PROPOSAL: Lightweight Properties
Reinier Zwitserloot
reinier at zwitserloot.com
Tue Mar 3 12:05:34 PST 2009
You call that lightweight?
How about following the beans spec more to the letter and just
generate the addPropertyChangeListener, removePropertyChangeListener,
setX(), and get/isX() method in response to seeing a keyword or
annotation on a given field. You'll have to work out the details, but
that sounds far, far simpler to understand.
You'll need to flesh this out, but it would look something like:
public class Foo {
private property int x;
}
Which would generate the addPropertyChangeListener,
removePropertyChangeListener, setX, getX methods, all public, along
with the required infrastructure to make it tick. If you don't like
the generation, for example because you want the setter to be package
private, you just add the setter in the source file; the keyword will
only generate the missing stuff. It doesn't cover every use case, but
there's always the alternative of doing whatever people do now with
beans. Something you didn't mention in your proposal, by the way.
I think there's also a fully fleshed out property proposal (including
a 'property' keyword) out there somewhere.
Possibly make a way to opt out of generating the property change
listener support, and just the getters/setters.
--Reinier Zwitserloot
On Mar 3, 2009, at 15:59, David Goodenough wrote:
> Below is my proposal for Lightweight Properties. I know that the
> syntax
> change is an abbomination to some people, but I have tried to reduce
> this to its absolute minimum, while still getting a significant
> benefit.
>
> PROJECT COIN SMALL LANGUAGE CHANGE PROPOSAL FORM v1.0
>
> AUTHOR(S):
>
> David Goodenough, long time Java user. I can be reached at
> david.goodenough at linkchoose.co.uk.
>
> OVERVIEW
>
> FEATURE SUMMARY:
>
> Lightweight Property support
>
> MAJOR ADVANTAGE:
>
> Both BeansBinding (whether JSR-295 or others such an JFace or the
> JGoodies
> binding) and the JPA Criteria API currently require field names (as
> Strings)
> as arguments, which an IDE/compiler can not check. With this
> proposal the
> strings would be abandoned, and the IDE/compiler will be able to
> check the
> correctness of the code.
>
> MAJOR BENEFIT:
>
> Manual checking no longer required. This proposal introduces a
> simple well
> defined IDE/compiler checkable solution.
>
> MAJOR DISADVANTAGE:
>
> It is a language change, and this seems to upset some people.
>
> ALTERNATIVES:
>
> None really, apart from using another language or continuing to use
> String
> names. The existing solutions all require String names which are
> uncheckable.
>
> EXAMPLES
>
> Lets assume we have a POJO called foo, of type Foo with a field bar
> of type
> Bar, which itself has a field of type Jim called jim.
>
> There are two forms of lightweight properties:-
>
> 1) foo#bar would be translated by the compiler into:-
>
> new Property<Foo,Bar>(foo,"bar");
>
> while foo#bar#jim would be translated into:-
>
> new Property<Foo,Jim>(foo,"bar","jim");
>
> 2) Foo#bar would be translated into:-
>
> new Property<Foo,Bar>(Foo.class,"bar");
>
> while Foo#bar#jim would be translated into:-
>
> new Property<Foo,Jim>(Foo.class,"bar","jim");
>
> These two forms create (1) a bound Property, or (2) an unbound one.
> Bound
> Properties are explicitly bound to a particular instance of a class
> (in this
> case foo), while unbound Properties are templates which can be
> applied to any
> instance of class Foo. Actually bound properties can also be used as
> unbound
> properties, but that is a harmless and useful side effect not a
> primary
> intent.
>
> The Property class would need to be added (it is appended below),
> and should
> be added either to the java.beans package or to the
> java.lang.reflect package
> (with which is probably has more in common).
>
> Syntactically a "#" can be placed wherever a "." can be placed
> (except inside
> a number), and the same checks need to be made (that each field to
> the right
> of a # is a field in the left hand side) as would be made for a ".".
> The only
> difference is in field visibility - For the "#" any field is
> visible, which
> follows the model currently available in the Field class with
> getDeclaredFields(). It also follows the model that while a field
> might be
> private and therefore not directly accessible from outside, getters
> and
> setters can provide access.
>
> The Property object provides type safe access to the field in the
> form of
> getters and setters. These come in pairs, one for bound and the
> other for
> unbound access. So for bound access no object is required to fetch
> the value,
> for an unbound object the parent object needs to be specified. So if
> we
> have:-
>
> Property<Foo,Bar>prop = foo#bar;
>
> we can later say:-
>
> Bar b = prop.get();
>
> or for an unbound one from a second Foo object foo2:-
>
> Bar b = prop.get(foo2);
>
> The getters and setters in the Property object will defer to
> explicitly coded
> getters and setters if present, otherwise they will use the Field
> getter and
> setter.
>
> If a setter is not explicitly coded, the implicit setter will look
> for a
> PropertyChangeSupport object in the parent object of the rightmost
> field and
> fire a PropertyChangeEvent to that object.
>
> There are also two Annotations provided by the Property class,
> ReadOnly and
> WriteOnly. These stop implicit getters and setters from trying to
> read/write
> the property.
>
> Talking of Annotations, this notation can also be used to get at the
> Annotations for a field. So to test for the presence of an
> Annotation Ann on
> Foo.bar we would use:-
>
> if(Foo#bar.getFields()[0].isAnnotationPresent(Ann.class)) ...
>
> SIMPLE EXAMPLE:
>
> To take an example from BeansBinding (taken from Shannon Hickey's
> blog):-
>
> // create a BeanProperty representing a bean's firstName
> Property firstP = BeanProperty.create("firstName");
> // Bind Duke's first name to the text property of a Swing JTextField
> BeanProperty textP = BeanProperty.create("text");
> Binding binding = Bindings.createAutoBinding(READ_WRITE, duke,
> firstP, textfield, textP);
> binding.bind();
>
>
> would instead be written:-
>
> Binding binding = Bindings.createAutoBinding(READ_WRITE,
> duke#firstName, textfield#text);
> binding.bind();
>
> which of course can be checked by the IDE/compiler, and will not
> wait until
> run time (not even instantiation time) to show up the error.
>
> ADVANCED EXAMPLE:
>
> For a JComboBox (or JList or JTable or JTree) there is a need to map
> a list of
> objects to the value strings (or column contents). For this we need
> to have
> an unbound Property which can be applied to each element of the list.
>
> Duke duke;
> List<Duke>dukes;
> BoundComboBox combo = new
> BoundComboBox(dukes,Duke#fullname,this#duke);
>
> and now the combo box will be populated from the list dukes, and the
> display
> values in the list will be taken from the fullname field of each Duke
> element, and the initial value will be set from the local class
> field duke
> and any changes to the combo box selected element will be reflected
> back to
> the duke field.
>
> DETAILS
>
> SPECIFICATION:
>
> This proposal adds a new syntactic element, "#", which can be used
> in the same
> way that "." can be used to qualify fields within a Java object.
>
> COMPILATION:
>
> This proposal requires no change to the class files, and is
> implemented by a
> simple generation of the required instance using the relevant Property
> constructor. Obviously the compiler would have to make sure that the
> use that
> the property object was being put to (in the examples above the left
> hand
> side of the assignment) had the correct Generic attributes.
>
> TESTING:
>
> How can the feature be tested?
>
> LIBRARY SUPPORT:
>
> The new Property class is required (see below).
>
> REFLECTIVE APIS:
>
> No changes are required to the reflective APIs although it makes
> extensive use
> of those APIs.
>
> OTHER CHANGES:
>
> No other changes are requires.
>
> MIGRATION:
>
> Fortunately there is no code that is formally part of J2SE 6 which
> uses such
> Properties. There are however two proposals which will need it
> (BeansBinding
> and JPA Criteria API), but neither of these seem to be destined to
> be part of
> J2SE 7 (BeansBinding seems to have died the death and the Criteria
> API would
> be part of the next J2EE which will follow J2SE 7), so this will
> provide a
> base for them to use and no existing code need to be updated.
>
> There are other extant Beans-Binding libraries, which could be
> modified to use
> this proposal, but as none of the existing features have been
> changed there
> is no need to change them (other than for type safety and compiler/IDE
> checkability).
>
> COMPATIBILITY
>
> BREAKING CHANGES:
>
> None. This change should not make any existing correct code fail to
> compile
> or run or change the way in which it compiles/runs.
>
> EXISTING PROGRAMS:
>
> No change required to any existing programs
>
> REFERENCES
>
> EXISTING BUGS:
>
> None
>
> URL FOR PROTOTYPE (optional):
>
> I do not have the knowledge to make changes to the compiler, and the
> only
> documentation making such changes concentrated on adding operators not
> changes at this level. So there is no prototype of the compiler
> part, but the
> Property class follows:-
>
> package java.lang.reflect;
>
> import java.beans.BeanInfo;
> import java.beans.Introspector;
> import java.beans.PropertyChangeSupport;
> import java.beans.PropertyDescriptor;
> import java.lang.reflect.Field;
> import java.lang.reflect.Method;
>
> /**
> * Property class
> * This is the support class for use with the # notation to provide
> lightweight
> * Property support for Java.
> *
> * @copyright Copyright(C) 2009 David Goodenough Linkchoose Ltd
> * @licence LPGL V2 : details of which can be found at http://fsf.org.
> * @author david.goodenough at linkchoose.co.uk
> *
> * @param <C> The Parent class for this field
> * @param <F> The Type of this field
> */
> public class Property<C,F> {
> private C parent;
> private Class<?> parentClass;
> private Field[] fields;
> private PropertyDescriptor[] pd = null;
> /**
> * Constructor used to create Property objects. The Parent object
> may be
> * null, but should normally be specified as it can be overridden
> anyway.
> * @param parent C object that contains the field
> * @param field Field describing this field
> */
> public Property(C parent, String ... fieldNames) {
> this.parent = parent;
> this(parent.getClass(), fieldNames);
> }
> /**
> * Constructor for unbound Properties, but also used internally
> after
> setting
> * the parent object by the bound Property objects.
> * @param parentClass Class of the parent object
> * @param fieldNames String[] of field names
> */
> public Property(Class<?>parentClass, String .. fieldNames) {
> this.parentClass = parentClass;
> fields = new Field[fieldNames.length];
> pd = new PropertyDescriptor[fieldNames.length];
> outer: for(int index = 0; index < fields.length; index++) {
> Field[]dclFields = parentClass.getDeclaredFields();
> for(Field field:dclFields) {
> if(field.getName().equals(fieldNames[index])) {
> fields[index] = field;
> field.setAccessible(true);
> try {
> BeanInfo beanInfo =
> Introspector.getBeanInfo(parent.getClass());
> PropertyDescriptor[]props =
> beanInfo.getPropertyDescriptors();
> for(PropertyDescriptor prop : props) {
> if(prop.getName().equals(field.getName())) {
> pd[index] = prop;
> break;
> }
> }
> } catch(Exception e) { /* assume can not find getter/
> setter
> */ }
> parentClass = field.getType();
> continue outer;
> }
> }
> throw new IllegalArgumentException("Field " + fieldNames[index] +
> " not found in class " +
> parentClass.getCanonicalName());
> }
> }
> /**
> * Getter from the field in the parent specified when this
> Property was
> created.
> * @see Property.get(C otherParent)
> * @return F the value of this field
> */
> public F get() {
> return get(parent);
> }
> /**
> * Getter with explicit parent.
> * This code will check see if this field is WriteOnly, and
> complain if it
> is.
> * It will then see if the use has provided am explicit getter,
> and call
> that
> * if present, otherwise it will just fetch the value through the
> Field
> provided
> * method.
> * @param otherParent C parent object
> * @return F value of the field
> */
> @SuppressWarnings("unchecked") // This should actually not be
> needed,
> // but the Field.get method is not
> typed
> public F get(C otherParent) {
> Object result = otherParent;
> try {
> for(int index = 0; index < fields.length; index++) {
>
> if(fields[index].getType().isAnnotationPresent(WriteOnly.class))
> throw new IllegalAccessException(
> "Can not get from a WriteOnly field - " +
> fields[index].getName());
> Method getter = pd[index] == null ? null :
> pd[index].getReadMethod();
> if(getter == null) result = fields[index].get(result);
> else result = getter.invoke(result);
> }
> } catch(Exception e) {
> throw new RuntimeException("Should not occur exception", e);
> }
> return (F)result;
> }
> /**
> * Setter to set the value of the field in the parent object
> declared with
> the
> * Property object
> * @param newValue F new value of this field
> */
> public void set(F newValue) {
> set(parent,newValue);
> }
> /**
> * Setter to set the value of the field to an explicit parent
> object.
> * If there is a ReadOnly annotation, then we object. If there is
> an
> explicit
> * setter then we use that, otherwise we set the field using the
> Field
> provided
> * set method and if there is a PropertyChangeSupport field, fire a
> property
> * change event to it.
> * We walk our way down the field chain, until we have the last
> object and
> its
> * field, and then we do the set.
> * @param parent C explicit parent object
> * @param newValue F new value for field in parent
> */
> public void set(C parent,F newValue) {
> try {
> Object last = parent;
> int index;
> for(index = 0; index < fields.length - 1; index++) {
>
> if(fields[index].getType().isAnnotationPresent(WriteOnly.class))
> throw new IllegalAccessException(
> "Can not get from a WriteOnly field - " +
> fields[index].getName());
> Method getter = pd[index] == null ? null :
> pd[index].getReadMethod();
> if(getter == null) last = fields[index].get(last);
> else last = getter.invoke(last);
> }
>
> if(fields[index].getType().isAnnotationPresent(ReadOnly.class))
> throw new IllegalAccessException(
> "Can not get from a WriteOnly field - " +
> fields[index].getName());
> Method setter = pd[index] == null ? null :
> pd[index].getWriteMethod();
> if(setter == null) {
> PropertyChangeSupport pcs = findPcs(last.getClass());
> fields[index].set(last,newValue);
> if(pcs != null)
> pcs.firePropertyChange(fields[index].getName(),
> newValue,
>
> fields[index].get(last));
> } else setter.invoke(last,newValue);
> } catch(Exception e) {
> throw new RuntimeException("Should not occur
> exception", e);
> }
> }
> /**
> * This is used so that the caller can view the Field name
> * @return String field name
> */
> public String[] getFieldName() {
> String[]names = new String[fields.length];
> for(int index = 0; index < fields.length; index++) {
> names[index] = fields[index].getName();
> }
> return names;
> }
> /**
> * This method is used to fetch the Field array, which is useful
> if you
> need to
> * access the Annotations of a field.
> * @return Field[] the array of Fields describing this Property.
> */
> public Field[] getFields() {
> return fields;
> }
> /**
> * This private method looks for a PropertyChangeSupport object in
> the
> class and
> * if one is found it will return it. It looks right the way up
> the class
> tree
> * by recurring up the superClasses.
> * @param parent Class to check for PropertyChangeSupport fields
> * @return PropertyChangeSupport first found object, or null if
> not found
> */
> private PropertyChangeSupport findPcs(Class<?> parent) {
> Field fields[] = parent.getDeclaredFields();
> for(Field field:fields) {
> field.setAccessible(true);
> try {
> if(field.getType() == PropertyChangeSupport.class)
> return (PropertyChangeSupport)field.get(parent);
> } catch(Exception e) { }
> }
> // If we did not find it then try the superclass
> Class<?>superClass = parent.getSuperclass();
> if(superClass == null) return null;
> return findPcs(parent.getClass().getSuperclass());
> }
> /**
> * This annotation is used to mark a field as WriteOnly, i.e. it
> can not
> be read.
> * This stops the automatic getter operation.
> */
> public @interface WriteOnly {
> }
> /**
> * This annotation is used to mark a field as ReadOnly, i.e. it
> can not be
> written.
> * This stops the automatic setter operation.
> */
> public @interface ReadOnly {
> }
> }
>
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