001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.beanutils.converters;
018
019import java.lang.reflect.Array;
020import java.util.Collection;
021
022import org.apache.commons.beanutils.BeanUtils;
023import org.apache.commons.beanutils.ConversionException;
024import org.apache.commons.beanutils.ConvertUtils;
025import org.apache.commons.beanutils.Converter;
026import org.apache.commons.logging.Log;
027import org.apache.commons.logging.LogFactory;
028
029/**
030 * Base {@link Converter} implementation that provides the structure
031 * for handling conversion <b>to</b> and <b>from</b> a specified type.
032 * <p>
033 * This implementation provides the basic structure for
034 * converting to/from a specified type optionally using a default
035 * value or throwing a {@link ConversionException} if a
036 * conversion error occurs.
037 * <p>
038 * Implementations should provide conversion to the specified
039 * type and from the specified type to a <code>String</code> value
040 * by implementing the following methods:
041 * <ul>
042 *     <li><code>convertToString(value)</code> - convert to a String
043 *        (default implementation uses the objects <code>toString()</code>
044 *        method).</li>
045 *     <li><code>convertToType(Class, value)</code> - convert
046 *         to the specified type</li>
047 * </ul>
048 * <p>
049 * The default value has to be compliant to the default type of this
050 * converter - which is enforced by the generic type parameter. If a
051 * conversion is not possible and a default value is set, the converter
052 * tries to transform the default value to the requested target type.
053 * If this fails, a {@code ConversionException} if thrown.
054 *
055 * @version $Id$
056 * @since 1.8.0
057 */
058public abstract class AbstractConverter implements Converter {
059
060    /** Debug logging message to indicate default value configuration */
061    private static final String DEFAULT_CONFIG_MSG =
062        "(N.B. Converters can be configured to use default values to avoid throwing exceptions)";
063
064    /** Current package name */
065    //    getPackage() below returns null on some platforms/jvm versions during the unit tests.
066//    private static final String PACKAGE = AbstractConverter.class.getPackage().getName() + ".";
067    private static final String PACKAGE = "org.apache.commons.beanutils.converters.";
068
069    /**
070     * Logging for this instance.
071     */
072    private transient Log log;
073
074    /**
075     * Should we return the default value on conversion errors?
076     */
077    private boolean useDefault = false;
078
079    /**
080     * The default value specified to our Constructor, if any.
081     */
082    private Object defaultValue = null;
083
084    // ----------------------------------------------------------- Constructors
085
086    /**
087     * Construct a <i>Converter</i> that throws a
088     * <code>ConversionException</code> if an error occurs.
089     */
090    public AbstractConverter() {
091    }
092
093    /**
094     * Construct a <i>Converter</i> that returns a default
095     * value if an error occurs.
096     *
097     * @param defaultValue The default value to be returned
098     * if the value to be converted is missing or an error
099     * 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 --&gt; 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}