GetField


1   // ============================================================================
2   // $Id: GetField.java,v 1.11 2006/08/05 21:34:32 davidahall Exp $
3   // Copyright (c) 2002-2005  David A. Hall
4   // ============================================================================
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17  // 
18  // Alternatively, the contents of this file may be used under the terms of the 
19  // GNU Lesser General Public License Version 2.1 or later (the "LGPL"), in which
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32  
33  package net.sf.jga.fn.property;
34  
35  import java.lang.reflect.InvocationTargetException  ;
36  import java.lang.reflect.Field  ;
37  import java.text.MessageFormat  ;
38  import net.sf.jga.fn.EvaluationException;
39  import net.sf.jga.fn.UnaryFunctor;
40  
41  /**
42   * Unary Functor that returns the value of the named field for the argument.
43   * The field name and type are set at construction.
44   * <p>
45   * Copyright &copy; 2002-2005  David A. Hall
46   *
47   * @author <a HREF="mailto:davidahall@users.sourceforge.net">David A. Hall</a>
48   **/
49  
50  public class GetField<T,R> extends UnaryFunctor<T,R> {
51      
52      static final long serialVersionUID = 5733484457689881481L;
53  
54      // The name of the field to be retrieved.
55      private String   _fieldName;
56  
57      // The class from which the field is retrieved.
58      private Class  <T> _argtype;
59      
60      // The Field whose valus is returned
61      private transient Field   _field;
62  
63      /**
64       * Builds a GetField for the given field
65       * @throws IllegalArgumentException if the field is null, or is not a field
66       * of the given class
67       */
68      public GetField(Field   field) {
69          if (field == null) {
70              throw new IllegalArgumentException  ("Must supply field");
71          }
72  
73          _field = field;
74          _fieldName = field.getName();
75              // @SuppressWarnings
76              // There's nothing we can do about this other than warn the users
77              // to make sure that they don't use an inappropriate argument type
78          _argtype = (Class  <T>) field.getDeclaringClass();
79      }
80      
81      /**
82       * Builds a GetField for the given field
83       * @throws IllegalArgumentException if the field is null, or is not a field
84       * of the given class
85       */
86      public GetField(Class  <T> argclass, Field   field) {
87          if (field == null) {
88              throw new IllegalArgumentException  ("Must supply field");
89          }
90  
91          if (!field.getDeclaringClass().isAssignableFrom(argclass)) {
92              throw new IllegalArgumentException  (buildNoSuchFieldMessage(field.getName(), argclass));
93          }
94          
95          _field = field;
96          _fieldName = field.getName();
97          _argtype = argclass;
98      }
99  
100     /**
101      * Builds a GetField for the given field, using the given class to
102      * find the desired field.
103      * @throws IllegalArgumentException if the field name is null or empty,
104      *    
105      */
106     public GetField(Class  <T> argclass, String   fieldName) {
107         if (fieldName == null || fieldName.length() == 0) {
108             throw new IllegalArgumentException  ("Must supply field name");
109         }
110 
111         _fieldName = fieldName;
112         _argtype = argclass;
113         
114         try {
115             _field = argclass.getField(_fieldName);
116         }
117         catch (NoSuchFieldException   x) {
118             IllegalArgumentException   iax =
119                 new IllegalArgumentException  (buildNoSuchFieldMessage(fieldName, argclass));
120             
121             iax.initCause(x);
122             throw iax;
123         }
124     }
125 
126     /**
127      * Returns the class on which the field is retrieved.
128      */
129     // UNTESTED
130     public Class  <T> getObjectType() {
131         return _argtype;
132     }
133 
134     /**
135      * Returns the name of the field that this functor retrieves.
136      */
137     public String   getFieldName() {
138         return _fieldName;
139     }
140 
141     /**
142      * Returns the type of field that this functor retrieves.  
143      */
144 
145     public Class  <R> getFieldType() {
146         return (Class  <R>) _field.getType();
147     }
148 
149     
150     /**
151      * Lazily loads the Field object.
152      */
153     
154     // Instances of Reflection classes are not serializable, so this must be loaded lazily
155     // in case this object was read from a stream (the first time the functor is invoked,
156     // the field object will be instantiated)
157     
158     public synchronized Field   getField() throws NoSuchFieldException   {
159         if (_field == null) 
160             _field = _argtype.getField(_fieldName);
161 
162         return _field;
163     }
164 
165     // Unary interface
166 
167     /**
168      * Returns the value of the designated field of the argument
169      * @return the value of the designated field of the argument
170      * @throws EvaluationException if the argument does not have the designated
171      *         public field, or if it is not of the correct type. 
172      */
173     public R fn(T arg) {
174         try {
175             // @SuppressWarnings("unchecked")
176             // There's nothing we can do about this other than warn the users
177             // to make sure that they don't use an inappropriate return type
178             R val = (R) getField().get(arg);
179             return val; 
180         }
181         catch (NoSuchFieldException   x) {
182             throw new EvaluationException(buildNoSuchFieldMessage(_fieldName, _argtype));
183         }
184         catch (ClassCastException   x) { 
185             String   msg = "{0}.{1} is of type {2}";
186             Object  [] args = new Object  []{ arg.getClass().getName(), _field.getName(),
187                                           _field.getType().getName() };
188             throw new EvaluationException(MessageFormat.format(msg,args), x);
189         }
190         catch (IllegalAccessException   x) {
191             String   msg = "{0}.{1} is not accessible";
192             Object  [] args = new Object  []{ _argtype.getName(), _fieldName};
193             throw new EvaluationException(MessageFormat.format(msg,args), x);
194         }
195     }
196 
197 
198     /**
199      */
200     private String   buildNoSuchFieldMessage(String   fieldname, Class   clasz) {
201         String   msg = "class {0} does not have field {1}";
202         Object  [] args = new Object  []{ clasz.getName(), fieldname };
203         return MessageFormat.format(msg,args);
204     }
205 
206     /**
207      * Calls the Visitor's <code>visit(GetField)</code> field, if it
208      * implements the nested Visitor interface.
209      */
210     public void accept(net.sf.jga.fn.Visitor v) {
211         if (v instanceof GetField.Visitor)
212             ((GetField.Visitor)v).visit(this);
213         else
214             v.visit(this);
215     }
216     
217     // Object overrides
218 
219     public String   toString() {
220         return "GetField["+_argtype.getName()+"."+_fieldName+"]";
221     }
222     
223    // AcyclicVisitor
224     
225     /**
226      * Interface for classes that may interpret a <b>GetField</b>
227      * function.
228      */
229     public interface Visitor extends net.sf.jga.fn.Visitor {
230         public void visit(GetField host);
231     }
232 }
233
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