原文： https://howtodoinjava.com/jaxb/marshal-without-xmlrootelement/

很多时候，我们将需要编组 Java 对象，而不使用 JAXB 注解，例如@XmlRootElement，并且我们不允许在源代码中进行任何更改。 当我们使用遗留代码或某些我们没有源代码的客户端 jar 时，可能会发生这种情况。

可能还有许多其他情况，但是想法是我们无法使用 JAXB 注解修改模型类。 这可能是将 Java 对象转换为 xml 而没有 jaxb 的示例。

1.不带@XmlRootElement的编组问题

在这种情况下，如果我们尝试将将 Java 对象直接编组为 XML，则将得到类似的错误。

  1. javax.xml.bind.MarshalException
  2.  - with linked exception:
  3. [com.sun.istack.internal.SAXException2: unable to marshal type "com.howtodoinjava.demo.model.Employee" 
  4. as an element because it is missing an @XmlRootElement annotation]
  5.   at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:311)
  6.   at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:236)
  7.   at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:95)
  8.   at com.howtodoinjava.demo.JaxbExample.jaxbObjectToXML(JaxbExample.java:45)
  9.   at com.howtodoinjava.demo.JaxbExample.main(JaxbExample.java:17)
  10. Caused by: com.sun.istack.internal.SAXException2: unable to marshal type "com.howtodoinjava.demo.model.Employee" 
  11. as an element because it is missing an @XmlRootElement annotation
  12.   at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.reportError(XMLSerializer.java:234)
  13.   at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:323)
  14.   at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:479)
  15.   at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:308)
  16.   ... 4 more

其中Employee.java类如下。 它没有任何@XmlRootElement这样的 JAXB 注解。

  1. package com.howtodoinjava.demo.model;
  3. import java.io.Serializable;
  5. public class Employee implements Serializable {
  7.   private static final long serialVersionUID = 1L;
  9.   private Integer id;
  10.   private String firstName;
  11.   private String lastName;
  12.   private Department department;
  14.   public Employee() {
  15.     super();
  16.   }
  18.   public Employee(int id, String fName, String lName, Department department) {
  19.     super();
  20.     this.id = id;
  21.     this.firstName = fName;
  22.     this.lastName = lName;
  23.     this.department = department;
  24.   }
  26.   //Getters and Setters
  28.   @Override
  29.   public String toString() {
  30.     return "Employee [id=" + id + ", firstName=" + firstName + ", 
  31.        lastName=" + lastName + ", department=" + department + "]";
  32.   }
  33. }

2.不带@XmlRootElement注解的编组的解决方案

缺少@XmlRootElement注解时，JAXB 无法为Employee对象构建JAXBElement实例。 这就是我们必须帮助 JAXB 手动构建它的地方。

2.1 语法

  1. /**
  2.   * @param name          Java binding of xml element tag name
  3.   * @param declaredType  Java binding of xml element declaration's type
  4.   * @param value         Java instance representing xml element's value
  5.   */
  6. JAXBElement<T> elem = new JAXBElement(QName name, Class<T> declaredType, T value );

例如：

  1. JAXBElement<Employee> jaxbElement 
  2.         = new JAXBElement<Employee>( new QName("", "employee"), Employee.class, employeeObj );

2.2 不带@XmlRootElement的 JAXB 编组示例

现在，让我们看看该编组代码的工作原理。

  1. package com.howtodoinjava.demo;
  3. import javax.xml.bind.JAXBContext;
  4. import javax.xml.bind.JAXBElement;
  5. import javax.xml.bind.JAXBException;
  6. import javax.xml.bind.Marshaller;
  7. import javax.xml.namespace.QName;
  9. import com.howtodoinjava.demo.model.Department;
  10. import com.howtodoinjava.demo.model.Employee;
  12. public class JaxbExample 
  13. {
  14.   public static void main(String[] args) 
  15.   {
  16.     Employee employee = new Employee(1, "Lokesh", "Gupta", new Department(101, "IT"));
  18.     jaxbObjectToXML( employee );
  19.   }
  21.   private static void jaxbObjectToXML(Employee employeeObj) 
  22.   {
  23.       try {
  24.           JAXBContext jaxbContext = JAXBContext.newInstance(Employee.class);
  25.           Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
  27.           // To format XML
  28.           jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE); 
  30.           //If we DO NOT have JAXB annotated class
  31.           JAXBElement<Employee> jaxbElement = 
  32.             new JAXBElement<Employee>( new QName("", "employee"), 
  33.                       Employee.class, 
  34.                       employeeObj);
  36.           jaxbMarshaller.marshal(jaxbElement, System.out);
  38.           //If we have JAXB annotated class
  39.           //jaxbMarshaller.marshal(employeeObj, System.out);  
  41.       } catch (JAXBException e) {
  42.           e.printStackTrace();
  43.       }
  44.   }
  45. }

程序输出：

  1. <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
  2. <employee>
  3.     <department>
  4.         <id>101</id>
  5.         <name>IT</name>
  6.     </department>
  7.     <firstName>Lokesh</firstName>
  8.     <id>1</id>
  9.     <lastName>Gupta</lastName>
  10. </employee>

请向我提供关于没有@XmlRootElement的 JAXB 编组示例的问题。

学习愉快！

参考： XmlRootElement Java 文档