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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.beanutils.converters;
18  
19  import java.lang.reflect.Array;
20  import java.util.Collection;
21  
22  import org.apache.commons.beanutils.BeanUtils;
23  import org.apache.commons.beanutils.ConversionException;
24  import org.apache.commons.beanutils.ConvertUtils;
25  import org.apache.commons.beanutils.Converter;
26  import org.apache.commons.logging.Log;
27  import org.apache.commons.logging.LogFactory;
28  
29  /**
30   * Base {@link Converter} implementation that provides the structure
31   * for handling conversion <b>to</b> and <b>from</b> a specified type.
32   * <p>
33   * This implementation provides the basic structure for
34   * converting to/from a specified type optionally using a default
35   * value or throwing a {@link ConversionException} if a
36   * conversion error occurs.
37   * <p>
38   * Implementations should provide conversion to the specified
39   * type and from the specified type to a <code>String</code> value
40   * by implementing the following methods:
41   * <ul>
42   *     <li><code>convertToString(value)</code> - convert to a String
43   *        (default implementation uses the objects <code>toString()</code>
44   *        method).</li>
45   *     <li><code>convertToType(Class, value)</code> - convert
46   *         to the specified type</li>
47   * </ul>
48   * <p>
49   * The default value has to be compliant to the default type of this
50   * converter - which is enforced by the generic type parameter. If a
51   * conversion is not possible and a default value is set, the converter
52   * tries to transform the default value to the requested target type.
53   * If this fails, a {@code ConversionException} if thrown.
54   *
55   * @version $Id$
56   * @since 1.8.0
57   */
58  public abstract class AbstractConverter implements Converter {
59  
60      /** Debug logging message to indicate default value configuration */
61      private static final String DEFAULT_CONFIG_MSG =
62          "(N.B. Converters can be configured to use default values to avoid throwing exceptions)";
63  
64      /** Current package name */
65      //    getPackage() below returns null on some platforms/jvm versions during the unit tests.
66  //    private static final String PACKAGE = AbstractConverter.class.getPackage().getName() + ".";
67      private static final String PACKAGE = "org.apache.commons.beanutils.converters.";
68  
69      /**
70       * Logging for this instance.
71       */
72      private transient Log log;
73  
74      /**
75       * Should we return the default value on conversion errors?
76       */
77      private boolean useDefault = false;
78  
79      /**
80       * The default value specified to our Constructor, if any.
81       */
82      private Object defaultValue = null;
83  
84      // ----------------------------------------------------------- Constructors
85  
86      /**
87       * Construct a <i>Converter</i> that throws a
88       * <code>ConversionException</code> if an error occurs.
89       */
90      public AbstractConverter() {
91      }
92  
93      /**
94       * Construct a <i>Converter</i> that returns a default
95       * value if an error occurs.
96       *
97       * @param defaultValue The default value to be returned
98       * if the value to be converted is missing or an error
99       * occurs converting the value.
100      */
101     public AbstractConverter(final Object defaultValue) {
102         setDefaultValue(defaultValue);
103     }
104 
105     // --------------------------------------------------------- Public Methods
106 
107     /**
108      * Indicates whether a default value will be returned or exception
109      * thrown in the event of a conversion error.
110      *
111      * @return <code>true</code> if a default value will be returned for
112      * conversion errors or <code>false</code> if a {@link ConversionException}
113      * will be thrown.
114      */
115     public boolean isUseDefault() {
116         return useDefault;
117     }
118 
119     /**
120      * Convert the input object into an output object of the
121      * specified type.
122      *
123      * @param <T> the target type of the conversion
124      * @param type Data type to which this value should be converted
125      * @param value The input value to be converted
126      * @return The converted value.
127      * @throws ConversionException if conversion cannot be performed
128      * successfully and no default is specified.
129      */
130     public <T> T convert(final Class<T> type, Object value) {
131 
132         if (type == null) {
133             return convertToDefaultType(type, value);
134         }
135 
136         Class<?> sourceType  = value == null ? null : value.getClass();
137         final Class<T> targetType  = ConvertUtils.primitiveToWrapper(type);
138 
139         if (log().isDebugEnabled()) {
140             log().debug("Converting"
141                     + (value == null ? "" : " '" + toString(sourceType) + "'")
142                     + " value '" + value + "' to type '" + toString(targetType) + "'");
143         }
144 
145         value = convertArray(value);
146 
147         // Missing Value
148         if (value == null) {
149             return handleMissing(targetType);
150         }
151 
152         sourceType = value.getClass();
153 
154         try {
155             // Convert --> String
156             if (targetType.equals(String.class)) {
157                 return targetType.cast(convertToString(value));
158 
159             // No conversion necessary
160             } else if (targetType.equals(sourceType)) {
161                 if (log().isDebugEnabled()) {
162                     log().debug("    No conversion required, value is already a "
163                                     + toString(targetType));
164                 }
165                 return targetType.cast(value);
166 
167             // Convert --> Type
168             } else {
169                 final Object result = convertToType(targetType, value);
170                 if (log().isDebugEnabled()) {
171                     log().debug("    Converted to " + toString(targetType) +
172                                    " value '" + result + "'");
173                 }
174                 return targetType.cast(result);
175             }
176         } catch (final Throwable t) {
177             return handleError(targetType, value, t);
178         }
179 
180     }
181 
182     /**
183      * Convert the input object into a String.
184      * <p>
185      * <b>N.B.</b>This implementation simply uses the value's
186      * <code>toString()</code> method and should be overriden if a
187      * more sophisticated mechanism for <i>conversion to a String</i>
188      * is required.
189      *
190      * @param value The input value to be converted.
191      * @return the converted String value.
192      * @throws Throwable if an error occurs converting to a String
193      */
194     protected String convertToString(final Object value) throws Throwable {
195         return value.toString();
196     }
197 
198     /**
199      * Convert the input object into an output object of the
200      * specified type.
201      * <p>
202      * Typical implementations will provide a minimum of
203      * <code>String --> type</code> conversion.
204      *
205      * @param <T> Target type of the conversion.
206      * @param type Data type to which this value should be converted.
207      * @param value The input value to be converted.
208      * @return The converted value.
209      * @throws Throwable if an error occurs converting to the specified type
210      */
211     protected abstract <T> T convertToType(Class<T> type, Object value) throws Throwable;
212 
213     /**
214      * Return the first element from an Array (or Collection)
215      * or the value unchanged if not an Array (or Collection).
216      *
217      * N.B. This needs to be overriden for array/Collection converters.
218      *
219      * @param value The value to convert
220      * @return The first element in an Array (or Collection)
221      * or the value unchanged if not an Array (or Collection)
222      */
223     protected Object convertArray(final Object value) {
224         if (value == null) {
225             return null;
226         }
227         if (value.getClass().isArray()) {
228             if (Array.getLength(value) > 0) {
229                 return Array.get(value, 0);
230             } else {
231                 return null;
232             }
233         }
234         if (value instanceof Collection) {
235             final Collection<?> collection = (Collection<?>)value;
236             if (collection.size() > 0) {
237                 return collection.iterator().next();
238             } else {
239                 return null;
240             }
241         }
242         return value;
243     }
244 
245     /**
246      * Handle Conversion Errors.
247      * <p>
248      * If a default value has been specified then it is returned
249      * otherwise a ConversionException is thrown.
250      *
251      * @param <T> Target type of the conversion.
252      * @param type Data type to which this value should be converted.
253      * @param value The input value to be converted
254      * @param cause The exception thrown by the <code>convert</code> method
255      * @return The default value.
256      * @throws ConversionException if no default value has been
257      * specified for this {@link Converter}.
258      */
259     protected <T> T handleError(final Class<T> type, final Object value, final Throwable cause) {
260         if (log().isDebugEnabled()) {
261             if (cause instanceof ConversionException) {
262                 log().debug("    Conversion threw ConversionException: " + cause.getMessage());
263             } else {
264                 log().debug("    Conversion threw " + cause);
265             }
266         }
267 
268         if (useDefault) {
269             return handleMissing(type);
270         }
271 
272         ConversionException cex = null;
273         if (cause instanceof ConversionException) {
274             cex = (ConversionException)cause;
275             if (log().isDebugEnabled()) {
276                 log().debug("    Re-throwing ConversionException: " + cex.getMessage());
277                 log().debug("    " + DEFAULT_CONFIG_MSG);
278             }
279         } else {
280             final String msg = "Error converting from '" + toString(value.getClass()) +
281                     "' to '" + toString(type) + "' " + cause.getMessage();
282             cex = new ConversionException(msg, cause);
283             if (log().isDebugEnabled()) {
284                 log().debug("    Throwing ConversionException: " + msg);
285                 log().debug("    " + DEFAULT_CONFIG_MSG);
286             }
287             BeanUtils.initCause(cex, cause);
288         }
289 
290         throw cex;
291 
292     }
293 
294     /**
295      * Handle missing values.
296      * <p>
297      * If a default value has been specified, then it is returned (after a cast
298      * to the desired target class); otherwise a ConversionException is thrown.
299      *
300      * @param <T> the desired target type
301      * @param type Data type to which this value should be converted.
302      * @return The default value.
303      * @throws ConversionException if no default value has been
304      * specified for this {@link Converter}.
305      */
306     protected <T> T handleMissing(final Class<T> type) {
307 
308         if (useDefault || type.equals(String.class)) {
309             Object value = getDefault(type);
310             if (useDefault && value != null && !(type.equals(value.getClass()))) {
311                 try {
312                     value = convertToType(type, defaultValue);
313                 } catch (final Throwable t) {
314                     throw new ConversionException("Default conversion to " + toString(type)
315                             + " failed.", t);
316                 }
317             }
318             if (log().isDebugEnabled()) {
319                 log().debug("    Using default "
320                         + (value == null ? "" : toString(value.getClass()) + " ")
321                         + "value '" + defaultValue + "'");
322             }
323             // value is now either null or of the desired target type
324             return type.cast(value);
325         }
326 
327         final ConversionException cex =  new ConversionException("No value specified for '" +
328                 toString(type) + "'");
329         if (log().isDebugEnabled()) {
330             log().debug("    Throwing ConversionException: " + cex.getMessage());
331             log().debug("    " + DEFAULT_CONFIG_MSG);
332         }
333         throw cex;
334 
335     }
336 
337     /**
338      * Set the default value, converting as required.
339      * <p>
340      * If the default value is different from the type the
341      * <code>Converter</code> handles, it will be converted
342      * to the handled type.
343      *
344      * @param defaultValue The default value to be returned
345      * if the value to be converted is missing or an error
346      * occurs converting the value.
347      * @throws ConversionException if an error occurs converting
348      * the default value
349      */
350     protected void setDefaultValue(final Object defaultValue) {
351         useDefault = false;
352         if (log().isDebugEnabled()) {
353             log().debug("Setting default value: " + defaultValue);
354         }
355         if (defaultValue == null) {
356            this.defaultValue  = null;
357         } else {
358            this.defaultValue  = convert(getDefaultType(), defaultValue);
359         }
360         useDefault = true;
361     }
362 
363     /**
364      * Return the default type this <code>Converter</code> handles.
365      *
366      * @return The default type this <code>Converter</code> handles.
367      */
368     protected abstract Class<?> getDefaultType();
369 
370     /**
371      * Return the default value for conversions to the specified
372      * type.
373      * @param type Data type to which this value should be converted.
374      * @return The default value for the specified type.
375      */
376     protected Object getDefault(final Class<?> type) {
377         if (type.equals(String.class)) {
378             return null;
379         } else  {
380             return defaultValue;
381         }
382     }
383 
384     /**
385      * Provide a String representation of this converter.
386      *
387      * @return A String representation of this converter
388      */
389     @Override
390     public String toString() {
391         return toString(getClass()) + "[UseDefault=" + useDefault + "]";
392     }
393 
394     // ----------------------------------------------------------- Package Methods
395 
396     /**
397      * Accessor method for Log instance.
398      * <p>
399      * The Log instance variable is transient and
400      * accessing it through this method ensures it
401      * is re-initialized when this instance is
402      * de-serialized.
403      *
404      * @return The Log instance.
405      */
406     Log log() {
407         if (log == null) {
408             log = LogFactory.getLog(getClass());
409         }
410         return log;
411     }
412 
413     /**
414      * Provide a String representation of a <code>java.lang.Class</code>.
415      * @param type The <code>java.lang.Class</code>.
416      * @return The String representation.
417      */
418     String toString(final Class<?> type) {
419         String typeName = null;
420         if (type == null) {
421             typeName = "null";
422         } else if (type.isArray()) {
423             Class<?> elementType = type.getComponentType();
424             int count = 1;
425             while (elementType.isArray()) {
426                 elementType = elementType .getComponentType();
427                 count++;
428             }
429             typeName = elementType.getName();
430             for (int i = 0; i < count; i++) {
431                 typeName += "[]";
432             }
433         } else {
434             typeName = type.getName();
435         }
436         if (typeName.startsWith("java.lang.") ||
437             typeName.startsWith("java.util.") ||
438             typeName.startsWith("java.math.")) {
439             typeName = typeName.substring("java.lang.".length());
440         } else if (typeName.startsWith(PACKAGE)) {
441             typeName = typeName.substring(PACKAGE.length());
442         }
443         return typeName;
444     }
445 
446     /**
447      * Performs a conversion to the default type. This method is called if we do
448      * not have a target class. In this case, the T parameter is not set.
449      * Therefore, we can cast to it (which is required to fulfill the contract
450      * of the method signature).
451      *
452      * @param <T> the type of the result object
453      * @param targetClass the target class of the conversion
454      * @param value the value to be converted
455      * @return the converted value
456      */
457     private <T> T convertToDefaultType(final Class<T> targetClass, final Object value) {
458         @SuppressWarnings("unchecked")
459         final
460         T result = (T) convert(getDefaultType(), value);
461         return result;
462     }
463 
464     /**
465      * Generates a standard conversion exception with a message indicating that
466      * the passed in value cannot be converted to the desired target type.
467      *
468      * @param type the target type
469      * @param value the value to be converted
470      * @return a {@code ConversionException} with a standard message
471      * @since 1.9
472      */
473     protected ConversionException conversionException(final Class<?> type, final Object value) {
474         return new ConversionException("Can't convert value '" + value
475                 + "' to type " + type);
476     }
477 }