Question

I’m thinking of implementing Objectify DAO with dependency injection, such that I can maintain my code to access the same “Dao” while the implementation may change from Objectify to Hibernate-MySQL or MongoDb in the future without me worrying on changing any code in the UI or client side.

UserDao is based on the example here:
http://turbomanage.wordpress.com/2010/01/28/simplify-with-objectify/

UserObjectifyDaoImpl implements Dao<User> {

 private UserDao dao = null; 

 public void put(User entity) {
    if (dao == null) {
       dao = new UserDao(); 
    }
    dao.put(entity);
 }
 // other put and set methods
}

Such that, I have the context.xml:

<bean id="userDao" class="com.example.server.daoimpl.UserObjectifyDaoImpl">
    <property name="dataSource" ref="dataSource"/>
</bean>

And if I need to change the implementation, I just need to change this bean from UserObjectifyDaoImpl to something like:

UserHibernateDaoImpl or UserMongoDBDaoImpl or whatever implementation saving to whatever database.

And still have my code in the UI / Client intact, like:

WebApplicationContext ctx = WebApplicationContextUtils.getWebApplicationContext(getServletContext());

Dao dao = (Dao) ctx.getBean("userDao");
dao.put(something);

One reason I need to do this right now, I need to develop using app engine (via objectify), however in the future I may need to change some data access objects to hibernate and some to mongodb (so its a mix).

I haven’t tested this code, will this strategy work?

Answer 1

Yes, this will work. In fact this is one of the major reasons why DI and coding to an interface was invented. Just make sure that all DAO implementations follow the same contract (DAOs very often introduce leaky abstractions).

Also you have several other options to achieve the same goal:

• Several @Service annotated classes with one marked as @Primary (if you are using autowiring)

• Spring profiles and selective activation of beans

BTW if you are considering switching to a different DAO implementation, have a look at CrudRepository from Spring Data. Spring Data project provides several modules that implement this interface for MongoDB, Neo4J, JPA, etc.

For the time being it seems like several Spring Data modules do not play together nicely (see: DATAJPA-146), so if you choose to implement CrudRepository make sure this issue is fixed or you can work it around. Thanks to @iddqd for pointing that out.