将动态ArrayList转换为Json

Question

I want to convert an array list to json string of a specific format. I get all the users emails in an array list and want to convert it into the following format of JSON.

 [
  {"email":"abc@gmail.com"},
  {"email":"xyz@gmail.com"}
 ]

My controller action is

 public static Result apiCustomers(){
    List<Customer> customerList = Model.coll(Customer.class).find().toArray();
    List<String> emails = new ArrayList<String>();

    for(Customer c : customerList){
        emails.add(c.email);
    }

    //ObjectNode result = Json.newObject();
    //result.put("emails", Json.toJson(emails));        
    return ok();
}

How can I convert the emails list to the above json format?

Thanks in advance

Answer 1

Why use another JSON ser/des lib? Play has one built-in (a wrapper around jackson - which is really fast).

Starting from your code:

public static Result apiCustomers(){
    List<Customer> customerList = Model.coll(Customer.class).find().toArray();
    List<String> emails = new ArrayList<String>();

    for(Customer c : customerList){
        emails.add(c.email);
    }

    return ok(Json.toJson(emails));
}

This uses some defaults but should be enough.

Or manually:

public static Result apiCustomers(){
        ArrayNode arrayNode = new ArrayNode(JsonNodeFactory.instance);

        List<Customer> customerList = Model.coll(Customer.class).find().toArray();

        for(Customer c : customerList){
            ObjectNode mail = Json.newObject();
            mail.put("email", c.email);
            arrayNode.add(mail);
        }

        return ok(arrayNode);
}

No need for Gson.

Answer 2

You can use this library: http://code.google.com/p/google-gson/

Pretty easy tutorial about it here: http://www.mkyong.com/java/how-do-convert-java-object-to-from-json-format-gson-api/

Or you can write a custom toJson method to your classes or an util Json (not a big deal)

in your case it should be something like that (i didn't test it):

public String toJson(List<String> emails) {
    StringBuilder result = new StringBuilder();
    result.append("[");
    result.append("\n");

    for (String s : emails) {
        result.append("{");
        result.append("\"email\":");
        result.append("\"");
        result.append(s);
        result.append("\"");
        result.append("}");
        result.append(",");
        result.append("\n");
    }
    result.append("]");
    return result.toString();
}

Answer 3

With the benefit of Java 8 (and I suspect a newer Jackson version):

private static final ObjectMapper mapper = new ObjectMapper();

...

List<Customer> customerList = Model.coll(Customer.class).find().toArray();
ArrayNode emails = mapper.createArrayNode();
customerList.forEach(c -> emails.add(c.email));