Sean February 2016

Marshall/Unmarshall Nested Map with JaxB

There are several previous questions around using JaxB to marshall/unmarshall a java.util.Map, many of which get pointed back to this example, which works great:

http://blog.bdoughan.com/2013/03/jaxb-and-javautilmap.html

However, I can't get JaxB to be able to marshall/unmarshall instances of Map if the map is not a member of the @XmlRootElement. For example, here's a root element class,

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public static class Customer {

    private MyField myField

    MyField getMyField() {
        return myField
    }

    void setMyField(MyField myField) {
        this.myField = myField
    }

}

The definition of it's field class:

@XmlAccessorType(XmlAccessType.FIELD)
public static class MyField{

    Map<String, String> getSomeMap() {
        return someMap
    }

    void setSomeMap(Map<String, String> someMap) {
        this.someMap = someMap
    }

    @XmlElement
    private Map<String, String> someMap = new HashMap<String, String>()
}

And some code to drive the marshalling:

    JAXBContext jc = JAXBContext.newInstance(Customer.class)

    Customer customer = new Customer()
    MyField myField1 = new MyField()
    myField1.someMap.put("foo", "bar")
    myField1.someMap.put("baz", "qux")
    customer.myField =  myField1

    Marshaller marshaller = jc.createMarshaller()
    marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true)
    marshaller.marshal(customer, System.out)

This example results in:

java.util.Map is an interface, and JAXB can't handle interfaces.
java.util.Map does not have a no-arg default constructor.

I am writing my code in Groovy rather than Java, but I don't think it should make much of a difference.

Answers


pczeus February 2016

I have experienced this before myself. Bottom line is that the warning is telling you exactly the problem. You have defined your field as type java.util.Map. JAXB does not support interfaces. To correct your problem, you need to change the declaration of your field to a concrete Map type like:

private HashMap<String, String> someMap = new HashMap<String, String>()

Your other option is described in the link you referenced. You need to have a

MapAdapter class as referenced in the link you provided and then include that in the annotation, hinting to JAXB how it should marshal/unmarshal the Map type.

I think this link gives a clearer example of how to create and implement the MapAdapter:

JAXB: how to marshall map into <key>value</key>


pczeus February 2016

I was able to encounter the same behavior using JAXB by creating a TestController of type @RestController, using Spring Boot.

import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController

@RestController
@RequestMapping(value = "test")
class TestController {

    @RequestMapping(value = "findList")
    List findList() {
        ["Test1", "Test2", "Test3"] as ArrayList<String>
    }

    @RequestMapping(value = "findMap")
    Map findMap() {
        ["T1":"Test1", "T2":"Test2", "T3":"Test3"] as HashMap<String,String>
    }

    @RequestMapping(value = "")
    String find(){
        "Test Something"
    }
}

With JAXB as the default implementation in SpringBoot, I could reproduce the issue that the /test/findList would correctly render XML, but /test/findMap would generate an error as described in the initial posting.

For me, the solution to the problem is to switch the XML rendering library to Jackson (there are others like XStream as well).

Using Gradle for the build file (build.gradle), I simply add the Jackson dependencies, very similar to how you would if using Maven:

'com.fasterxml.jackson.core:jackson-core:2.7.1', 'com.fasterxml.jackson.core:jackson-annotations:2.7.1', 'com.fasterxml.jackson.core:jackson-databind:2.7.1-1', 'com.fasterxml.jackson.dataformat:jackson-dataformat-xml:2.7.1', 'org.codehaus.woodstox:woodstox-core-asl:4.4.1',


Sean February 2016

The answer to the specific issue I was having ended up being removing the @XmlElement annotation from the Map field like so:

@XmlAccessorType(XmlAccessType.FIELD)
public static class MyField{

  Map<String, String> getSomeMap() {
      return someMap
  }

  void setSomeMap(Map<String, String> someMap) {
      this.someMap = someMap
  }

  //@XmlElement Remove this annotation
  private Map<String, String> someMap = new HashMap<String, String>()
}  

Without that annotation, the marshalling/unmarshalling works fine, and still interprets the Map as an XmlElement - there seems to be a bug with that annotation specifically. However, as @dlcole points out, an alternative (that would allow you to have more control over the format of your serialized representation) is to use Jackson rather than JAXB.

Post Status

Asked in February 2016
Viewed 2,751 times
Voted 5
Answered 3 times

Search




Leave an answer