Dependency Injection into your Singleton

Question

I have a singleton that has a spring injected Dao (simplified below):

public class MyService<T> implements Service<T> {
    private final Map<String, T> objects;
    private static MyService instance;

    MyDao myDao;

    public void set MyDao(MyDao myDao) {
        this. myDao = myDao;
    }

    private MyService() {
        this.objects = Collections.synchronizedMap(new HashMap<String, T>());
        // start a background thread that runs for ever
    }

    public static synchronized MyService getInstance() {
        if(instance == null) {
            instance = new MyService();
        }
        return instance;
    }

    public void doSomething() {
        myDao.persist(objects);
    }
}

My spring config will probably look like this:

 <bean id="service" class="MyService" factory-method="getInstance"/>

But this will instantiate the MyService during startup.

Is there a programmatic way to do a dependency injection of MyDao into MyService, but not have spring manage the MyService?

Basically I want to be able to do this from my code:

MyService.getInstance().doSomething();

while having spring inject the MyDao for me.

Answer 1

Here is a solution, create a class with a static factory method:

public class MyService {
    private static MyService instance;

    private MyDao myDao;

    public static MyService createInstance(final MyDao myDao) {
      instance = new MyService(myDao);
      return instance;
    }

    private MyService(final MyDao myDao) {
      this.myDao = myDao;
    }

    public static synchronized MyService getInstance() {
      return instance;
    }

    public void doSomething() {
      // just do it!
      myDao.justDoIt();
    }
}

and use spring to initilize it:

  <bean class="my.path.MyService" factory-method="createInstance" scope="singleton">
    <constructor-arg ref="reference.to.myDao" />
  </bean>

and now you should be able to do:

MyService.getInstance().doSomething();

without any problems.

Answer 2

If you want a singleton, why not just define that one class in the Spring configs, and it's automatically a singleton (by default).

To avoid initialising at start up, have you looked at Spring lazy initialisation ? Basically you need:

lazy-init="true"

in your bean definition.