上面的代碼從aLocal對象復制屬性到aValue對象。它相當簡單！它不管local（或對應的value）對象有多少個屬性，只管進行復制。我們假設local對象有100個屬性。上面的代碼使我們可以無需鍵入至少100行的冗長、容易出錯和反復的get和set方法調用。這太棒了！太強大了！太有用了！
現在，還有一個壞消息：使用BeanUtils的成本驚人地昂貴！我做了一個簡單的測試，BeanUtils所花費的時間要超過取數據、將其復制到對應的value對象（通過手動調用get和set方法），以及通過串行化將其返回到遠程的客戶機的時間總和。所以要小心使用這種威力！
Beanutils用了魔術般的反射技術，實現了很多夸張有用的功能，都是C/C++時代不敢想的。無論誰的項目，始終一天都會用得上它。我算是后知后覺了，第一回看到它的時候居然錯過。
其中BeanUtils會使用ConvertUtils類把字符串轉為Bean屬性的真正類型，方便從HttpServletRequest等對象中提取bean，或者把bean輸出到頁面。
六、ConstructorUtils補遺
創建對象還有一個方法：invokeExactConstructor，該方法對參數要求
更加嚴格，傳遞進去的參數必須嚴格符合構造方法的參數列表。
例如：
Object[] args={new Integer(1), "Jan"};
Class[] argsType={int.class, String.class};
Object obj;
//下面這句調用將不會成功，因為args[0]的類型為Integer，而不是int
//obj = ConstructorUtils.invokeExactConstructor(Month.class, args);
//這一句就可以，因為argsType指定了類型。
obj = ConstructorUtils.invokeExactConstructor(Month.class, args, argsType);
Month month=(Month)obj;
System.out.println(BeanUtils.getProperty(month,"value"));
七、MethodUtils
與ConstructorUtils類似，不過調用的時候，通常需要再指定一個method name的參數。
八、DynaClass/DynaBean
這似乎是BeanUtils中最有趣的部分之一了，很簡單，簡單到光看這兩個接口中的方法會不明白
為什么要設計這兩個接口。不過看到ResultSetDynaClass后，就明白了。下面是java doc中的代碼：
???ResultSet rs = ...;
???ResultSetDynaClass rsdc = new ResultSetDynaClass(rs);
???Iterator rows = rsdc.iterator();
???while (rows.hasNext())??{
?????DynaBean row = (DynaBean) rows.next();
?????... process this row ...
???}
???rs.close();
原來這是一個ResultSet的包裝器，ResultSetDynaClass實現了DynaClass，它的iterator方法返回一個
ResultSetIterator，則是實現了DynaBean接口。
在獲得一個DynaBean之后，我們就可以用
?????DynaBean row = (DynaBean) rows.next();
?????System.out.println(row.get("field1")); //field1是其中一個字段的名字
再看另一個類RowSetDynaClass的用法，代碼如下：
String driver="com.mysql.jdbc.Driver";
String url="jdbc:mysql://localhost/2hu?useUnicode=true&characterEncoding=GBK";
String username="root";
String password="";
java.sql.Connection con=null;
PreparedStatement ps=null;
ResultSet rs=null;
try {
Class.forName(driver).newInstance();
con = DriverManager.getConnection(url);
ps=con.prepareStatement("select * from forumlist");
rs=ps.executeQuery();
//先打印一下，用于檢驗后面的結果。
while(rs.next()){
System.out.println(rs.getString("name"));
}
rs.beforeFirst();//這里必須用beforeFirst，因為RowSetDynaClass只從當前位置向前滾動
RowSetDynaClass rsdc = new RowSetDynaClass(rs);
rs.close();
ps.close();
List rows = rsdc.getRows();//返回一個標準的List，存放的是DynaBean
for (int i = 0; i <rows.size(); i++) {
DynaBean b=(DynaBean)rows.get(i);
System.out.println(b.get("name"));
}
} catch (Exception e) {
e.printStackTrace();
}
finally{
try {
con.close();
} catch (Exception e) {
}
}
是不是很有趣？封裝了ResultSet的數據，代價是占用內存。如果一個表有10萬條記錄，rsdc.getRows()
就會返回10萬個記錄。@_@
需要注意的是ResultSetDynaClass和RowSetDynaClass的不同之處：
1，ResultSetDynaClass是基于Iterator的，一次只返回一條記錄，而RowSetDynaClass是基于
List的，一次性返回全部記錄。直接影響是在數據比較多時ResultSetDynaClass會比較的快速，
而RowSetDynaClass需要將ResultSet中的全部數據都讀出來（并存儲在其內部），會占用過多的
內存，并且速度也會比較慢。
2，ResultSetDynaClass一次只處理一條記錄，在處理完成之前，ResultSet不可以關閉。
3，ResultSetIterator的next()方法返回的DynaBean其實是指向其內部的一個固定
對象，在每次next()之后，內部的值都會被改變。這樣做的目的是節約內存，如果你需要保存每
次生成的DynaBean，就需要創建另一個DynaBean，并將數據復制過去，下面也是java doc中的代碼：
???ArrayList results = new ArrayList(); // To hold copied list
???ResultSetDynaClass rsdc = ...;
???DynaProperty properties[] = rsdc.getDynaProperties();
???BasicDynaClass bdc =
?????new BasicDynaClass("foo", BasicDynaBean.class,
????????????????????????rsdc.getDynaProperties());
???Iterator rows = rsdc.iterator();
???while (rows.hasNext()) {
?????DynaBean oldRow = (DynaBean) rows.next();
?????DynaBean newRow = bdc.newInstance();
?????PropertyUtils.copyProperties(newRow, oldRow);
?????results.add(newRow);
???}
事實上DynaClass/DynaBean可以用于很多地方，存儲各種類型的數據。自己想吧。嘿嘿。
九、自定義的CustomRowSetDynaClass
兩年前寫過一個與RowSetDynaClass目標相同的類，不過多一個功能，就是分頁，只取需要的數據，
這樣內存占用就會減少。
先看一段代碼：
String driver="com.mysql.jdbc.Driver";
String url="jdbc:mysql://localhost/2hu?useUnicode=true&characterEncoding=GBK";
String username="root";
String password="";
java.sql.Connection con=null;
PreparedStatement ps=null;
ResultSet rs=null;
try {
Class.forName(driver).newInstance();
con = DriverManager.getConnection(url);
ps=con.prepareStatement("select * from forumlist order by name");
rs=ps.executeQuery();
/*
while(rs.next()){
System.out.println(rs.getString("name"));
}
rs.beforeFirst();
*/
//第二個參數表示第幾頁，第三個參數表示頁的大小
CustomRowSetDynaClass rsdc = new CustomRowSetDynaClass(rs, 2, 5);
//RowSetDynaClass rsdc = new RowSetDynaClass(rs);
rs.close();
ps.close();
List rows = rsdc.getRows();
for (int i = 0; i <rows.size(); i++) {
DynaBean b=(DynaBean)rows.get(i);
System.out.println(b.get("name"));
}
} catch (Exception e) {
e.printStackTrace();
}
finally{
try {
con.close();
} catch (Exception e) {
}
}
在這里用到了一個CustomRowSetDynaClass類，構造方法中增加了page和pageSize兩個參數，
這樣，不管數據庫里有多少條記錄，最多只取pageSize條記錄，若pageSize==-1，則功能和
RowSetDynaClass一樣。這在大多數情況下是適用的。該類的代碼如下：
package test.jakarta.commons.beanutils;
import java.io.*;
import java.sql.*;
import java.util.*;
import org.apache.commons.beanutils.*;
/**
* @author SonyMusic
*
* To change this generated comment edit the template variable "typecomment":
* Window>Preferences>Java>Templates.
* To enable and disable the creation of type comments go to
* Window>Preferences>Java>Code Generation.
*/
public class CustomRowSetDynaClass implements DynaClass, Serializable {
// ----------------------------------------------------------- Constructors
/**
* <p>Construct a new {@link RowSetDynaClass} for the specified
* <code>ResultSet</code>.??The property names corresponding
* to column names in the result set will be lower cased.</p>
*
* @param resultSet The result set to be wrapped
*
* @exception NullPointerException if <code>resultSet</code>
*??is <code>null</code>
* @exception SQLException if the metadata for this result set
*??cannot be introspected
*/
public CustomRowSetDynaClass(ResultSet resultSet) throws SQLException {
this(resultSet, true);
}
/**
* <p>Construct a new {@link RowSetDynaClass} for the specified
* <code>ResultSet</code>.??The property names corresponding
* to the column names in the result set will be lower cased or not,
* depending on the specified <code>lowerCase</code> value.</p>
*
* <p><strong>WARNING</strong> - If you specify <code>false</code>
* for <code>lowerCase</code>, the returned property names will
* exactly match the column names returned by your JDBC driver.
* Because different drivers might return column names in different
* cases, the property names seen by your application will vary
* depending on which JDBC driver you are using.</p>
*
* @param resultSet The result set to be wrapped
* @param lowerCase Should property names be lower cased?
*
* @exception NullPointerException if <code>resultSet</code>
*??is <code>null</code>
* @exception SQLException if the metadata for this result set
*??cannot be introspected
*/
public CustomRowSetDynaClass(ResultSet resultSet, boolean lowerCase)
throws SQLException {
this(resultSet, 1, -1, lowerCase);
}
public CustomRowSetDynaClass(
ResultSet resultSet,
int page,
int pageSize,
boolean lowerCase)
throws SQLException {
if (resultSet == null) {
throw new NullPointerException();
}
this.lowerCase = lowerCase;
this.page = page;
this.pageSize = pageSize;
introspect(resultSet);
copy(resultSet);
}
public CustomRowSetDynaClass(ResultSet resultSet, int page, int pageSize)
throws SQLException {
this(resultSet, page, pageSize, true);
}
// ----------------------------------------------------- Instance Variables
/**
* <p>Flag defining whether column names should be lower cased when
* converted to property names.</p>
*/
protected boolean lowerCase = true;
protected int page = 1;
protected int pageSize = -1;
/**
* <p>The set of dynamic properties that are part of this
* {@link DynaClass}.</p>
*/
protected DynaProperty properties[] = null;
/**
* <p>The set of dynamic properties that are part of this
* {@link DynaClass}, keyed by the property name.??Individual descriptor
* instances will be the same instances as those in the
* <code>properties</code> list.</p>
*/
protected Map propertiesMap = new HashMap();
/**
* <p>The list of {@link DynaBean}s representing the contents of
* the original <code>ResultSet</code> on which this
* {@link RowSetDynaClass} was based.</p>
*/
protected List rows = new ArrayList();
// ------------------------------------------------------ DynaClass Methods
/**
* <p>Return the name of this DynaClass (analogous to the
* <code>getName()</code> method of <code>java.lang.Class</code), which
* allows the same <code>DynaClass</code> implementation class to support
* different dynamic classes, with different sets of properties.</p>
*/
public String getName() {
return (this.getClass().getName());
}
/**
* <p>Return a property descriptor for the specified property, if it
* exists; otherwise, return <code>null</code>.</p>
*
* @param name Name of the dynamic property for which a descriptor
*??is requested
*
* @exception IllegalArgumentException if no property name is specified
*/
public DynaProperty getDynaProperty(String name) {
if (name == null) {
throw new IllegalArgumentException("No property name specified");
}
return ((DynaProperty) propertiesMap.get(name));
}
/**
* <p>Return an array of <code>ProperyDescriptors</code> for the properties
* currently defined in this DynaClass.??If no properties are defined, a
* zero-length array will be returned.</p>
*/
public DynaProperty[] getDynaProperties() {
return (properties);
}
/**
* <p>Instantiate and return a new DynaBean instance, associated
* with this DynaClass.??<strong>NOTE</strong> - This operation is not
* supported, and throws an exception.</p>
*
* @exception IllegalAccessException if the Class or the appropriate
*??constructor is not accessible
* @exception InstantiationException if this Class represents an abstract
*??class, an array class, a primitive type, or void; or if instantiation
*??fails for some other reason
*/
public DynaBean newInstance()
throws IllegalAccessException, InstantiationException {
throw new UnsupportedOperationException("newInstance() not supported");
}
// --------------------------------------------------------- Public Methods
/**
* <p>Return a <code>List</code> containing the {@link DynaBean}s that
* represent the contents of each <code>Row</code> from the
* <code>ResultSet</code> that was the basis of this
* {@link RowSetDynaClass} instance.??These {@link DynaBean}s are
* disconnected from the database itself, so there is no problem with
* modifying the contents of the list, or the values of the properties
* of these {@link DynaBean}s.??However, it is the application's
* responsibility to persist any such changes back to the database,
* if it so desires.</p>
*/
public List getRows() {
return (this.rows);
}
// ------------------------------------------------------ Protected Methods
/**
* <p>Copy the column values for each row in the specified
* <code>ResultSet</code> into a newly created {@link DynaBean}, and add
* this bean to the list of {@link DynaBean}s that will later by
* returned by a call to <code>getRows()</code>.</p>
*
* @param resultSet The <code>ResultSet</code> whose data is to be
*??copied
*
* @exception SQLException if an error is encountered copying the data
*/
protected void copy(ResultSet resultSet) throws SQLException {
int abs = 0;
int rowsCount = 0;
int currentPageRows = 0;
resultSet.last();
rowsCount = resultSet.getRow();
if (pageSize != -1) {
int totalPages = (int) Math.ceil(((double) rowsCount) / pageSize);
if (page > totalPages)
page = totalPages;
if (page < 1)
page = 1;
abs = (page - 1) * pageSize;
//currentPageRows=(page==totalPages?rowsCount-pageSize*(totalPages-1):pageSize);
} else
pageSize = rowsCount;
if (abs == 0)
resultSet.beforeFirst();
else
resultSet.absolute(abs);
//int
while (resultSet.next() && ++currentPageRows <= pageSize) {
DynaBean bean = new BasicDynaBean(this);
for (int i = 0; i < properties.length; i++) {
String name = properties[i].getName();
bean.set(name, resultSet.getObject(name));
}
rows.add(bean);
}
}
/**
* <p>Introspect the metadata associated with our result set, and populate
* the <code>properties</code> and <code>propertiesMap</code> instance
* variables.</p>
*
* @param resultSet The <code>resultSet</code> whose metadata is to
*??be introspected
*
* @exception SQLException if an error is encountered processing the
*??result set metadata
*/
protected void introspect(ResultSet resultSet) throws SQLException {
// Accumulate an ordered list of DynaProperties
ArrayList list = new ArrayList();
ResultSetMetaData metadata = resultSet.getMetaData();
int n = metadata.getColumnCount();
for (int i = 1; i <= n; i++) { // JDBC is one-relative!
DynaProperty dynaProperty = createDynaProperty(metadata, i);
if (dynaProperty != null) {
list.add(dynaProperty);
}
}
// Convert this list into the internal data structures we need
properties =
(DynaProperty[]) list.toArray(new DynaProperty[list.size()]);
for (int i = 0; i < properties.length; i++) {
propertiesMap.put(properties[i].getName(), properties[i]);
}
}
/**
* <p>Factory method to create a new DynaProperty for the given index
* into the result set metadata.</p>
*
* @param metadata is the result set metadata
* @param i is the column index in the metadata
* @return the newly created DynaProperty instance
*/
protected DynaProperty createDynaProperty(
ResultSetMetaData metadata,
int i)
throws SQLException {
String name = null;
if (lowerCase) {
name = metadata.getColumnName(i).toLowerCase();
} else {
name = metadata.getColumnName(i);
}
String className = null;
try {
className = metadata.getColumnClassName(i);
} catch (SQLException e) {
// this is a patch for HsqlDb to ignore exceptions
// thrown by its metadata implementation
}
// Default to Object type if no class name could be retrieved
// from the metadata
Class clazz = Object.class;
if (className != null) {
clazz = loadClass(className);
}
return new DynaProperty(name, clazz);
}
/**
* <p>Loads and returns the <code>Class</code> of the given name.
* By default, a load from the thread context class loader is attempted.
* If there is no such class loader, the class loader used to load this
* class will be utilized.</p>
*
* @exception SQLException if an exception was thrown trying to load
*??the specified class
*/
protected Class loadClass(String className) throws SQLException {
try {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
if (cl == null) {
cl = this.getClass().getClassLoader();
}
return (cl.loadClass(className));
} catch (Exception e) {
throw new SQLException(
"Cannot load column class '" + className + "': " + e);
}
}
}
大部分代碼從BeanUtils的源碼中取得，只做了簡單的修改，沒有加多余的注釋。如果要正式使用，
需要再做精加工。