JAXB 深入显出 - JAXB 教程 XML转Java对象深入（Unmarshaller）-蒲公英云

摘要： JAXB 作为JDK的一部分，能便捷地将Java对象与XML进行相互转换，本教程从实际案例出发来讲解JAXB 2 的那些事儿。完整版目录

前情回顾

上一节以简单介绍了 UnMarshaller 的过程，主要介绍了多种数据源如何处理。这一节将深入介绍XML数据转换为JAVA对象，将涉及更加复杂的XML结构。

Unmarshaller Callback

在JAXB反序列化时，有两个方法可以自定义一些行为，它们有固定的形式：

void beforeUnmarshal(Unmarshaller unmarshaller, Object parent) {}

void afterUnmarshal(Unmarshaller unmarshaller, Object parent) {}

上一节中的Employe经过改造：

package com.example.demo.lesson17;
import java.io.Serializable;
import javax.xml.bind.Unmarshaller;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement(name= "Employe")
@XmlAccessorType(XmlAccessType.FIELD)
public class Employe implements Serializable{ 
    private static final long serialVersionUID = 1L;
    private String id;
    private String name;
    public String getId() { 
        return id;
    }
    public void setId(String id) { 
        this.id = id;
    }
    public String getName() { 
        return name;
    }
    public void setName(String name) { 
        this.name = name;
    }
    void beforeUnmarshal(Unmarshaller unmarshaller, Object parent) { 
        System.out.println("调用Unmarshaller之前");
    }
    void afterUnmarshal(Unmarshaller unmarshaller, Object parent) { 
        System.out.println("调用Unmarshaller之后");
    }
    @Override
    public String toString() { 
        return "Employee [id=" + id + ", name=" + name + "]";
    }
}

测试一下：

@Test
    public void test1() throws JAXBException { 
        JAXBContext context = JAXBContext.newInstance(Employe.class);
        Unmarshaller unmarshaller = context.createUnmarshaller();
        String xmlStr = "<Employe><id>1504</id><name>Test</name></Employe>";
        Employe employe = (Employe)unmarshaller.unmarshal(new StringReader(xmlStr));
        System.out.println(employe);//Employee [id=1504, name=Test]
    }

结果：

调用Unmarshaller之前
调用Unmarshaller之后
Employee [id=1504, name=Test]

这两个方法在某些场合可以起到自定义 Unmarshaller 的效果。

复杂XML转化

之前见到的XML都是一层结构，真实场景中的XML肯定复杂的多。但是对于特别复杂的XML结构，我不会深入，因为XML都是一级一级的结构，再复杂的XML也是多级拼接而成，于是正确的拆分XML便是反序列化中重要的一环。

<employee id="17">
    <department>
        <id>101</id>
        <name>IT</name>
    </department>
    <firstName>Lokesh</firstName>
    <lastName>Gupta</lastName>
</employee>

对于这个XML，需要把XML拆分成两部分：最外层的employe和嵌套的department。如果遇到XML中是属性的，在Java bean的字段上添加注解@XmlAttribute，需要名称不一样的，添加别名@XmlElement(name="FirstName")。

Employee的部分重要代码：

@XmlRootElement(name = "employee")
@XmlAccessorType(XmlAccessType.FIELD)
public class Employee { 
    @XmlAttribute
    private Integer id;
    @XmlElement(name="FirstName")
    private String firstName;
    private String lastName;
    private Department department;
// ignore setters/getters,toString
}

Department的完整代码：

package com.example.demo.lesson17;
public class Department { 
    public String id;
    public String name;
    @Override
    public String toString() { 
        return "Department [id=" + id + ", name=" + name + "]";
    }
}

因为比较简单，不需要任何注解，看起来好像和JAXB无关的Department。

测试一下：

@Test
    public void test2() throws JAXBException { 
        JAXBContext context = JAXBContext.newInstance(Employee.class);
        Unmarshaller unmarshaller = context.createUnmarshaller();
        String xmlStr = "<employee id='17'>\r\n" + 
                " <department>\r\n" + 
                " <id>101</id>\r\n" + 
                " <name>IT</name>\r\n" + 
                " </department>\r\n" + 
                " <FirstName>Lokesh</FirstName>\r\n" + 
                " <id>1</id>\r\n" + 
                " <lastName>Gupta</lastName>\r\n" + 
                "</employee>";
        Employee employee = (Employee)unmarshaller.unmarshal(new StringReader(xmlStr));
        System.out.println(employee);//Employee [id=17, firstName=Lokesh, lastName=Gupta, department=Department [id=101, name=IT]]
    }

得到的结果：

Employee [id=17, firstName=Lokesh, lastName=Gupta, department=Department [id=101, name=IT]]

所有的字段都反序列化成功了，不仅仅得到了Employee,还得到了Department数据。

有的XML显得更为复杂，像下面这种：

<MUSEUMS>
 <MUSEUM children_allowed="false">
   <MUSEUM_NAME>Reina Sofia Museum</MUSEUM_NAME>
   <CITY>Madrid</CITY>
   <PERMANENT_EXHIBITION>
      <NAME>Permanent Exhibition - Reina Sofia Museum</NAME>
      <ARTIST>Picasso</ARTIST>
      <ARTIST>Dali</ARTIST>
      <ARTIST>Miro</ARTIST>
      <FROM>1900-01-01</FROM>
      <TO>2014-12-31</TO>
   </PERMANENT_EXHIBITION>
 </MUSEUM>
 <MUSEUM>
   <MUSEUM_NAME>Louvre Museum</MUSEUM_NAME>
   <CITY>Paris</CITY>
   <PERMANENT_EXHIBITION>
      <NAME>Permanent Exhibition - Louvre Museum</NAME>
      <ARTIST>Leonardo da Vinci</ARTIST>
      <ARTIST>Caravaggio</ARTIST>
      <ARTIST>Delacroix</ARTIST>
   </PERMANENT_EXHIBITION>
 </MUSEUM>
 <TOTAL>2</TOTAL>
</MUSEUMS>

这种XML结构在业务场景更普遍，它们很长，看着很复杂，其实并没有想象的那么难。这个数据看着复杂，其实更多的是重复，既然都是一样的，那就需要巧用‘循环’了。而Java bean中的循环当然要考虑List，它里面包含的数据都是相同类型，结构必然一样，把上面这段XML拆分后，就得到了3个Java bean。

最外层的Java bean：

@XmlRootElement(name = "MUSEUMS")
@XmlAccessorType(XmlAccessType.FIELD)
public class Museums { 
    @XmlElement(name = "MUSEUM")
    List<Museum> museums;
    @XmlElement(name = "TOTAL")
    String total;
// ignore setters/getters,toString
}

接下来的Meseum：

@XmlRootElement( name = "MUSEUM" )
@XmlType( propOrder = {  "name", "city", "special" } )
@XmlAccessorType(XmlAccessType.FIELD)
public class Museum { 
    @XmlElement(name = "MUSEUM_NAME")
    String name;
    @XmlAttribute(name = "children_allowed", required=false)
    Boolean childrenAllowed;
    @XmlElement(name = "CITY")
    String city;
    @XmlElement(name = "PERMANENT_EXHIBITION")
    Exhibition special;
// ignore setters/getters,toString
}

接下来的一层：

@XmlAccessorType(XmlAccessType.FIELD)
public class Exhibition { 
    @XmlElement(name = "NAME")
    String name;
    @XmlElement(name = "ARTIST")
    List<String> artist;
    @XmlElement(name = "FROM")
    String from;
    @XmlElement(name = "TO")
    String to;
// ignore setters/getters,toString
}

演示代码比较长：

@Test
    public void test3() throws JAXBException { 
        JAXBContext context = JAXBContext.newInstance(Museums.class);
        Unmarshaller unmarshaller = context.createUnmarshaller();
        String xmlStr = "<MUSEUMS>\r\n" + 
                " <MUSEUM children_allowed=\"false\">\r\n" + 
                " <MUSEUM_NAME>Reina Sofia Museum</MUSEUM_NAME>\r\n" + 
                " <CITY>Madrid</CITY>\r\n" + 
                " <PERMANENT_EXHIBITION>\r\n" + 
                " <NAME>Permanent Exhibition - Reina Sofia Museum</NAME>\r\n" + 
                " <ARTIST>Picasso</ARTIST>\r\n" + 
                " <ARTIST>Dali</ARTIST>\r\n" + 
                " <ARTIST>Miro</ARTIST>\r\n" + 
                " <FROM>1900-01-01</FROM>\r\n" + 
                " <TO>2014-12-31</TO>\r\n" + 
                " </PERMANENT_EXHIBITION>\r\n" + 
                " </MUSEUM>\r\n" + 
                " <MUSEUM>\r\n" + 
                " <MUSEUM_NAME>Louvre Museum</MUSEUM_NAME>\r\n" + 
                " <CITY>Paris</CITY>\r\n" + 
                " <PERMANENT_EXHIBITION>\r\n" + 
                " <NAME>Permanent Exhibition - Louvre Museum</NAME>\r\n" + 
                " <ARTIST>Leonardo da Vinci</ARTIST>\r\n" + 
                " <ARTIST>Caravaggio</ARTIST>\r\n" + 
                " <ARTIST>Delacroix</ARTIST>\r\n" + 
                " </PERMANENT_EXHIBITION>\r\n" + 
                " </MUSEUM>\r\n" + 
                " <TOTAL>2</TOTAL>\r\n" + 
                "</MUSEUMS>";
        Museums museums = (Museums)unmarshaller.unmarshal(new StringReader(xmlStr));
        System.out.println(museums);
        //Museums [museums=[Museum [name=Reina Sofia Museum, childrenAllowed=false, city=Madrid, special=Exhibition [name=Permanent Exhibition - Reina Sofia Museum, artist=[Picasso, Dali, Miro], from=1900-01-01, to=2014-12-31]], Museum [name=Louvre Museum, childrenAllowed=null, city=Paris, special=Exhibition [name=Permanent Exhibition - Louvre Museum, artist=[Leonardo da Vinci, Caravaggio, Delacroix], from=null, to=null]]], total=2]
    }