`XmlMapper` serializes `@JsonAppend` property twice

[cowtowncoder]

Discussed in https://github.com/FasterXML/jackson-databind/discussions/3806

^{Originally posted by **stepince** March 5, 2023} XmlMapper is serializing jsonAppend virtual property twice. ObjectMapper for json works correctly. jackson version: 2.14.1 ``` public class VirtualBeanPropertyWriterTest { @Test public void testJsonAppend() throws Exception { ObjectMapper mapper = new XmlMapper(); String xml = mapper.writeValueAsString(new Pojo("foo")); assertEquals("foobar",xml); } @JsonAppend(props = @JsonAppend.Prop(name = "virtual", value = MyVirtualPropertyWriter.class)) public static class Pojo { private final String name; public Pojo(String name) { this.name = name; } public String getName() { return name; } } public static class MyVirtualPropertyWriter extends VirtualBeanPropertyWriter { public MyVirtualPropertyWriter() {} protected MyVirtualPropertyWriter(BeanPropertyDefinition propDef, Annotations contextAnnotations, JavaType declaredType) { super(propDef, contextAnnotations, declaredType); } @Override protected Object value(Object bean, JsonGenerator jgen, SerializerProvider prov) throws Exception { return "bar"; } @Override public VirtualBeanPropertyWriter withConfig(MapperConfig config, AnnotatedClass declaringClass, BeanPropertyDefinition propDef, JavaType type) { return new MyVirtualPropertyWriter(propDef, declaringClass.getAnnotations(), type); } } } ``` output ``` org.opentest4j.AssertionFailedError: Expected :`foobar` Actual :`foobarbar`

[cowtowncoder]

Sounds like a flaw. Thank you for reporting @stepince

[stepince]

Any workaround for this issue? I am actually using a mixin class for the serialization. Maybe in VirtualBeanPropertyWriter Impl check if the property has been written. Not sure if this is possible?

[cowtowncoder]

@stepince Without knowing why it's failing it is hard to suggest any workaround. But no, writer should not really have to check for anything, backend should not write duplicates.

I hope I'll have time to look into this in near future -- existence of unit test should be great help.

[mukham12]

Hi @cowtowncoder,

Not sure if this helps, but I was able to track down where the property is getting set twice. It is in this file (AnnotationIntrospectorPair.java) lines 594 and 595. The first virtual prop is getting set when _primary calls findAndAddVirtualProperties and also the second time when _secondary calls findAndAddVirtualProperties. Thanks.

[cowtowncoder]

@mukham12 that is indeed helpful! Question then being why do we have 2 JacksonAnnotationIntrospector instances registered (-Pair is used to combine 2 AIs).

[cowtowncoder]

Ah. That is because JacksonXmlModule inserts our special JacksonXmlAnnotationIntrospector; it won't (and basically can't) replace plain JacksonAnnotationIntrospector. But what that means, then, is that it doubles up this call.

Hmmh. In a way fix is easy, although I need to think it through -- basically have a switch to prevent some of the calls to super class. The reason for a switch (as opposed to just blocking) is that this implementation may be used directly as well, as a replacement of JacksonAnnotationIntrospector (instead of augmenting) -- if so, calls should be handled by base implementation.

But at least I now know how to implement this.

[mukham12]

Happy to help.

I may be able to help with the implementation if you can just nudge me in the right direction and lay out the overall plan. Thanks.

[cowtowncoder]

Fixed by simply overriding and blocking findAndAddVirtualProperties() in JacksonXmlAnnotationIntrospector. Hopefully works well enough; it's bit of a design flaw (this particular method in AnnotationIntrospector)

[stepince]

Thx Tatu. I see that the fix will be released in 2.15 . I don't see a release date for 2.15.

[cowtowncoder]

@stepince Project does not have pre-planned release dates; releases occur when versions are considered ready.

Having said that, general expectations are updated sometimes on project release page:

https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.15