返回列表时“找不到MessageBodyWriter” <Long> 与JAX-RS中的响应对象

Question

I am currently trying to create a rest-webservice which returns an object of the type List< Long> . The service is annotated with @Produces(MediaType.APPLICATION_JSON) .

I don't have any problems with the other services in this project.

If I write the code as following:

List<Long> list = new ArrayList<Long>();
            list.add(100L);
            list.add(200L);
            list.add(300L);

return Response.ok(list)
               .build();

Then I get the following message in the log:

MessageBodyWriter not found for media type=application/json, type=class java.util.ArrayList, genericType=class java.util.ArrayList.

If I instead write it as following (same as I did for another service in the project, but where it is a List of pojo objects):

List<Long> list = new ArrayList<Long>();
            list.add(100L);
            list.add(200L);
            list.add(300L);

GenericEntity<List<Long>> genericEntity = new GenericEntity<List<Long>>(list){};

return Response.ok(genericEntity)
               .build();

Then I get the following message instead:

MessageBodyWriter not found for media type=application/json, type=class java.util.ArrayList, genericType=java.util.List<java.lang.Long>.

After googling I found the following suggestion to convert the List to an array as seen below, but this didn't work either.

List<Long> list = new ArrayList<Long>();
            list.add(100L);
            list.add(200L);
            list.add(300L);

            return Response.ok(list.toArray()).build();

Then I get:

MessageBodyWriter not found for media type=application/json, type=class [Ljava.lang.Object;, genericType=class [Ljava.lang.Object;.

We are using jackson-core version 2.3.2 and the jax-rs implementation is Jersey which is included with Weblogic 12c.

Is there any good solution for this (without using any other libraries)?

Answer 1

As you are producing json type data, Json related jar is required for parsing Object in to json form.

Add Jackson jar to your pom.xml if it is Maven project.

add this dependency..

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.9.6</version>
</dependency>

Or

download the jar and add it to class path.

Answer 2

You did not mention which JAX-RS framework you're playing with, but according to Apache CXF documentation for instance, if you want to use Jackson as JSON Provider, you need this dependency:

<dependency>
  <groupId>com.fasterxml.jackson.jaxrs</groupId>
  <artifactId>jackson-jaxrs-json-provider</artifactId>
  <version>2.9.3</version>
</dependency>

Then register one of the Jackson Providers on your JAX-RS service, I guess the one for POJOs in your case, not the one for JAXB beans (WARNING: the package name in CXF documentation is for Jackson 1.x, I replaced with the one for Jackson 2.x):

<jaxrs:providers>
   <bean class="com.fasterxml.jackson.jaxrs.json.JacksonJsonProvider"/>
</jaxrs:providers>

For Jersey, it would be a different dependency .

Answer 3

While returning a List< Long> in a Response object doesn't work for me I have managed to come around it by embedding the list in a new class.

The new class looks as following:

public class LongArrayList {

    private List<Long> list;

    public LongArrayList() {

    }

    public LongArrayList(List<Long> list) {
        this.setList(list);
    }

    public List<Long> getList() {
        return list;
    }

    public void setList(List<Long> list) {
        this.list = list;
    }
}

I then return this list as following:

return Response.ok(new LongArrayList(theOldLongList))
               .build();

Unfortunatelly this returns a little bigger json than I would want:

{"list":[5,12,29,30]}

If anybody finds a better solution I will accept it as an answer for this question instead.