关于commons dbutils组件的一个小缺陷分析

非常喜欢这种轻量级的JDBC封装，比起Hibernate和iBatis，可以非常自由和灵活地运用和自行二次封装，由于dbutils的 BeanHandler转换方式采取了反射技术，在性能上肯定有所损失，所以项目中基本上都使用MapHandler方式来转换数据，当然就是自己写的代 码多一点，也无所谓。一般的查询、子查询、联合查询、包括视图查询等等都很正常，但是发现一个比较小的问题，就是在使用聚合函数的场所，例 如：select user_type, count(*) as count from `user` group by user_type这种类型查询的时候，MapHandler方式不起作用，as列都变成key为空串的K-V对，导致有许多地方使用 map.get("")代码的情况出现，这种写法当然是不太好的，容易出问题。
        鉴于前面没有时间了解，就都粗略使用了上面那种粗暴的map.get("")来处理，最好的情况是让dbutils组件能自动识别到as类型的列名。于是有空了就专门看了看它的源代码，发现最主要的一段代码如下：

 1 public  Map < String, Object >  toMap(ResultSet rs)  throws  SQLException  {
 2         Map < String, Object >  result  =   new  CaseInsensitiveHashMap();
 3         ResultSetMetaData rsmd  =  rs.getMetaData();
 4          int  cols  =  rsmd.getColumnCount();
 5
 6          for  ( int  i  =   1 ; i  <=  cols; i ++ )  {
 7             result.put(rsmd.getColumnName(i), rs.getObject(i));
 8         }
 9
10          return  result;
11     }

        CaseInsensitiveHashMap是dbutils自定义的一个Map，忽略键大小写的K-V字典，但是key使用的是 ResultSetMetaData.getColumnName()，我想问题大概出在这里，于是认真翻了翻java的api文档（开发做久了容易遗忘 基础），果然，原来getColumnName() 是：获取指定列的名称； 而as关键字之后，使列名称变成用于显示的意义，这个时候应该使用getColumnLabel ()：获取用于打印输出和显示的指定列的建议标题。建议标题通常由 SQL AS 子句来指定。如果未指定 SQL AS ，则从 getColumnLabel 返回的值将和 getColumnName 方法返回的值相同 。自己手动试验了一下，果然如所料，问题就出在这里。
        所以呢，如果想要dbutils在自动转换Map及MapList时能识别聚合函数的列名，那么最好的做法就是重载这种方式，懒一点的，你就干脆修改上面 那段代码，让它判断是否使用了as关键字。个人暂时搞不清楚官方为什么没有考虑这一步，有时间再思考一下！

刚进场的时候戏就落幕

评论

2 楼 lgh1992314 2017-08-17  

pkptzx 写道

额,误人子弟...
CaseInsensitiveMap是apache-commons-collections.jar里的.
难道你不知道dbutils依赖collections????

/*
 * 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.dbutils;

import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Locale;
import java.util.Map;

/**
 * Basic implementation of the <code>RowProcessor</code> interface.
 *
 * <p>
 * This class is thread-safe.
 * </p>
 *
 * @see RowProcessor
 */
public class BasicRowProcessor implements RowProcessor {

    /**
     * The default BeanProcessor instance to use if not supplied in the
     * constructor.
     */
    private static final BeanProcessor defaultConvert = new BeanProcessor();

    /**
     * The Singleton instance of this class.
     */
    private static final BasicRowProcessor instance = new BasicRowProcessor();

    /**
     * Returns the Singleton instance of this class.
     *
     * @return The single instance of this class.
     * @deprecated Create instances with the constructors instead.  This will
     * be removed after DbUtils 1.1.
     */
    @Deprecated
    public static BasicRowProcessor instance() {
        return instance;
    }

    /**
     * Use this to process beans.
     */
    private final BeanProcessor convert;

    /**
     * BasicRowProcessor constructor.  Bean processing defaults to a
     * BeanProcessor instance.
     */
    public BasicRowProcessor() {
        this(defaultConvert);
    }

    /**
     * BasicRowProcessor constructor.
     * @param convert The BeanProcessor to use when converting columns to
     * bean properties.
     * @since DbUtils 1.1
     */
    public BasicRowProcessor(BeanProcessor convert) {
        super();
        this.convert = convert;
    }

    /**
     * Convert a <code>ResultSet</code> row into an <code>Object[]</code>.
     * This implementation copies column values into the array in the same
     * order they're returned from the <code>ResultSet</code>.  Array elements
     * will be set to <code>null</code> if the column was SQL NULL.
     *
     * @see org.apache.commons.dbutils.RowProcessor#toArray(java.sql.ResultSet)
     * @param rs ResultSet that supplies the array data
     * @throws SQLException if a database access error occurs
     * @return the newly created array
     */
    @Override
    public Object[] toArray(ResultSet rs) throws SQLException {
        ResultSetMetaData meta = rs.getMetaData();
        int cols = meta.getColumnCount();
        Object[] result = new Object[cols];

        for (int i = 0; i < cols; i++) {
            result[i] = rs.getObject(i + 1);
        }

        return result;
    }

    /**
     * Convert a <code>ResultSet</code> row into a JavaBean.  This
     * implementation delegates to a BeanProcessor instance.
     * @see org.apache.commons.dbutils.RowProcessor#toBean(java.sql.ResultSet, java.lang.Class)
     * @see org.apache.commons.dbutils.BeanProcessor#toBean(java.sql.ResultSet, java.lang.Class)
     * @param <T> The type of bean to create
     * @param rs ResultSet that supplies the bean data
     * @param type Class from which to create the bean instance
     * @throws SQLException if a database access error occurs
     * @return the newly created bean
     */
    @Override
    public <T> T toBean(ResultSet rs, Class<T> type) throws SQLException {
        return this.convert.toBean(rs, type);
    }

    /**
     * Convert a <code>ResultSet</code> into a <code>List</code> of JavaBeans.
     * This implementation delegates to a BeanProcessor instance.
     * @see org.apache.commons.dbutils.RowProcessor#toBeanList(java.sql.ResultSet, java.lang.Class)
     * @see org.apache.commons.dbutils.BeanProcessor#toBeanList(java.sql.ResultSet, java.lang.Class)
     * @param <T> The type of bean to create
     * @param rs ResultSet that supplies the bean data
     * @param type Class from which to create the bean instance
     * @throws SQLException if a database access error occurs
     * @return A <code>List</code> of beans with the given type in the order
     * they were returned by the <code>ResultSet</code>.
     */
    @Override
    public <T> List<T> toBeanList(ResultSet rs, Class<T> type) throws SQLException {
        return this.convert.toBeanList(rs, type);
    }

    /**
     * Convert a <code>ResultSet</code> row into a <code>Map</code>.
     *
     * <p>
     * This implementation returns a <code>Map</code> with case insensitive column names as keys. Calls to
     * <code>map.get("COL")</code> and <code>map.get("col")</code> return the same value. Furthermore this implementation
     * will return an ordered map, that preserves the ordering of the columns in the ResultSet, so that iterating over
     * the entry set of the returned map will return the first column of the ResultSet, then the second and so forth.
     * </p>
     *
     * @param rs ResultSet that supplies the map data
     * @return the newly created Map
     * @throws SQLException if a database access error occurs
     * @see org.apache.commons.dbutils.RowProcessor#toMap(java.sql.ResultSet)
     */
    @Override
    public Map<String, Object> toMap(ResultSet rs) throws SQLException {
        Map<String, Object> result = new CaseInsensitiveHashMap();
        ResultSetMetaData rsmd = rs.getMetaData();
        int cols = rsmd.getColumnCount();

        for (int i = 1; i <= cols; i++) {
            String columnName = rsmd.getColumnLabel(i);
            if (null == columnName || 0 == columnName.length()) {
              columnName = rsmd.getColumnName(i);
            }
            result.put(columnName, rs.getObject(i));
        }

        return result;
    }

    /**
     * A Map that converts all keys to lowercase Strings for case insensitive
     * lookups.  This is needed for the toMap() implementation because
     * databases don't consistently handle the casing of column names.
     *
     * <p>The keys are stored as they are given [BUG #DBUTILS-34], so we maintain
     * an internal mapping from lowercase keys to the real keys in order to
     * achieve the case insensitive lookup.
     *
     * <p>Note: This implementation does not allow <tt>null</tt>
     * for key, whereas {@link LinkedHashMap} does, because of the code:
     * <pre>
     * key.toString().toLowerCase()
     * </pre>
     */
    private static class CaseInsensitiveHashMap extends LinkedHashMap<String, Object> {
        /**
         * The internal mapping from lowercase keys to the real keys.
         *
         * <p>
         * Any query operation using the key
         * ({@link #get(Object)}, {@link #containsKey(Object)})
         * is done in three steps:
         * <ul>
         * <li>convert the parameter key to lower case</li>
         * <li>get the actual key that corresponds to the lower case key</li>
         * <li>query the map with the actual key</li>
         * </ul>
         * </p>
         */
        private final Map<String, String> lowerCaseMap = new HashMap<String, String>();

        /**
         * Required for serialization support.
         *
         * @see java.io.Serializable
         */
        private static final long serialVersionUID = -2848100435296897392L;

        /** {@inheritDoc} */
        @Override
        public boolean containsKey(Object key) {
            Object realKey = lowerCaseMap.get(key.toString().toLowerCase(Locale.ENGLISH));
            return super.containsKey(realKey);
            // Possible optimisation here:
            // Since the lowerCaseMap contains a mapping for all the keys,
            // we could just do this:
            // return lowerCaseMap.containsKey(key.toString().toLowerCase());
        }

        /** {@inheritDoc} */
        @Override
        public Object get(Object key) {
            Object realKey = lowerCaseMap.get(key.toString().toLowerCase(Locale.ENGLISH));
            return super.get(realKey);
        }

        /** {@inheritDoc} */
        @Override
        public Object put(String key, Object value) {
            /*
             * In order to keep the map and lowerCaseMap synchronized,
             * we have to remove the old mapping before putting the
             * new one. Indeed, oldKey and key are not necessaliry equals.
             * (That's why we call super.remove(oldKey) and not just
             * super.put(key, value))
             */
            Object oldKey = lowerCaseMap.put(key.toLowerCase(Locale.ENGLISH), key);
            Object oldValue = super.remove(oldKey);
            super.put(key, value);
            return oldValue;
        }

        /** {@inheritDoc} */
        @Override
        public void putAll(Map<? extends String, ?> m) {
            for (Map.Entry<? extends String, ?> entry : m.entrySet()) {
                String key = entry.getKey();
                Object value = entry.getValue();
                this.put(key, value);
            }
        }

        /** {@inheritDoc} */
        @Override
        public Object remove(Object key) {
            Object realKey = lowerCaseMap.remove(key.toString().toLowerCase(Locale.ENGLISH));
            return super.remove(realKey);
        }
    }

}

1 楼 pkptzx 2012-12-24  

额,误人子弟...
CaseInsensitiveMap是apache-commons-collections.jar里的.
难道你不知道dbutils依赖collections????