Apache Commons Beanutils為開源軟件，可在Apache官網http://commons.apache.org/proper/commons-beanutils/download_beanutils.cgi下載，使用它還需另一個Apache開源軟件Apache

Commons Logging，可在Apache官網http://commons.apache.org/proper/commons-logging/download_logging.cgi下載，我使用的是commons-beanutils-1.9.1-bin.zip和commons-logging-1.1.3-bin.zip

public class User {

private String name;

private Integer age;

private boolean single;

private Map map;

public User(){}

public User(String name,Integer age,boolean single,

Map map){

this.name = name;

this.age = age;

this.single = single;

this.map = map;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public boolean isSingle() {

return single;

}

public void setSingle(boolean single) {

this.single = single;

}

public Map getMap() {

return map;

}

public void setMap(Map map) {

this.map = map;

}

public String toString(){

String str = "name:"+name+" age:"+age+" single:"+single+" ";

if(null != map && map.size() > 0){

str += "map[";

Iterator it = map.keySet().iterator();

while(it.hasNext()){

Integer key = it.next();

Integer value = map.get(key);

str += key+":"+value+" ";

}

str += "]";

}

return str;

}

}package com.sean;

import java.util.HashMap;

import java.util.Map;

import org.apache.commons.beanutils.BeanUtils;

public class Test {

public static void main(String[] args) throws Exception {

Map map = new HashMap();

map.put(1, 1);

User u1 = new User("tom",123,true,map);

User u2 = new User();

BeanUtils.copyProperties(u2, u1);

System.out.println(u1.toString());

System.out.println(u2.toString());

}

}

運行結果為(比較復雜的屬性也可以被復制并且只有擁有get/set方法的屬性才可以被復制)：

name:tom age:123 single:true map[1:1 ]

name:tom age:null single:true map[1:1 ]

更詳細的使用說明就不介紹了，接下來看看BeanUtils是如何實現的，好戲剛剛開始

BeanUtils：

.......

public static void copyProperties(Object dest, Object orig)

throws IllegalAccessException, InvocationTargetException {

BeanUtilsBean.getInstance().copyProperties(dest, orig);

}

......

/**

* Gets the instance which provides the functionality for {@link BeanUtils}.

* This is a pseudo-singleton - an single instance is provided per (thread) context classloader.

* This mechanism provides isolation for web apps deployed in the same container.

*

* @return The (pseudo-singleton) BeanUtils bean instance

*/

public static BeanUtilsBean getInstance() {

return BEANS_BY_CLASSLOADER.get();

}

......

這里特意帶上了方法說明，getInstance()方法并不是一個簡單的單例模式，而是一個“偽單例”模式

ContextClassLoaderLocal:

......

private static final ContextClassLoaderLocal BEANS_BY_CLASSLOADER

= new ContextClassLoaderLocal() {

// Creates the default instance used when the context classloader is unavailable

@Override

protected BeanUtilsBean initialValue() {

return new BeanUtilsBean();

}

};

......

/**

* Gets the instance which provides the functionality for {@link BeanUtils}.

* This is a pseudo-singleton - an single instance is provided per (thread) context classloader.

* This mechanism provides isolation for web apps deployed in the same container.

* @return the object currently associated with the context-classloader of the current thread.

*/

public synchronized T get() {

// synchronizing the whole method is a bit slower

// but guarantees no subtle threading problems, and there's no

// need to synchronize valueByClassLoader

// make sure that the map is given a change to purge itself

valueByClassLoader.isEmpty();

try {

ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();

if (contextClassLoader != null) {

T value = valueByClassLoader.get(contextClassLoader);

if ((value == null) && !valueByClassLoader.containsKey(contextClassLoader)) {

value = initialValue();

valueByClassLoader.put(contextClassLoader, value);

}

return value;

}

} catch (SecurityException e) { /* SWALLOW - should we log this? */ }

// if none or exception, return the globalValue

if (!globalValueInitialized) {

globalValue = initialValue();

globalValueInitialized = true;

}//else already set

return globalValue;

}

.......

和ContextClassLoader配合實現了一個線程中單例的“偽單例”模式，真正的亮點

總結

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