a completely arbitrary set of properties and corresponding
  values, with the ability to add and remove properties dynamically.

* Support the ability to dynamically register the set of property
  names that are allowed, and then enforce storing only those
  property names.

* Support the ability to register data types (Java classes) for each
  property, along with the names, with automatic type checking.

* Support for properties with scalar types, array types, and
  "mapped" types (as currently supported in Beanutils).

* Support "simple" and "indexed" property getters and setters
  in a style similar to standard JavaBeans.

* Support registration of property change listeners and broadcast
  of property change events

* Provide the basic DynaBean API as a Java interface, as well as a
  convenience base class, so that applications can adopt it however
  they wish.

* Base implementation class should implement Serializable so that
  DynaBeans can be serialized and deserialized (assuming that the
  actual property values can be).

* Transparently support DynaBean objects in the existing BeanUtils
  and PropertyUtils classes of the Bean Utilities package.

* To the maximum extent feasible, provide JavaBeans-like introspection
  capabilities of the available properties.  May require an
  IntrospectionUtils addition to the Beanutils package.


CONCEPTUAL API:

public interface DynaBean {


    // ----------------------------------------------------------------
    // Dynamic property configuration
    // ----------------------------------------------------------------

    void addDynamic(String name);             // Unrestricted type
    void addDynamic(String name, Class type); // Restricted type
    void removeDyanamic(String name);         // Remove property & value
    void restrictDynamic(boolean restrict);   // Are property names
                                              // restricted to registered
                                              // ones?  (default=false)
                                              // (can be changed)

    // ----------------------------------------------------------------
    // Property getters and setters.  Behavior on previously
    // unknown property names depends on the restrict setting
    // ----------------------------------------------------------------

    // Does this bean have a property of the specified name?
    boolean contains(String name);

    // Getters return null on unknown property names
    Object get(String name);                  // Simple
    Object get(String name, int index);       // Indexed
    Object get(String name, String key);      // Mapped

    // Setters are allowed to set null unless property is native.
    // Optionally throw IllegalArgumentException if type is restricted
    // and the value argument is not assignment-compatible
    void set(String name, Object value);      // Simple
    void set(String name, int index,          // Indexed
             Object value);
    void set(String name, String key,         // Mapped

}


Craig McClanahan




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