How to wrap a List as top level element in JSON generated by Jackson

Question

I am running into a problem where I am trying to include a List as the root node, but I can't seem to be able to get this. Let me explain. Let's say we have a class "TestClass"

class TestClass{
    String propertyA;       
}

Now, in some utility method this is what I do

String utilityMethod(){
   List<TestClass> list = someService.getList();
   new ObjectMapper().writeValueAsString(list); 
}

The output I am trying to get in JSON is

{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}

I have tried to use

objMapper.getSerializationConfig().set(Feature.WRAP_ROOT_VALUE, true);

But, I still don't seem to get it right.

Right now, I am just creating a Map < String,TestClass > and I write that to achieve what I am trying to do, which works but clearly this is a hack. Could someone please help me with a more elegant solution? Thanks

Answer 1

Unfortunately, even with the WRAP_ROOT_VALUE feature enabled you still need extra logic to control the root name generated when serializing a Java collection (see this answer for details why). Which leaves you with the options of:

• using a holder class to define the root name
• using a map.
• using a custom ObjectWriter

Here is some code illustrating the three different options:

public class TestClass {
    private String propertyA;

    // constructor/getters/setters
}

public class TestClassListHolder {

    @JsonProperty("ListOfTestClasses")
    private List<TestClass> data;

    // constructor/getters/setters
}

public class TestHarness {

    protected List<TestClass> getTestList() {
        return Arrays.asList(new TestClass("propertyAValue"), new TestClass(
                "someOtherPropertyValue"));
    }

    @Test
    public void testSerializeTestClassListDirectly() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        mapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
        System.out.println(mapper.writeValueAsString(getTestList()));
    }

    @Test
    public void testSerializeTestClassListViaMap() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        final Map<String, List<TestClass>> dataMap = new HashMap<String, List<TestClass>>(
                4);
        dataMap.put("ListOfTestClasses", getTestList());
        System.out.println(mapper.writeValueAsString(dataMap));
    }

    @Test
    public void testSerializeTestClassListViaHolder() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        final TestClassListHolder holder = new TestClassListHolder();
        holder.setData(getTestList());
        System.out.println(mapper.writeValueAsString(holder));
    }

    @Test
    public void testSerializeTestClassListViaWriter() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        final ObjectWriter writer = mapper.writer().withRootName(
                "ListOfTestClasses");
        System.out.println(writer.writeValueAsString(getTestList()));
    }
}

Output:

{"ArrayList":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}
{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}
{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}
{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}

Using an ObjectWriter is very convenient - just bare in mind that all top level objects serialized with it will have the same root name. If thats not desirable then use a map or holder class instead.

Answer 2

I'd expect the basic idea to be something like:

class UtilityClass {
    List listOfTestClasses;
    UtilityClass(List tests) {
        this.listOfTestClasses = tests;
    }
}

String utilityMethod(){
    List<TestClass> list = someService.getList();
    UtilityClass wrapper = new UtilityClass(list);
    new ObjectMapper().writeValueAsString(wrapper); 
}