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Table of Content

Java REST Backend

This is a utility library for a Google AppEngine Java REST backend. It’s fully managed by Maven.

The API specification which is implemented can be found here http://cloudburo.github.com/docs/opensource/jsonapispec/

Source Code Location

Fork me on GitHub: https://github.com/talfco/clb-appEngineBackend

Maven Repository

Add the following entry to your POM:

        <name>Cloudburo Maven Repo Snapshot on Github</name>  
        <name>Cloudburo Release Repo on Github</name>  

And the following dependency


Implementation Aspects

GSON Converters

The Cloudburo library uses Gson as a library to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of. It’s a open source library (Apache License 2.0).

Within the package con.cloudburo.servlet there is the GsonWrapper class which provides specific serializer- and deserializer for the following conversions

  • DateTime Type
  • Date Type
  • LocalDateTime Type
  • Objectify Key Type

Special care has to be taken for the Objectify Key Type, which cannot be deserialized by the default GSON implementation. There are various questions posted to this topic in the Google Groups Support forum, as well as Stack Overflow, as one comment stated

Private fields, immutable objects, and odd constructors are all common problems in the world of deserializers.

I’ve chosen the following approach to deserialized Objectify Key attributes via the GSON library.

The following sample shows a complex Objectify class which will got persisted. It also contains a Key attribute referencing an entity Service.

    public class ProductType {
      public @Id Long _id;
      public String name = "";
      public String category = "";
      public String description = "Test this out";

      public ProductType.Relations relations;
      static class Relations {
          static class ProductTypeServiceConfig {
              public @Id Long _id;
              public enum Periodicity {  M, W, V, D};   
              public Periodicity periodicity;
              public Integer count;
                public Key<Service> service;
          public List<ProductTypeServiceConfig> productTypeServiceConfigs;

In order to deserialize this object the following strategy was chosen.

Objectify key with 2 constructor parameters

  • The creation of a key element is dependent on a Long id or a String name as well as a kind, e.g Key<Car> rootKey = Key.create(Car.class, 959);
  • In the above example a Objectify a service identifier would look like something like ‘[Service(“5066549580791808”)]’, a combination of type and identifier.

The serialization step will now only return the identifier or name to the consumer, to be inline with any other identifier type used in the protocol.

       public JsonElement serialize(Key key, Type type, JsonSerializationContext serialContext) {
         logger.log(Level.INFO,"Serialize "+key);
         if (key.getId() == 0)
           return new JsonPrimitive(key.getName());
           return new JsonPrimitive(key.getId());

The deserialization step has now to take into consideration the type information as well in order to create a correkt Key instance, by inspecting the type input parameter.

    public Key deserialize(JsonElement element, Type type,  JsonDeserializationContext deserialContext) throws JsonParseException {
      // When we have a Parent Key dependency we have to pass in the parent key - tbd
          StringTokenizer tok = new StringTokenizer(element.getAsString(),";");
        String className = ((ParameterizedType)type).getActualTypeArguments()[0].toString(); 
        if (className.startsWith("class ")) { className = className.substring("class ".length()); }
        logger.log(Level.INFO,"Deserizalize Objectify key:"+element.getAsString()+" "+className);
        try {
           Class clazz = Class.forName(className.toString());
              if (tok.countTokens() > 0) 
                 String parentKey = tok.nextToken();
                 // TODO We have to include here the parent !!!
                 return Key.create(clazz,Long.parseLong(element.getAsString()));
              else {
                 return Key.create(clazz,Long.parseLong(element.getAsString())); 
      } catch (Exception e) {
             throw new JsonParseException("ObjectifKey conversion failed "+e.getMessage());

Objectify key with 3 constructor parameters (Parent Key)

Will be provided in a later build


A simple example code can be found in the directory `src/test/java/com/cloudburo/entity'

Entity Class

Simply define for each Entity to be persisted an Objectify annotated based class . Make sure to have each attribute defined as public

    package com.cloudburo.entity;

    import com.googlecode.objectify.annotation.Entity;
    import com.googlecode.objectify.annotation.Id;
    import com.googlecode.objectify.annotation.Index;
    import java.util.Date;
    import org.joda.time.LocalDateTime;

    public class Customer {
        @Id public Long _id;
        @Index public String name;
        @Index public String surname;
        @Index public String email;
        public String address;
        public String plz;
        public String location;
        public Date date;
        public LocalDateTime date1;
        public String telephone;
        public String birthdate;
        public String citizenship;
        public String mobile;

OfyService Class

As a second class extend the OfyService class and register your class.

    package com.cloudburo.entity;
    import com.googlecode.objectify.Objectify;
    import com.googlecode.objectify.ObjectifyService;
    import com.cloudburo.servlet.OfyService;

    public class MyOfyService extends OfyService {
        static {
        /** Mandatory to override the method **/
        public static Objectify ofy() { return ObjectifyService.ofy(); }


Servlet Class

Finally provide a Servlet class which extends the abstract RestAPIServlet class and you are done.

    package com.cloudburo.entity;
    import com.cloudburo.entity.Customer;
    import com.cloudburo.entity.MyOfyService;
    import com.cloudburo.servlet.RestAPIServlet;
    import com.googlecode.objectify.Objectify;

    public class CustomerServlet extends RestAPIServlet {

        protected Class getPersistencyClass() {
            return Customer.class;

        protected   Objectify ofy() {
            return MyOfyService.ofy();