Apache Commons Betwixt2 的陷阱

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。

 

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