How to serialize a POJO into query params with Jersey

Question

I have been playing around and creating multiple small Java RESTful client libraries for different services at my company. Most of the time, I am unable to change anything on the server side and I need to write the Jersey pieces of code to interact with existing RESTful APIs.

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Context

Up to know, I have been using Jersey with Jackson to use JSON: when I query a POJO I deserialize it from JSON, and when I need to send a POJO, I serialize it into a JSON body. This two kinds of snippets have been doing the job for me up to now...

Querying and de-serialization

ClientResponse response = webResource
    .path("/path/to/resource")
    .queryParam("key", "value")
    .accept(Mediatype.APPLICATION_JSON)
    .get(ClientResponse.class);

// (...) Check response status code

MyClassPojo pojo = response.getEntity(MyClassPojo.class);

Serialization and sending

ClientResponse response = webResource
    .path("/path/to/resource")
    .type(Mediatype.APPLICATION_JSON_TYPE)
    .accept(Mediatype.APPLICATION_JSON)
    .post(ClientResponse.class, pojo)

// (...) Check response status code

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Issue

I am now facing a RESTful server that does not accept JSON bodies for sending my POJOs. The only thing that seem to work is to use query parameters.

For instance, if I want to send the object

public MyClassPojo {
    public int attr1;
    public String attr2;
}

MyClassPojo pojo = new MyClassPojo();
pojo.attr1 = 42;
pojo.attr2 = "Foo bar";

I would have loved to serialize it in JSON:

{
    "attr1": 42,
    "attr2": "Foo bar"
}

But this specific RESTful server is expecting query params:

?attr1=42&attr2=Foo+bar

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Question

This kinda sucks, but I don't really have a choice... and I am now hoping that there is an easy way to achieve this with Jersey: how can I automatically serialize objects as query parameters, to send to a RESTful server?

Note: I closed this question as @Jukka answered it. Don't hesitate referring to a new question you create, if like me you are actually looking for a way to send x-www-form-urlencoded data. I am about to get something working...

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Update

Following an idea from @Jukka I wrote the following method:

public MultivaluedMap<String, String> toQueryParams() {
    final MultivaluedMap<String, String> queryParams = new Form();
    final Field[] fields = getClass().getDeclaredFields();
    for (Field field : fields) {
        final boolean accessible = field.isAccessible();
        try {
            field.setAccessible(true);
            final Object value = field.get(this);
            if (value != null) {
                final String name = field.getName();
                queryParams.add(name, value.toString());
            }
        } catch (IllegalAccessException e) {
            LOGGER.error("Error accessing a field", e);
        } finally {
            field.setAccessible(accessible);
        }
    }
    return queryParams;
}

This is a great starting point, and would work perfectly if you actually do need Query Params. In my case, I got confused and I actually need a x-www-form-urlencoded! For that purpose I had to write a MessageBodyWriter!

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My form encoding provider

@Produces(MediaType.APPLICATION_FORM_URLENCODED)
public class MyFormEncodingProvider implements MessageBodyWriter<Object> {
    private static final String ENCODING = "UTF-8";

    @Override
    public boolean isWriteable(Class<?> aClass, Type type, Annotation[] annotations, MediaType mediaType) {
        return true;
    }

    @Override
    public long getSize(Object obj, Class<?> aClass, Type type, Annotation[] annotations, MediaType mediaType) {
        return -1;
    }

    @Override
    public void writeTo(Object obj, Class<?> aClass, Type type, Annotation[] annotations, MediaType mediaType,
                        MultivaluedMap<String, Object> stringObjectMultivaluedMap, OutputStream outputStream) throws IOException, WebApplicationException {
        final Writer osWriter = new OutputStreamWriter(outputStream);
        final MultivaluedMap<String, String> fieldsAndValues = getFieldsAndValues(obj);

        boolean firstVal = true;
        for (Entry<String, List<String>> entry : fieldsAndValues.entrySet()) {
            final List<String> values = entry.getValue();
            if (values == null || values.size() == 0) {
                if (!firstVal) {
                    osWriter.write("&");
                }
                osWriter.write(entry.getKey() + "=");
                firstVal = false;
            } else {
                for (String value : values) {
                    if (!firstVal) {
                        osWriter.write("&");
                    }
                    osWriter.write(entry.getKey() + "=" + URLEncoder.encode(value, ENCODING));
                    firstVal = false;
                }
            }
        }
        osWriter.flush();
        osWriter.close();
    }

    private static MultivaluedMap<String, String> getFieldsAndValues(Object obj) {
        // Find all available fields
        final Collection<Field> allFields = new ArrayList<>();
        Class<?> clazz = obj.getClass();
        while (clazz != null && clazz != Object.class) {
            Collections.addAll(allFields, clazz.getDeclaredFields());
            clazz = clazz.getSuperclass();
        }

        // Get all non-null values
        final MultivaluedMap<String, String> queryParams = new Form();
        for (Field field : allFields) {
            final boolean accessible = field.isAccessible();
            try {
                field.setAccessible(true);
                final Object value = field.get(obj);
                if (value != null) {
                    final String name = field.getName();
                    queryParams.add(name, value.toString());
                }
            } catch (IllegalAccessException e) {
                Logger.getLogger(AbstractIMSPojo.class).error("Error accessing a field", e);
            } finally {
                field.setAccessible(accessible);
            }
        }
        return queryParams;
    }
}

Answer 1

I would just implement this kind of view to your POJO:

class Pojo {
    ...
   public MultiValuedMap<String,String> asQueryParams() {
       ...
   }
}

and pass the result to WebResource.queryParams(..).