abstractconverter.java

APACHE 公司出的java bean 工具包

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.beanutils.converters;

import java.lang.reflect.Array;
import java.util.Collection;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.commons.beanutils.BeanUtils;
import org.apache.commons.beanutils.ConversionException;
import org.apache.commons.beanutils.Converter;

/**
 * Base {@link Converter} implementation that provides the structure
 * for handling conversion <b>to</b> and <b>from</b> a specified type.
 * <p>
 * This implementation provides the basic structure for
 * converting to/from a specified type optionally using a default
 * value or throwing a {@link ConversionException} if a
 * conversion error occurs.
 * <p>
 * Implementations should provide conversion to the specified
 * type and from the specified type to a <code>String</code> value
 * by implementing the following methods:
 * <ul>
 *     <li><code>convertToString(value)</code> - convert to a String
 *        (default implementation uses the objects <code>toString()</code>
 *        method).</li>
 *     <li><code>convertToType(Class, value)</code> - convert
 *         to the specified type</li>
 * </ul>
 *
 * @version $Revision: 555824 $ $Date: 2007-07-13 01:27:15 +0100 (Fri, 13 Jul 2007) $
 * @since 1.8.0
 */
public abstract class AbstractConverter implements Converter {

    /** Debug logging message to indicate default value configuration */
    private static final String DEFAULT_CONFIG_MSG =
        "(N.B. Converters can be configured to use default values to avoid throwing exceptions)";

    /** Current package name */
    //    getPackage() below returns null on some platforms/jvm versions during the unit tests.
//    private static final String PACKAGE = AbstractConverter.class.getPackage().getName() + ".";
    private static final String PACKAGE = "org.apache.commons.beanutils.converters.";

    /**
     * Logging for this instance.
     */
    private transient Log log;

    /**
     * The default type this <code>Converter</code> handles.
     */
    private Class defaultType = null;

    /**
     * Should we return the default value on conversion errors?
     */
    private boolean useDefault = false;

    /**
     * The default value specified to our Constructor, if any.
     */
    private Object defaultValue = null;

    // ----------------------------------------------------------- Constructors

    /**
     * Construct a <i>Converter</i> that throws a
     * <code>ConversionException</code> if an error occurs.
     *
     * @param defaultType The default type this <code>Converter</code>
     * handles
     */
    public AbstractConverter(Class defaultType) {
        this.defaultType = defaultType;
        if (defaultType == null) {
            throw new IllegalArgumentException("Default type is missing.");
        }
    }

    /**
     * Construct a <i>Converter</i> that returns a default
     * value if an error occurs.
     *
     * @param defaultType The default type this <code>Converter</code>
     * handles
     * @param defaultValue The default value to be returned
     * if the value to be converted is missing or an error
     * occurs converting the value.
     */
    public AbstractConverter(Class defaultType, Object defaultValue) {
        this(defaultType);
        setDefaultValue(defaultValue);
    }

    // --------------------------------------------------------- Public Methods

    /**
     * Indicates whether a default value will be returned or exception
     * thrown in the event of a conversion error.
     *
     * @return <code>true</code> if a default value will be returned for
     * conversion errors or <code>false</code> if a {@link ConversionException}
     * will be thrown.
     */
    public boolean isUseDefault() {
        return useDefault;
    }

    /**
     * Convert the input object into an output object of the
     * specified type.
     *
     * @param type Data type to which this value should be converted
     * @param value The input value to be converted
     * @return The converted value.
     * @throws ConversionException if conversion cannot be performed
     * successfully and no default is specified.
     */
    public Object convert(Class type, Object value) {

        Class sourceType  = value == null ? null : value.getClass();
        Class targetType  = primitive(type  == null ? getDefaultType() : type);

        if (log().isDebugEnabled()) {
            log().debug("Converting"
                    + (value == null ? "" : " '" + toString(sourceType) + "'")
                    + " value '" + value + "' to type '" + toString(targetType) + "'");
        }

        value = convertArray(value);

        // Missing Value
        if (value == null) {
            return handleMissing(targetType);
        }

        sourceType = value.getClass();

        try {
            // Convert --> String
            if (targetType.equals(String.class)) {
                return convertToString(value);

            // No conversion necessary
            } else if (targetType.equals(sourceType)) {
                if (log().isDebugEnabled()) {
                    log().debug("    No conversion required, value is already a "
                                    + toString(targetType));
                }
                return value;

            // Convert --> Type
            } else {
                Object result = convertToType(targetType, value);
                if (log().isDebugEnabled()) {
                    log().debug("    Converted to " + toString(targetType) +
                                   " value '" + result + "'");
                }
                return result;
            }
        } catch (Throwable t) {
            return handleError(targetType, value, t);
        }

    }

    /**
     * Convert the input object into a String.
     * <p>
     * <b>N.B.</b>This implementation simply uses the value's
     * <code>toString()</code> method and should be overriden if a
     * more sophisticated mechanism for <i>conversion to a String</i>
     * is required.
     *
     * @param value The input value to be converted.
     * @return the converted String value.
     * @throws Throwable if an error occurs converting to a String
     */
    protected String convertToString(Object value) throws Throwable {
        return value.toString();
    }

    /**
     * Convert the input object into an output object of the
     * specified type.
     * <p>
     * Typical implementations will provide a minimum of
     * <code>String --> type</code> conversion.
     *
     * @param type Data type to which this value should be converted.
     * @param value The input value to be converted.
     * @return The converted value.
     * @throws Throwable if an error occurs converting to the specified type
     */
    protected abstract Object convertToType(Class type, Object value) throws Throwable;

    /**
     * Return the first element from an Array (or Collection)
     * or the value unchanged if not an Array (or Collection).
     *
     * N.B. This needs to be overriden for array/Collection converters.
     *
     * @param value The value to convert
     * @return The first element in an Array (or Collection)
     * or the value unchanged if not an Array (or Collection)
     */
    protected Object convertArray(Object value) {
        if (value == null) {
            return null;
        }
        if (value.getClass().isArray()) {
            if (Array.getLength(value) > 0) {
                return Array.get(value, 0);
            } else {
                return null;
            }
        }
        if (value instanceof Collection) {
            Collection collection = (Collection)value;
            if (collection.size() > 0) {
                return collection.iterator().next();