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This is essentially a JAXB/XJC plugin that adds automatic validation to the generated Java classes. You define business objects in an XSD file, pass it through XJC and the plugin will add the validation code. It uses information in the XSD to pick up validation, and you can specify extensions to that in the XSD. The resulting Java classes check for validity when the setter is called and they reject attempts to set invalid values (this is a difference from other validation frameworks). The Java classes also expose a metadata interface to make it easy for UIs to generate, say, lists of options for a select box. The validation framework handles single field validation but you can inject a rule engine (or several) to handle cross field validation. But to any Java code the objects still look like ordinary Java beans. The surrounding application is unaware that they are anything different until they throw an exception. This makes it easy to use with frameworks that expect Java beans, and most of them do.

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/*******************************************************************************
 * Copyright (c)2014 Prometheus Consulting
 *
 * Licensed 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.springframework.context.support;

import java.text.MessageFormat;
import java.util.HashMap;
import java.util.Locale;
import java.util.Map;

import org.springframework.beans.factory.BeanInitializationException;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.dao.IncorrectResultSizeDataAccessException;
import org.springframework.jdbc.core.JdbcTemplate;

/**
 * Reading of messages using data in databases. It must be provided with a
 * datasource and sql code which must be passed 2 jdbc parameters which will be
 * respectively filled with the code and the string representation of the
 * locale. 

 * TODO: improve the caching part, either switching to a real cache (but isn't
 * it too heavy to ask in the configuration to specify something like EHCache
 * etc..) or caching per entry (it is not much useful to cache depending on the
 * date of the last query whatever it is) 

 * TODO: check that synchronization isn't needed on the cache part
 * 
 * @author Olivier Jolly
 */
public class JdbcMessageSource extends AbstractMessageSource implements
        InitializingBean {

    private static class CacheKey {
        @SuppressWarnings("unused")
		public String code;

        @SuppressWarnings("unused")
		public Locale locale;

        public CacheKey(String code, Locale locale) {
            this.code = code;
            this.locale = locale;
        }
    }

    private JdbcTemplate jdbcTemplate;

    private String sqlStatement;

    private Map cachedMessages = new HashMap<>();

    private long cachedMilliSecond = 0;

    private long lastQuery = 0;

    /*
     * (non-Javadoc)
     * 
     * @see org.springframework.context.support.AbstractMessageSource#resolveCode(java.lang.String,
     *      java.util.Locale)
     */
    protected MessageFormat resolveCode(String code, Locale locale) {
        MessageFormat result = null;

        if (cachedMilliSecond == 0
                || ((result = cachedMessages.get(new CacheKey(
                        code, locale))) == null)
                || lastQuery + cachedMilliSecond < System.currentTimeMillis()) {
            result = resolveCodeInternal(code, locale);
            cachedMessages.put(new CacheKey(code, locale), result);
        }

        if (result != null)
            return result;

        return resolveCodeInternal(code, locale);
    }

    /**
     * Check in base the message associated with the given code and locale
     * 
     * @param code
     *            the code of the message to solve
     * @param locale
     *            the locale to check against
     * @return a MessageFormat if one were found, either for the given locale or
     *         for the default on, or null if nothing could be found
     */
    protected MessageFormat resolveCodeInternal(String code, Locale locale) {
        String result;

        lastQuery = System.currentTimeMillis();

        try {
            result = (String) jdbcTemplate.queryForObject(sqlStatement,
                    new Object[] { code, locale.toString() }, String.class);
        } catch (IncorrectResultSizeDataAccessException e) {
            if (locale != null) {
                // Retry without a locale if we checked with one before
                try {
                    result = (String) jdbcTemplate.queryForObject(sqlStatement,
                            new Object[] { code, null }, String.class);
                } catch (IncorrectResultSizeDataAccessException ex) {
                    return null;
                }
            } else {
                return null;
            }
        }

        return new MessageFormat(result, locale);
    }

    /**
     * @param jdbcTemplate
     *            The jdbcTemplate to set.
     */
    public void setJdbcTemplate(JdbcTemplate jdbcTemplate) {
        this.jdbcTemplate = jdbcTemplate;
    }

    /**
     * @param sqlStatement
     *            The sqlStatement to set.
     */
    public void setSqlStatement(String sqlStatement) {
        this.sqlStatement = sqlStatement;
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet()
     */
    public void afterPropertiesSet() throws Exception {
        if (sqlStatement == null) {
            throw new BeanInitializationException(
                    "Sql statement should be filled");
        }

        if (jdbcTemplate == null) {
            throw new BeanInitializationException(
                    "Jdbc template should be filled");
        }

    }

    /**
     * Empty the cache so that all future message resolving request ends in
     * database queries
     */
    public void clearCache() {
        cachedMessages.clear();
    }

}