java - JAXB 编码 : treat empty object like it's null

Question

我想用一个简单的例子来解释我的问题:

Foo:

@SomeXMLAnnotations
public class Foo {
    // Bar is just a random class with its own XML annotations
    @XmlElement(required = true)
    Bar someBarObj;

    boolean chosen = true;
    boolean required = true;

    public Foo(){ 
        chosen = false;
    }

    public Foo(Bar someBarObj){
        this.someBarObj = someBarObj;
    }
}

我的类(class):

@SomeXMLAnnotations
public class MyClass {

    @XmlElement(required = false)
    Foo anyFooObj;

    @XmlElement(required = true)
    Foo anyFooObjRequired;

    public MyClass (){ }

    public MyClass (Foo anyFooObj, Foo anyFooObjRequired){
        this.anyFooObj = anyFooObj;
        if(anyFooObj == null)
            this.anyFooObj = new Foo();
        /*
         * This is the reason why i can't let 'anyFooObj' be 'null'.
         * So 'anyFooObj' MUST be initialized somehow.
         * It's needed for some internal logic, not JAXB.
         */
        anyFooObj.required = false;

        this.anyFooObjRequired = anyFooObjRequired;
    }
}

示例对象:

Foo fooRequired = new Foo(new Bar());
MyClass myObj = new MyClass(null, fooRequired); 

当我现在尝试编码(marshal) myObj 时，它会抛出如下异常:

org.eclipse.persistence.oxm.record.ValidatingMarshalRecord$MarshalSAXParseException; 
cvc-complex-type.2.4.b: The content of element 'n0:anyFooObj ' is not complete.
One of '{"AnyNamespace":someBarObj}' is expected.

发生这种情况是因为 anyFooObj 已初始化，但它是必需的，成员 someBarObj 不是。

可能的解决方案:

我知道我可以将此方法添加到MyClass:

void beforeMarshal(Marshaller m){
    if(! anyFooObj.chosen)
        anyFooObj= null;
    }
}

但是我有很多类，并且这些类有很多非必填字段。 因此，这个解决方案需要很长时间，而且看起来也不是一个合适的解决方案。

我的问题:

有没有办法告诉 JAXB 它应该像对待 null 一样对待空对象？或者当元素设置不正确时它应该忽略它。例如这样的事情:

@XmlElement(required = false, ingnoreWhenNotMarshallable = true)
Foo anyFooObj;

注意:

我不是代码的开发人员。我只需将 JAXB 添加到项目中，并使所有内容都与给定的 XSD 文件兼容。我不允许更改类之间的关系。

Answer 1

我认为你试图让 JAXB 编码器做一些它实际上不是设计要做的事情，所以我想说你在这里进入了黑客领域。我建议推迟要求，以便从一开始就避免出现此问题。

也就是说，如果您必须这样做，那么考虑到您要求避免为每个类/字段编写代码，我认为您会希望为此使用反射 - 我在下面提供了一个示例，该示例可以反射性地检查值所有领域。

有用的扩展是:

• 让它也考虑 getter 方法
• 通过要求字段具有附加注释来选择启用 null 设置行为 - 您可以将其命名为 @JAXBNullIfEmpty

示例.java:

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import java.io.StringWriter;
import java.lang.reflect.Field;

public class Example
{
    public abstract static class JAXBAutoNullifierForEmptyOptionalFields
    {
        void beforeMarshal(Marshaller x)
        {
            try
            {
                for (Field field : this.getClass().getFields())
                {
                    final XmlElement el = field.getAnnotation(XmlElement.class);

                    // If this is an optional field, it has a value & it has no fields populated then we should replace it with null
                    if (!el.required())
                    {
                        if (JAXBAutoNullifierForEmptyOptionalFields.class.isAssignableFrom(field.getType()))
                        {
                            final JAXBAutoNullifierForEmptyOptionalFields val = (JAXBAutoNullifierForEmptyOptionalFields) field.get(
                                    this);

                            if (val != null && !val.hasAnyElementFieldsPopulated())
                                field.set(this, null); // No fields populated, replace with null
                        }
                    }
                }
            }
            catch (IllegalAccessException e)
            {
                throw new RuntimeException("Error determining if class has all required fields: " + this, e);
            }
        }


        boolean hasAnyElementFieldsPopulated()
        {
            for (Field field : this.getClass().getFields())
            {
                try
                {
                    if (field.isAnnotationPresent(XmlElement.class))
                    {
                        // Retrieve value
                        final Object val = field.get(this);

                        // If the value is non-null then at least one field has been populated
                        if (val != null)
                        {
                            return true;
                        }
                    }
                }
                catch (IllegalAccessException e)
                {
                    throw new RuntimeException("Error determining if class has any populated JAXB fields: " + this, e);
                }
            }

            // There were no fields with a non-null value
            return false;
        }
    }

    @XmlRootElement
    public static class MyJAXBType extends JAXBAutoNullifierForEmptyOptionalFields
    {
        @XmlElement
        public String someField;

        @XmlElement
        public MyJAXBType someOtherField;


        public MyJAXBType()
        {
        }


        public MyJAXBType(final String someField, MyJAXBType someOtherField)
        {
            this.someField = someField;
            this.someOtherField = someOtherField;
        }
    }


    public static void main(String[] args) throws Exception
    {
        final Marshaller marshaller = JAXBContext.newInstance(MyJAXBType.class).createMarshaller();

        MyJAXBType innerValue = new MyJAXBType(); // Unpopulated inner value
        MyJAXBType value = new MyJAXBType("some text value", innerValue);

        final StringWriter sw = new StringWriter();

        marshaller.marshal(value, sw); // Omits "someOtherField"

        System.out.println(sw.toString());
    }
}