Create and initialize a singleton with Guice

Question

I have a the following classes StorageProxy.java (Interface) StorageProxyImpl1.java (Implementation1 of StorageProxy Interface)

public class StorageProxyImpl1 extends StorageProxy{
    public void init(String key){
      ....
    }
}

public class Resource1{
    private StorageProxy sp;
    ...
}

public class Resource2{
    private StorageProxy sp;
    ...
}

I have a main method in the Application class that initializes and uses Resource1 and Resource2.

public class MyApp{
  public static void main(String args[]){
    Resource1 r1 = new Resource1();
    Resource2 r2 = new Resource2();
    runApp(r1,r2);
  }
  public void runApp(Resource1 r1, Resource2 r2){ // Makes use of r1 and r2
    ...
  }
}

I want to guice for dependency injection for the StorageProxy so that I do not have much pain when I create new implementations. I want Resource1 and Resource2 to use the same object of StorageProxy (i.e it should be Singleton). It is critical that init is separate from the constructor. Summing up, I need to create an instance of StorageProxyImpl1 and call StorageProxyImpl1.init(String) on it in the main method and this instance should be shared by Resource1 and Resource2. How to do this?

The question asked in Guice: How to get instance of Singleton without injector or using Constructor Injection is little bit on the high level and I find it difficult to understand without a concrete example.

Answer 1

You can use the provider interface

public class StorageProxyProvider implements Provider<StorageProxy> {
   public StorageProxy get() {
      StorageProxy storageProxy = new StorageProxy();
      storageProxy.init();
      return storageProxy;
   }
}

public class StorageProxyModule extends AbstractModule {
   protected void configure() {
     bind(StorageProxy.class).toProvider(StorageProxyProvider.class).in(Singleton.class);
   }
}

A working example:

public class StorageProxyProvider implements Provider<StorageProxy> {

    public StorageProxy get() {
        StorageProxy storageProxy = new StorageProxy();
        storageProxy.init();
        return storageProxy;
    }

}


public class StorageProxy {

    public void init(){
        System.out.println("init");
    }
    public void proxy(){
        System.out.println("proxy");
    }

}

public class StorageProxyModule extends AbstractModule {
    protected void configure() {
        bind(StorageProxy.class).toProvider(StorageProxyProvider.class).in(Singleton.class);
    }
}


public class Example {
    @Inject
    StorageProxy storageProxy;

    public void doSomethingThatNeedStorageProxy() {
        System.out.println(storageProxy);
        storageProxy.proxy();
    }

}

public class Main {
    public static void main(String[] args) {
        Injector injector = Guice.createInjector(new StorageProxyModule());
        Example e = injector.getInstance(Example.class);
        e.doSomethingThatNeedStorageProxy();
        e = injector.getInstance(Example.class);
        e.doSomethingThatNeedStorageProxy();
    }
}