How to convert json String to java List?

Question

My application must consume a web service, that produces an array of specific type, in json format. This is the method:

public Unidade[] getUnidades(){
    Session s = ConnectDb.getSession();
    try {
        List<Unidade> lista = new ArrayList<Unidade>(s.createQuery("from Unidade order by unidSigla").list());
        Unidade[] unidades = lista.toArray(new Unidade[0]);
        return unidades;
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    } finally {
        s.close();
    }
}

Then, in my client, it's consumed like this:

    WebResource webResource = client.resource("http://localhost:8080/RestauranteWeb/rest/unidades/");
    ClientResponse response = webResource.accept("application/json").get(ClientResponse.class);
    String output = response.getEntity(String.class);
    System.out.println(output);
    Unidade[] unidades = new Gson().fromJson(output, Unidade[].class);
    System.out.println(unidades);

The json string is retrieved correctly, but i get the following error when converting it to array:

{"unidade":[{"unidCodigo":"17","unidSigla":"BOSTA"},{"unidCodigo":"18","unidSigla":"BOSTE"},{"unidCodigo":"13","unidSigla":"bumerangui"},{"unidCodigo":"15","unidSigla":"HHH"},{"unidCodigo":"16","unidSigla":"HHH2"},{"unidCodigo":"6","unidSigla":"papapa"},{"unidCodigo":"9","unidSigla":"pobrena"},{"unidCodigo":"7","unidSigla":"sei la"},{"unidCodigo":"3","unidSigla":"TAÇINHA"},{"unidCodigo":"5","unidSigla":"tanque"},{"unidCodigo":"1","unidSigla":"UNIDADE"},{"unidCodigo":"14","unidSigla":"zerao"}]}
Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2
    at com.google.gson.Gson.fromJson(Gson.java:815)
    at com.google.gson.Gson.fromJson(Gson.java:768)
    at com.google.gson.Gson.fromJson(Gson.java:717)
    at com.google.gson.Gson.fromJson(Gson.java:689)
    at br.mviccari.client.UnidadeClient.main(UnidadeClient.java:40)
Caused by: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2
    at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:338)
    at com.google.gson.internal.bind.ArrayTypeAdapter.read(ArrayTypeAdapter.java:70)
    at com.google.gson.Gson.fromJson(Gson.java:803)
    ... 4 more

What am i doing wrong?

I see you're using the jersey client and layering GSON on top of that. Not sure if that's intentional, but the JSON integration with JAXB annotations is pretty cool. The Jersey client will automatically parse the response for you can return your object. It'd make your code a bit simpler. — Jonathan S. Fisher
– Jonathan S. Fisher, Commented Aug 4, 2013 at 14:37

jtahlborn · Accepted Answer · 2013-08-04 13:59:14Z

2

As you can see from the data and the error, the top-level json type is an object, not an array. what you actually have is a top-level object which has an array of Unidade elements. you should define something like this for your top-level class:

public class UnidadeWrapper {
    public Unidade[] unidade;
}

answered Aug 4, 2013 at 13:59

jtahlborn

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2 Comments

Mateus Viccari Over a year ago

Well, that works! I think it's odd the need to create a wrapper for the class, but if it's the best solution this is what i'm gonna do.

jtahlborn Over a year ago

@MateusViccari - it's due to how the json is written. if the top-level of the jso was an array, you wouldn't need the wrapper.

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