03-dbutils源碼之BeanProcessor、RowProcessor、BasicRo...

    在dbutils中，BeanProcessor是一個很重要的類：將列和bean對象的屬性進行匹配，將列的值賦予bean的對象。這個是使用了反射來進行的。

    來看下類的outline：

    

    從上圖能夠看出，有「建立對象」、「調用setter」、「得到類的屬性描述符」等方法，還有一個最重要的方法mapColumnsToProperties(ResultSetMetaData,PropertyDescriptor)，這個方法是將結果集和類的屬性進行一個匹配。toBean是將一行記錄轉換成一個bean對象。toBeanList就是將多行記錄變成bean對象的List集合。

    下面看一下它的具體實現。

/*
 * 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.beans.BeanInfo;
import java.beans.IntrospectionException;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.SQLXML;
import java.sql.Timestamp;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * <p>
 * <code>BeanProcessor</code> matches column names to bean property names
 * and converts <code>ResultSet</code> columns into objects for those bean
 * properties.  Subclasses should override the methods in the processing chain
 * to customize behavior.
 * </p>
 * BeanProcessor將列名和bean的屬性名進行匹配；將ResultSet的列轉換成對象的屬性。想要定製行爲，能夠覆蓋這些方法。
 * 
 * <p>
 * This class is thread-safe.
 * </p>
 *
 * @see BasicRowProcessor
 *
 * @since DbUtils 1.1
 */
public class BeanProcessor {

    /**
     * Special array value used by <code>mapColumnsToProperties</code> that
     * indicates there is no bean property that matches a column from a
     * <code>ResultSet</code>.
     * <p>
     * 特殊的數組值，給mapColumnsToProperties使用。代表了在ResultSet中某一列
     * 在javabean中沒有對應的屬性
     * </p>
     */
    protected static final int PROPERTY_NOT_FOUND = -1;

    /**
     * Set a bean's primitive properties to these defaults when SQL NULL
     * is returned.  These are the same as the defaults that ResultSet get*
     * methods return in the event of a NULL column.
     * <p>
     * 以基本數據類型爲key，以0和FALSE爲默認值
     * </p>
     */
    private static final Map<Class<?>, Object> primitiveDefaults = new HashMap<Class<?>, Object>();

    /**
     * ResultSet column to bean property name overrides.
     */
    private final Map<String, String> columnToPropertyOverrides;
    
    static {
        primitiveDefaults.put(Integer.TYPE, Integer.valueOf(0));
        primitiveDefaults.put(Short.TYPE, Short.valueOf((short) 0));
        primitiveDefaults.put(Byte.TYPE, Byte.valueOf((byte) 0));
        primitiveDefaults.put(Float.TYPE, Float.valueOf(0f));
        primitiveDefaults.put(Double.TYPE, Double.valueOf(0d));
        primitiveDefaults.put(Long.TYPE, Long.valueOf(0L));
        primitiveDefaults.put(Boolean.TYPE, Boolean.FALSE);
        primitiveDefaults.put(Character.TYPE, Character.valueOf((char) 0));
    }

    /**
     * Constructor for BeanProcessor.
     */
    public BeanProcessor() {
        this(new HashMap<String, String>());
    }

    /**
     * Constructor for BeanProcessor configured with column to property name overrides.
     *
     * @param columnToPropertyOverrides ResultSet column to bean property name overrides
     * @since 1.5
     */
    public BeanProcessor(Map<String, String> columnToPropertyOverrides) {
        super();
        if (columnToPropertyOverrides == null) {
            throw new IllegalArgumentException("columnToPropertyOverrides map cannot be null");
        }
        this.columnToPropertyOverrides = columnToPropertyOverrides;
    }

    /**
     * Convert a <code>ResultSet</code> row into a JavaBean.  This
     * implementation uses reflection and <code>BeanInfo</code> classes to
     * match column names to bean property names.  Properties are matched to
     * columns based on several factors:
     * <br/>
     * <ol>
     *     <li>
     *     The class has a writable property with the same name as a column.
     *     The name comparison is case insensitive.
     *     </li>
     *
     *     <li>
     *     The column type can be converted to the property's set method
     *     parameter type with a ResultSet.get* method.  If the conversion fails
     *     (ie. the property was an int and the column was a Timestamp) an
     *     SQLException is thrown.
     *     </li>
     * </ol>
     *
     * <p>
     * Primitive bean properties are set to their defaults when SQL NULL is
     * returned from the <code>ResultSet</code>.  Numeric fields are set to 0
     * and booleans are set to false.  Object bean properties are set to
     * <code>null</code> when SQL NULL is returned.  This is the same behavior
     * as the <code>ResultSet</code> get* methods.
     * </p>
     * @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
     */
    public <T> T toBean(ResultSet rs, Class<T> type) throws SQLException {
    	//得到類的屬性描述符
        PropertyDescriptor[] props = this.propertyDescriptors(type);
        //得到ResultSet的元數據
        ResultSetMetaData rsmd = rs.getMetaData();
        
        int[] columnToProperty = this.mapColumnsToProperties(rsmd, props);

        return this.createBean(rs, type, props, columnToProperty);
    }

    /**
     * Convert a <code>ResultSet</code> into a <code>List</code> of JavaBeans.
     * This implementation uses reflection and <code>BeanInfo</code> classes to
     * match column names to bean property names. Properties are matched to
     * columns based on several factors:
     * <br/>
     * <ol>
     *     <li>
     *     The class has a writable property with the same name as a column.
     *     The name comparison is case insensitive.
     *     </li>
     *
     *     <li>
     *     The column type can be converted to the property's set method
     *     parameter type with a ResultSet.get* method.  If the conversion fails
     *     (ie. the property was an int and the column was a Timestamp) an
     *     SQLException is thrown.
     *     </li>
     * </ol>
     *
     * <p>
     * Primitive bean properties are set to their defaults when SQL NULL is
     * returned from the <code>ResultSet</code>.  Numeric fields are set to 0
     * and booleans are set to false.  Object bean properties are set to
     * <code>null</code> when SQL NULL is returned.  This is the same behavior
     * as the <code>ResultSet</code> get* methods.
     * </p>
     * @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 List of beans
     */
    public <T> List<T> toBeanList(ResultSet rs, Class<T> type) throws SQLException {
        List<T> results = new ArrayList<T>();
        
        //若是ResultSet中沒有記錄，那麼就返回一個size爲0的集合List
        if (!rs.next()) {
            return results;
        }

        PropertyDescriptor[] props = this.propertyDescriptors(type);
        ResultSetMetaData rsmd = rs.getMetaData();
        int[] columnToProperty = this.mapColumnsToProperties(rsmd, props);
        
        // TODO 這裏爲何要使用do while，使用while也能夠實現啊
        do {
        	//調用一次createBean，就產生一個bean對象
            results.add(this.createBean(rs, type, props, columnToProperty));
        } while (rs.next());
        
        //返回一個有內容的集合List
        return results;
    }

    /**
     * Creates a new object and initializes its fields from the ResultSet.
     * 建立一個object，實例化object的字段，值來自ResultSet的一行記錄
     * @param <T> The type of bean to create
     * @param rs The result set.
     * @param type The bean type (the return type of the object).
     * @param props The property descriptors.
     * @param columnToProperty The column indices in the result set.
     * @return An initialized object.
     * @throws SQLException if a database error occurs.
     */
    private <T> T createBean(ResultSet rs, Class<T> type,
            PropertyDescriptor[] props, int[] columnToProperty)
            throws SQLException {
    	//建立出一個對象
        T bean = this.newInstance(type);
        
        /*
         * 從1開始計算，由於0存儲的沒意義
         * i爲列數
         */
        for (int i = 1; i < columnToProperty.length; i++) {
        	//若是值爲-1，那麼就說明此列在屬性描述符數組中沒有對應的屬性
            if (columnToProperty[i] == PROPERTY_NOT_FOUND) {
                continue;
            }
            
            //根據列數拿到對應的屬性描述符
            PropertyDescriptor prop = props[columnToProperty[i]];
            //拿到屬性的類型
            Class<?> propType = prop.getPropertyType();
            //得到列值
            Object value = this.processColumn(rs, i, propType);
            //屬性的類型不爲null，值爲null，且爲基本類型，就給予一個默認的值
            if (propType != null && value == null && propType.isPrimitive()) {
                value = primitiveDefaults.get(propType);
            }
            
            //而後調用此屬性的setter方法，設置值
            this.callSetter(bean, prop, value);
        }
        //當循環結束後，就將bean中能賦值的屬性都賦值了
        return bean;
    }

    /**
     * Calls the setter method on the target object for the given property.
     * If no setter method exists for the property, this method does nothing.
     * <p>
     * 調用目標對象指定屬性的setter方法。若是沒有setter方法，那麼此方法就終止了
     * </p>
     * @param target The object to set the property on.
     * @param prop The property to set.
     * @param value The value to pass into the setter.
     * @throws SQLException if an error occurs setting the property.
     */
    private void callSetter(Object target, PropertyDescriptor prop, Object value)
            throws SQLException {
    	
    	//得到指定屬性的setter方法
        Method setter = prop.getWriteMethod();
        
        //setter爲null，那麼就終止了，什麼也不幹
        if (setter == null) {
            return;
        }
        //得到setter方法的參數類型
        Class<?>[] params = setter.getParameterTypes();
        try {
            // convert types for some popular ones
            if (value instanceof java.util.Date) {
            	//拿出setter方法的參數類型名字
                final String targetType = params[0].getName();
                if ("java.sql.Date".equals(targetType)) {
                    value = new java.sql.Date(((java.util.Date) value).getTime());
                } else
                if ("java.sql.Time".equals(targetType)) {
                    value = new java.sql.Time(((java.util.Date) value).getTime());
                } else
                if ("java.sql.Timestamp".equals(targetType)) {
                    value = new java.sql.Timestamp(((java.util.Date) value).getTime());
                }
            }

            // Don't call setter if the value object isn't the right type
            if (this.isCompatibleType(value, params[0])) {
                setter.invoke(target, new Object[]{value});
            } else {
              throw new SQLException(
                  "Cannot set " + prop.getName() + ": incompatible types, cannot convert "
                  + value.getClass().getName() + " to " + params[0].getName());
                  // value cannot be null here because isCompatibleType allows null
            }

        } catch (IllegalArgumentException e) {
            throw new SQLException(
                "Cannot set " + prop.getName() + ": " + e.getMessage());

        } catch (IllegalAccessException e) {
            throw new SQLException(
                "Cannot set " + prop.getName() + ": " + e.getMessage());

        } catch (InvocationTargetException e) {
            throw new SQLException(
                "Cannot set " + prop.getName() + ": " + e.getMessage());
        }
    }

    /**
     * ResultSet.getObject() returns an Integer object for an INT column.  The
     * setter method for the property might take an Integer or a primitive int.
     * This method returns true if the value can be successfully passed into
     * the setter method.  Remember, Method.invoke() handles the unwrapping
     * of Integer into an int.
     *
     * @param value The value to be passed into the setter method.
     * @param type The setter's parameter type (non-null)
     * @return boolean True if the value is compatible (null => true)
     */
    private boolean isCompatibleType(Object value, Class<?> type) {
        // Do object check first, then primitives
        if (value == null || type.isInstance(value)) {
            return true;

        } else if (type.equals(Integer.TYPE) && Integer.class.isInstance(value)) {
            return true;

        } else if (type.equals(Long.TYPE) && Long.class.isInstance(value)) {
            return true;

        } else if (type.equals(Double.TYPE) && Double.class.isInstance(value)) {
            return true;

        } else if (type.equals(Float.TYPE) && Float.class.isInstance(value)) {
            return true;

        } else if (type.equals(Short.TYPE) && Short.class.isInstance(value)) {
            return true;

        } else if (type.equals(Byte.TYPE) && Byte.class.isInstance(value)) {
            return true;

        } else if (type.equals(Character.TYPE) && Character.class.isInstance(value)) {
            return true;

        } else if (type.equals(Boolean.TYPE) && Boolean.class.isInstance(value)) {
            return true;

        }
        return false;

    }

    /**
     * Factory method that returns a new instance of the given Class.  This
     * is called at the start of the bean creation process and may be
     * overridden to provide custom behavior like returning a cached bean
     * instance.
     * 工廠方法，根據給定的Class返回一個新的實例（使用了反射）。在bean建立流程的開始
     * 會調用此方法。
     * @param <T> The type of object to create
     * @param c The Class to create an object from.
     * @return A newly created object of the Class.
     * @throws SQLException if creation failed.
     */
    protected <T> T newInstance(Class<T> c) throws SQLException {
        try {
        	//使用反射建立對象。使用的是默認無參構造函數。
            return c.newInstance();

        } catch (InstantiationException e) {
            throw new SQLException(
                "Cannot create " + c.getName() + ": " + e.getMessage());

        } catch (IllegalAccessException e) {
            throw new SQLException(
                "Cannot create " + c.getName() + ": " + e.getMessage());
        }
    }

    /**
     * Returns a PropertyDescriptor[] for the given Class.
     *
     * @param c The Class to retrieve PropertyDescriptors for.
     * @return A PropertyDescriptor[] describing the Class.
     * @throws SQLException if introspection failed.
     */
    private PropertyDescriptor[] propertyDescriptors(Class<?> c)
        throws SQLException {
        // Introspector caches BeanInfo classes for better performance
        BeanInfo beanInfo = null;
        try {
            beanInfo = Introspector.getBeanInfo(c);

        } catch (IntrospectionException e) {
            throw new SQLException(
                "Bean introspection failed: " + e.getMessage());
        }

        return beanInfo.getPropertyDescriptors();
    }

    /**
     * The positions in the returned array represent column numbers.  The
     * values stored at each position represent the index in the
     * <code>PropertyDescriptor[]</code> for the bean property that matches
     * the column name.  If no bean property was found for a column, the
     * position is set to <code>PROPERTY_NOT_FOUND</code>.
     * <p>
     * 返回的int數組中的位置表明着列的號數。每一個位置上的值表明着一個bean的
     * PropertyDescriptor[]中的第幾個元素
     * </p>
     * 能夠說columnToProperty中的index表明着列的號數（除了0）
     * 每一個元素的值表明的PropertyDescriptor[]的第幾個元素
     * 如：columnToProperty[1] = 0;那麼就是列1對應着PropertyDescriptor[]
     * 的第0個元素。這個int[]數組將列數和屬性描述符之間的對應關係保存了起來
     * @param rsmd The <code>ResultSetMetaData</code> containing column
     * information.
     *
     * @param props The bean property descriptors.
     *
     * @throws SQLException if a database access error occurs
     *
     * @return An int[] with column index to property index mappings.  The 0th
     * element is meaningless because JDBC column indexing starts at 1.
     */
    protected int[] mapColumnsToProperties(ResultSetMetaData rsmd,
            PropertyDescriptor[] props) throws SQLException {
    	//得到ResultSet的列數
        int cols = rsmd.getColumnCount();
        /*
         * 數組的lenght比列數大1，數組的第0個元素不放置東西
         * 數組的第1個元素與列1對應，一次類推
         */
        int[] columnToProperty = new int[cols + 1];
        //將數組中的全部元素填充爲-1
        Arrays.fill(columnToProperty, PROPERTY_NOT_FOUND);

        for (int col = 1; col <= cols; col++) {
            //先得到列的ColumnLabel，若是沒有，就改得到列名
        	String columnName = rsmd.getColumnLabel(col);
            if (null == columnName || 0 == columnName.length()) {
              columnName = rsmd.getColumnName(col);
            }
            //columnToPropertyOverrides默認在無參構造函數內有賦值
            String propertyName = columnToPropertyOverrides.get(columnName);
            if (propertyName == null) {
                propertyName = columnName;
            }
            for (int i = 0; i < props.length; i++) {
            	//propertyName若是等於屬性描述符的name，使用0以上的數字覆蓋掉原來的-1
                if (propertyName.equalsIgnoreCase(props[i].getName())) {
                    columnToProperty[col] = i;
                    break;
                }
            }
        }
        
        return columnToProperty;
    }

    /**
     * Convert a <code>ResultSet</code> column into an object.  Simple
     * implementations could just call <code>rs.getObject(index)</code> while
     * more complex implementations could perform type manipulation to match
     * the column's type to the bean property type.
     *
     * <p>
     * This implementation calls the appropriate <code>ResultSet</code> getter
     * method for the given property type to perform the type conversion.  If
     * the property type doesn't match one of the supported
     * <code>ResultSet</code> types, <code>getObject</code> is called.
     * </p>
     *
     * @param rs The <code>ResultSet</code> currently being processed.  It is
     * positioned on a valid row before being passed into this method.
     *
     * @param index The current column index being processed.
     *
     * @param propType The bean property type that this column needs to be
     * converted into.
     *
     * @throws SQLException if a database access error occurs
     *
     * @return The object from the <code>ResultSet</code> at the given column
     * index after optional type processing or <code>null</code> if the column
     * value was SQL NULL.
     */
    protected Object processColumn(ResultSet rs, int index, Class<?> propType)
        throws SQLException {
    	//若是屬性不是基本類型的，並且對應的列的值爲null，那麼返回null
        if ( !propType.isPrimitive() && rs.getObject(index) == null ) {
            return null;
        }
        /*
         * 若是屬性是String類型的，就取出值
         * 若是是基本類型或基本類型的包裝類，那麼就取出值
         * 其餘的Object類型，直接getObject
         */
        if (propType.equals(String.class)) {
            return rs.getString(index);

        } else if (
            propType.equals(Integer.TYPE) || propType.equals(Integer.class)) {
            return Integer.valueOf(rs.getInt(index));

        } else if (
            propType.equals(Boolean.TYPE) || propType.equals(Boolean.class)) {
            return Boolean.valueOf(rs.getBoolean(index));

        } else if (propType.equals(Long.TYPE) || propType.equals(Long.class)) {
            return Long.valueOf(rs.getLong(index));

        } else if (
            propType.equals(Double.TYPE) || propType.equals(Double.class)) {
            return Double.valueOf(rs.getDouble(index));

        } else if (
            propType.equals(Float.TYPE) || propType.equals(Float.class)) {
            return Float.valueOf(rs.getFloat(index));

        } else if (
            propType.equals(Short.TYPE) || propType.equals(Short.class)) {
            return Short.valueOf(rs.getShort(index));

        } else if (propType.equals(Byte.TYPE) || propType.equals(Byte.class)) {
            return Byte.valueOf(rs.getByte(index));

        } else if (propType.equals(Timestamp.class)) {
            return rs.getTimestamp(index);

        } else if (propType.equals(SQLXML.class)) {
            return rs.getSQLXML(index);

        } else {
            return rs.getObject(index);
        }

    }

}

 

 總的來講：BeanProcessor就是對bean的操做。

 

---------------------------mini分割線-----------------------------

下面來看一個接口：RowProcessor。它定義了以下方法：

toArray：將記錄變成數組

toBean：將記錄變成對象

toBeanList：將記錄變成bean的集合

toMap：將記錄變成Map對象

它有一個實現類：BasicRowProcessor。

 

BasicRowProcessor實現了對ResultSet的處理。其實是BasicRowProcessor和BeanProcessor一塊兒合做的。

下面看下BasicRowProcessor的源碼：

/*
 * 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.List;
import java.util.Locale;
import java.util.Map;

/**
 * Basic implementation of the <code>RowProcessor</code> interface.
 * RowProcessor接口的基本實現
 * 有些方法實現委託給了BeanProcessor對象實現
 * <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.
     * 一個BeanProcessor對象
     */
    private static final BeanProcessor defaultConvert = new BeanProcessor();

    /**
     * The Singleton instance of this class.
     * 單例
     */
    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.
     * <p>
     * BasicRowProcessor構造器。Bean處理默認使用了一個BeanProcessor對象
     * </p>
     */
    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.
     * 將一個ResultSet中的一行轉換成一個Object[]，每個列值就是數組中的一個元素。
     * 這個實現將列的值拷貝到數組中，二者時間
     * 的順序一致。若是列值爲SQL NULL，那麼在數組中就會設置爲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();
        //得到列數
        int cols = meta.getColumnCount();
        //根據列數，建立一個對應大小的數組
        Object[] result = new Object[cols];
        //將列值放到數組中
        for (int i = 0; i < cols; i++) {
            result[i] = rs.getObject(i + 1);
        }
        //返回結果
        return result;
    }

    /**
     * Convert a <code>ResultSet</code> row into a JavaBean.  This
     * implementation delegates to a BeanProcessor instance.
     * 將ResultSet中的一行轉換成一個javaBean。具體實現委派給一個BeanProcessor實例。
     * @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.
     * 將一個ResultSet轉換成一個javaBean集合。具體實現委派給一個BeanProcessor實例。
     * @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>.  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.
     * 將一個ResultSet中的額一行轉換成一個Map對象。列名爲key，值爲value。Map的key不區分
     * 大小寫。map.get("col")和map.get("COL")都返回同一個值
     * @see org.apache.commons.dbutils.RowProcessor#toMap(java.sql.ResultSet)
     * @param rs ResultSet that supplies the map data
     * @throws SQLException if a database access error occurs
     * @return the newly created Map
     */
    @Override
    public Map<String, Object> toMap(ResultSet rs) throws SQLException {
    	/*
    	 * 建立一個Map對象，能夠看到使用了一個內部類CaseInsensitiveHashMap
    	 * 由於key無論大小寫，都將返回同一個值。JDK中沒有提供這種類型的Map，
    	 * 因此要自定義了
    	 */
        Map<String, Object> result = new CaseInsensitiveHashMap();
        //得到ResultSet的元數據
        ResultSetMetaData rsmd = rs.getMetaData();
        //得到列數
        int cols = rsmd.getColumnCount();

        for (int i = 1; i <= cols; i++) {
        	//以列名爲key，列值爲value存放到Map中
            result.put(rsmd.getColumnName(i), 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 HashMap} does, because of the code:
     * <pre>
     * key.toString().toLowerCase()
     * </pre>
     */
    private static class CaseInsensitiveHashMap extends HashMap<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);
        }
    }

}

 

BasicRowProcessor實現了RowProcessor接口，下面簡單講下方法的實現

一、toArray：將一行記錄變成一個數組。根據列數來建立一個對應大小的Object[]。而後將列值取出放到Object[]中。

二、toBean：將一行記錄變成一個bean對象。其實它內部調用了BeanProcessor中的toBean方法，具體的能夠看看上面BeanProcessor的源碼。

三、toBeanList：此方法跟toBean方法相似，也是調用了Beanprocessor中的toBeanList方法。

四、toMap：將一行記錄變成一個Map對象。列名爲key，列值爲value。它使用了一個自定義的CaseInsensitiveHashMap。此map實現了key不區分大小寫。這對於咱們的使用特別方便。

PS：從01-03，把dbutils中是核心類都講了，以後的各類Handler主要是調用了BasicRowProcessor和BeanProcessor類。以後會大概講些Handler。