@JacksonXmlRootElement malfunction when using it with XmlMapper in multi-thread environment

[wanglingsong]

This is the sample to reproduce the bug. The "localName" override is failed in some of thread, printing original class name "TestModel" instead of "model". XmlMapper seems not thread safe.

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlRootElement;

@JacksonXmlRootElement(localName = "model")
public class TestModel {


    public static void main(String[] s) throws Exception {

        final ObjectMapper xmlMapper = new XmlMapper();

        for (int i=0;i < 100;i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        System.out.println(xmlMapper.writeValueAsString(new TestModel()));
                    } catch (JsonProcessingException e) {
                        e.printStackTrace();
                    }
                }
            }).start();
        }

        Thread.sleep(10000L);

    }

}

Output:

<model/>
<model/>
<model/>
<model/>
<model/>
<model/>
<model/>
<TestModel/>
<model/>
<model/>
<model/>
<model/>

[cowtowncoder]

Whoa. That is interesting... I assume this is with 2.6.3?

[wanglingsong]

Yes

[cowtowncoder]

I can reproduce this. No obvious cause, looking at code, but should not be too difficult to find.

[cowtowncoder]

Ok. Yes, interesting. Bug comes from a combination of factors, and was introduced in 2.5, most likely.
Basically, a subset of AnnotatedClass instances (ones for which only class annotations are checked, like for use with root name discovery) may be cached (start with 2.5.0). Further, class annotation lookup is actually done lazily. And finally, since (earlier) assumption was that access to AnnotatedClass is not multi-threaded, this becomes problematic.

I'll try to get this fixed; 2.6 probably needs bit different fix than what may be done for 2.7, but both should be addressable.