Apache Commons Betwixt2 的陷阱
 
环境:
Betwixt2 0.8
Java5
 
Apache Commons Betwixt2 真是个变态的家伙,当你用Betwixt2在java与xml之间做转换的时候,如果Bean中有集合属性,就千万要注意了。----如果你搞不好,在xml转向java的时候,集合属性就丢失了。
 
追其原因,是Betwixt2对带有集合的javabean转换有些约定:
1、集合属性的名字要以集合元素类的名字开头(大小写不敏感);
2、每个Bean都必须有一个不带参数的构造方法。或者bean不要写任何构造方法(这样实际上有一个默认的不带参数的构造方法)。
3、每个对于包含其他Bean集合的Bean,要有一个addXxxBean(XxxBean bean)的方法,只有这样,才能保证将xml转换为java的时候,集合属性不丢失。
 
由于这个原因,我差点将原来费了好大劲写好的代码推翻重做了。现在写个简单的例子放出来,希望对遇到同样问题的博友有所帮助。下面看个例子吧:
 
例子是这样的:一个人Person可以有多个宠物Pet,现在创建一个Person,将其转换为XML,然后做个逆转。实现代码如下:
 
package bt;

import java.util.ArrayList;
import java.util.List;

/**
* 人
*
* @author leizhimin 2008-10-22 10:16:06
*/

public class Person {
        private String name;                //姓名
        private int age;                        //年龄
        private List<Pet> petList;    //拥有的宠物

        //------------constructor-----------

        /**
         * 这个默认的构造方法不可少,否则转换出错
         */

        public Person() {
                petList = new ArrayList<Pet>();
        }

        public Person(String name, int age) {
                petList = new ArrayList<Pet>();
                this.name = name;
                this.age = age;
        }

        //------------add集合成员的方法-----------

        /**
         * 添加集合属性元素的方法,add后的单词必须决定了xml中元素的名字
         *
         * @param pet
         */

        public void addPet(Pet pet) {
                petList.add(pet);
        }

        //------------getter/setter-----------

        public String getName() {
                return name;
        }

        public void setName(String name) {
                this.name = name;
        }

        public int getAge() {
                return age;
        }

        public void setAge(int age) {
                this.age = age;
        }

        public List<Pet> getPetList() {
                return petList;
        }

        public void setPetList(List<Pet> petList) {
                this.petList = petList;
        }

        public String toString() {
                StringBuffer sb = new StringBuffer();
                sb.append("Person{" +
                                "name='" + name + '\'' +
                                ", age=" + age +
                                ", petList=\n");
                for (Pet pet : petList) {
                        sb.append("\t\t" + pet.toString()).append(";\n");
                }
                sb.append('}');
                return sb.toString();
        }
}
 
package bt;

/**
* 宠物
*
* @author leizhimin 2008-10-22 10:17:03
*/

public class Pet {
        private String nikename;        //昵称
        private String color;             //颜色

        /**
         * 这个默认的构造方法不可少
         */

        public Pet() {
        }

        public Pet(String nikename, String color) {
                this.nikename = nikename;
                this.color = color;
        }

        public String getNikename() {
                return nikename;
        }

        public void setNikename(String nikename) {
                this.nikename = nikename;
        }

        public String getColor() {
                return color;
        }

        public void setColor(String color) {
                this.color = color;
        }

        public String toString() {
                return "Pet{" +
                                "nikename='" + nikename + '\'' +
                                ", color='" + color + '\'' +
                                '}';
        }
}
 
package bt;

import org.apache.commons.betwixt.io.BeanReader;
import org.apache.commons.betwixt.io.BeanWriter;
import org.xml.sax.SAXException;

import java.beans.IntrospectionException;
import java.io.IOException;
import java.io.StringReader;
import java.io.StringWriter;

/**
* 超变态的Betwixt测试
*
* @author leizhimin 2008-10-22 11:28:15
*/

public class TestBetwixt {
        public static void main(String[] args) throws IOException, SAXException, IntrospectionException {
                String xml = java2XML();
                System.out.println(xml);

                Person person = xml2Java(xml);
                System.out.println(person);
        }

        public static String java2XML() throws IOException, SAXException, IntrospectionException {
                String reslutXml;

                //创建一个输出流,将用来输出Java转换的XML文件
                StringWriter outputWriter = new StringWriter();

                //输出XML的文件头
                outputWriter.write("<?xml version='1.0' ?>\n");

                //创建一个BeanWriter实例,并将BeanWriter的输出重定向到指定的输出流
                BeanWriter beanWriter = new BeanWriter(outputWriter);

                //配置BeanWriter对象
                beanWriter.getXMLIntrospector().getConfiguration().setAttributesForPrimitives(false);
                beanWriter.getBindingConfiguration().setMapIDs(false); //不自动生成ID
                beanWriter.setWriteEmptyElements(true);         //输出空元素
                beanWriter.enablePrettyPrint();         //格式化输出

                //构建要转换的对象
                Person person = new Person("唐伯虎", 24);
                Pet pet1 = new Pet("旺财", "黄色");
                Pet pet2 = new Pet("小强", "灰色");
                person.getPetList().add(pet1);
                person.getPetList().add(pet2);

                //将对象转换为XML
                beanWriter.write(person);
                //获取转换后的结果
                reslutXml = outputWriter.toString();

                //关闭输出流
                outputWriter.close();

                return reslutXml;
        }

        public static Person xml2Java(String xml) throws IntrospectionException, IOException, SAXException {
                //创建一个读取xml文件的流
                StringReader xmlReader = new StringReader(xml);
                //创建一个BeanReader实例,相当于转换器
                BeanReader beanReader = new BeanReader();

                //配置BeanReader实例
                beanReader.getXMLIntrospector().getConfiguration().setAttributesForPrimitives(false);
                beanReader.getBindingConfiguration().setMapIDs(false); //不自动生成ID
                //注册要转换对象的类,并指定根节点名称
                beanReader.registerBeanClass("Person", Person.class);

                //将XML解析Java Object
                Person person = (Person) beanReader.parse(xmlReader);

                return person;
        }
}
 
运行结果:
log4j:WARN No appenders could be found for logger (org.apache.commons.betwixt.io.AbstractBeanWriter).
log4j:WARN Please initialize the log4j system properly.
<?xml version='1.0' ?>
    <Person>
        <age>24</age>
        <name>唐伯虎</name>
        <petList>
            <pet>
                <color>黄色</color>
                <nikename>旺财</nikename>
            </pet>
            <pet>
                <color>灰色</color>
                <nikename>小强</nikename>
            </pet>
        </petList>
    </Person>

Person{name='唐伯虎', age=24, petList=
    Pet{nikename='旺财', color='黄色'};
    Pet{nikename='小强', color='灰色'};
}

Process finished with exit code 0
 
看到了吧,双向转换成功了。
 
但是修改下petList属性的名字,修改下getPetList、setPetList的名字,去掉默认的构造方法,使得不符合上面的约定原则,双向转换就失败了。
 
另外,经过Betwixt2还有一个陷阱,就是在将xml转Java过程中,如果日期数据元素为空,则导致转换失败。解决办法是,将java转xml的时候,设置忽略空元素beanWriter.setWriteEmptyElements(false);,在将xml转java 的时候,空日期元素删除掉即可。