How to de-serialize XMLGregorianCalender with Gson?

问题

When processing a Json string with XMLGregorianCalender using Gson I get the exception:

java.lang.RuntimeException: Failed to invoke public javax.xml.datatype.XMLGregorianCalendar() with no args

The object which is de-serialized by fromJson(..) with Gson has a XMLGregorianCalender object.

What can be the possible solution for the above error?

回答1:

Abstract class javax.xml.datatype.XMLGregorianCalendar can not be a instantiated by its default/no-args constructor which makes GSON to fail.

If you resolve the class that extends above mentioned and that class has a public no-args constructor you can de-serialize that directly. For example, in my case:

XMLGregorianCalendar xmlGC = gson.fromJson(strXMLGC,
        com.sun.org.apache.xerces.internal
            .jaxp.datatype.XMLGregorianCalendarImpl.class);

A generic way to get it working everywhere - without caring the implementing class - is to define a custom JsonDeserializer. To make de-serialing easy you could first create an adapter class that holds the data that XMLGregorianCalendar's JSON has:

@Getter
public class XMLGregoriancalendarAdapterClass {
    private BigInteger year;
    private int month, day, timezone,  hour, minute, second;
    private BigDecimal fractionalSecond;        
}

Data types of each field in above class are chosen to match one specific method for constructing XMLGregorianCalendar with javax.xml.datatype.DatatypeFactory.

Having above adapter class create a de-serialiazer like:

public class XMLGregorianCalendarDeserializer
        implements JsonDeserializer<XMLGregorianCalendar> {
    @Override
    public XMLGregorianCalendar deserialize(JsonElement json, Type typeOfT,
            JsonDeserializationContext context) throws JsonParseException {
        // Easily parse the adapter class first
        XMLGregoriancalendarAdapterClass ac = 
                new Gson().fromJson(json, 
                        XMLGregoriancalendarAdapterClass.class);
        try {
            // Then return a new newXMLGregorianCalendar
            // using values in adapter class
            return DatatypeFactory.newInstance()
                    .newXMLGregorianCalendar(ac.getYear(), ac.getMonth(), 
                            ac.getDay(), ac.getHour(),        
                            ac.getMinute(), ac.getSecond(),
                            ac.getFractionalSecond(), ac.getTimezone());
        } catch (DatatypeConfigurationException e) {
            e.printStackTrace();
        }
        return null;
    }
}

Using above you can construct GSON like:

Gson gson = new GsonBuilder().setPrettyPrinting()
                .registerTypeAdapter(
                     XMLGregorianCalendar.class,
                     new XMLGregorianCalendarDeserializer() )
                .create();

after which it is just:

XMLGregorianCalendar xmlGC2 = 
        gson.fromJson(json, YourClassHavingXMLGregorianCalendar.class);

来源：https://stackoverflow.com/questions/48704930/how-to-de-serialize-xmlgregoriancalender-with-gson

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xmlgregoriancalendar