package com.bjpowernode.test;

public class ClassA {
  private int classAId;
  private String classAName;

  private ClassB classB;

  public int getClassAId() {
    return classAId;
  }

  public void setClassAId(int classAId) {
    this.classAId = classAId;
  }

  public String getClassAName() {
    return classAName;
  }

  public void setClassAName(String classAName) {
    this.classAName = classAName;
  }

  public ClassB getClassB() {
    return classB;
  }

  public void setClassB(ClassB classB) {
    this.classB = classB;
  }
}

package com.bjpowernode.test;

public class ClassB {
  private int classBId;
  private String classBName;

  public int getClassBId() {
    return classBId;
  }

  public void setClassBId(int classBId) {
    this.classBId = classBId;
  }

  public String getClassBName() {
    return classBName;
  }

  public void setClassBName(String classBName) {
    this.classBName = classBName;
  }
}

package com.bjpowernode.test;

import java.io.StringReader;
import java.io.StringWriter;

import javax.xml.bind.*;

public class XmlUtil {

  public static String toXML(Object obj) {
    try {
      JAXBContext context = JAXBContext.newInstance(obj.getClass());

      Marshaller marshaller = context.createMarshaller();
      marshaller.setProperty(Marshaller.JAXB_ENCODING, "UTF-8");// //编码格式
      marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);// 是否格式化生成的xml串
      marshaller.setProperty(Marshaller.JAXB_FRAGMENT, false);// 是否省略xm头声明信息
      StringWriter writer = new StringWriter();
      marshaller.marshal(obj, writer);
      return writer.toString();
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }

  @SuppressWarnings("unchecked")
  public static <T> T fromXML(String xml, Class<T> valueType) {
    try {
      JAXBContext context = JAXBContext.newInstance(valueType);
      Unmarshaller unmarshaller = context.createUnmarshaller();
      return (T) unmarshaller.unmarshal(new StringReader(xml));
    } catch (Exception e) {
      throw new RuntimeException(e.getMessage());
    }
  }
}

package com.bjpowernode.test;

public class MainRun {

  /**
   * @param args
   */
  public static void main(String[] args) {

    ClassB classB = new ClassB();
    classB.setClassBId(22);
    classB.setClassBName("B2");

    ClassA classA = new ClassA();
    classA.setClassAId(11);
    classA.setClassAName("A1");
    classA.setClassB(classB);

    System.out.println(XmlUtil.toXML(classA));
  }

}

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<classA>
  <classAId>11</classAId>
  <classAName>A1</classAName>
  <classB>
    <classBId>22</classBId>
    <classBName>B2</classBName>
  </classB>
</classA>

@XmlRootElement(namespace="com.bjpowernode.test")
@XmlAccessorType(XmlAccessType.FIELD)
public class ClassA {
  private int classAId;
  
  @XmlElement(name="ClassAName")
  private String classAName;

  private ClassB classB;

  public int getClassAId() {
    return classAId;
  }
  public void setClassAId(int classAId) {
    this.classAId = classAId;
  }

  public String getClassAName() {
    return classAName;
  }

  public void setClassAName(String classAName) {
    this.classAName = classAName;
  }

  public ClassB getClassB() {
    return classB;
  }

  public void setClassB(ClassB classB) {
    this.classB = classB;
  }
}

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class ClassB {
  private int ClassBId;
  private String ClassBName;

  public int getClassBId() {
    return ClassBId;
  }

  public void setClassBId(int classBId) {
    this.ClassBId = classBId;
  }

  public String getClassBName() {
    return ClassBName;
  }

  public void setClassBName(String classBName) {
    this.ClassBName = classBName;
  }
}

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:classA xmlns:ns2="com.bjpowernode.test">
  <classAId>11</classAId>
  <ClassAName>A1</ClassAName>
  <classB>
    <ClassBId>22</ClassBId>
    <ClassBName>B2</ClassBName>
  </classB>
</ns2:classA>