PROPOSAL: Lightweight Properties (original) (raw)

Neal Gafter neal at gafter.com
Tue Mar 3 12:17:30 PST 2009

Joe Darcy sort of ruled out adding property support in project coin in http://blogs.sun.com/darcy/entry/guidance_measure_language_change_size

Regards, Neal

On Tue, Mar 3, 2009 at 12:05 PM, Reinier Zwitserloot <reinier at zwitserloot.com> wrote:

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(READWRITE, duke, firstP, textfield, textP); binding.bind();

would instead be written:- Binding binding = Bindings.createAutoBinding(READWRITE, 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; Listdukes; 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 The Parent class for this field * @param 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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