JAXB 忽略 HashMap 属性？

Question

I'm using the JAXB implementation that comes with J2SE to serialize a bean that contains a HashMap property. I would assume that this should work out of the box since this states

JAXB spec defines a special handling for Map when it's used as a propety of a bean. For example, the following bean would produce XMLs like the following: ...

This more or less works unless the structure has more than one level, ie the HashMap is a property of a bean that is a property of a bean - like this:

import java.util.HashMap;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.JAXB;

class bean { 
    @XmlElement public HashMap<String,String> map;
}

@XmlRootElement class b2 { 
    @XmlElement public bean b;
}


class foo { 
    public static void main(String args[]) { 
        try { 
            bean b = new bean();
            b.map = new HashMap<String,String>();
            b.map.put("a","b");
            b2 two = new b2();
            two.b=b;
            JAXB.marshal(two, System.out);  

        } catch (Exception e) { 
            System.out.println("Exception: " + e.toString());
        }
    }
}

This outputs <?xml version="1.0" encoding="UTF-8" standalone="yes"?><b2><b><map/></b></b2> instead of a correctly formatted HashMap. It works if I annotate bean with @XmlRootElement and remove the @XmlElement from map , but I don't see why that should be necessary. Is it supposed to be like that?

Answer 1

The explanation is given on the website you linked:

Unfortunately, as of 2.1, this processing is only defined for bean properties and not when you marshal HashMap as a top-level object (such as a value in JAXBElement.) In such case, HashMap will be treated as a Java bean, and when you look at HashMap as a bean it defines no getter/setter property pair, so the following code would produce the following XML:

Bean with Map:

m = new HashMap();
m.put("abc",1);
marshaller.marshal(new JAXBElement(new QName("root"),HashMap.class,m),System.out);

XML representation:

<root />

This issue has been recorded as #223 and the fix needs to happen in later versions of the JAXB spec.