What's the difference between @Singleton and @ApplicationScoped? Confused.

[seto]

public static void main(String[] args) {

  CdiContainer cdiContainer = CdiContainerLoader.getCdiContainer();
   cdiContainer.boot();

   // Starting the application-context enables use of @ApplicationScoped beans
   ContextControl contextControl = cdiContainer.getContextControl();
   contextControl.startContext(ApplicationScoped.class);

   // You can use CDI here
   Kernel k = BeanProvider.getContextualReference(Kernel.class,false);
   k.show();
   System.out.println(k);
   System.out.println(k.nameCache);
   k.show();
   cdiContainer.shutdown();
}

Here's a simple application with DeltaSpike and Weld. I'm using it in Java SE.

public static void main(String[] args) {

  Weld weld = new Weld();

   WeldContainer container = weld.initialize();
// BeanManager beanManager = container.getBeanManager();
// Bean<Kernel> bean = (Bean<Kernel>) beanManager.resolve(beanManager.getBeans(Kernel.class));
// Kernel k = (Kernel) beanManager.getReference(bean, bean.getBeanClass(), beanManager.createCreationalContext(bean));

// Kernel k = beanManager.getContext(bean.getScope()).get(bean, beanManager.createCreationalContext(bean)); // this will work because it refer to the actual object instead of proxy

   Kernel k = container.select(Kernel.class).get();
   k.show();
   System.out.println(k);
   System.out.println(k.nameCache);
   k.show();

   container.shutdown();
   }

Or maybe use this without DeltaSpike, Weld only. The output is the same.

@Inject
@NameCache
Cache<String, String> nameCache;

public void show(){

  System.out.println(this);
   System.out.println(this.nameCache);
}

Here's the main code of Kernel.

co.kaiba.blueeyes.impl.kernel.Kernel@2432c72c

Cache 'name-cache'@DESKTOP-7SRGBKP-36592

co.kaiba.blueeyes.impl.kernel.Kernel@2432c72c

null

co.kaiba.blueeyes.impl.kernel.Kernel@2432c72c

Cache 'name-cache'@DESKTOP-7SRGBKP-36592

Here's the output of @ApplicationScoped Kernel.

co.kaiba.blueeyes.impl.kernel.Kernel@2121d0ed

Cache 'name-cache'@DESKTOP-7SRGBKP-37141

co.kaiba.blueeyes.impl.kernel.Kernel@2121d0ed

Cache 'name-cache'@DESKTOP-7SRGBKP-37141

co.kaiba.blueeyes.impl.kernel.Kernel@2121d0ed

Cache 'name-cache'@DESKTOP-7SRGBKP-37141

Here's the output of @Singleton Kernel.

The output of k.nameCache is different in this case.

I'd like to know the actual difference between @Singleton and @ApplicationScoped.

The documentation doesn't clearly clarify the difference.

I think the proxy object do the trick.

Then how can I access to the field of proxy object?

Or I can do it only with a getter method?

Why is the injection in the Kernel only correct with show method in @ApplicationScoped?

But why is the injection in the Kernel always correct with @Singleton?

Please clarify it. Thanks.

[mkouba]

Hi Seto,

the  difference is that @Singleton is pseudo-scope, so if you inject a @Singleton you never get a client proxy but a direct reference to the bean instance (or rather an internal container construct in case of interceptors or decorators are associated). And this may cause unexpected problems during serialization - see also Weld docs - 5.4. The singleton pseudo-scope and CDI spec - 5.4. Client proxies. Also note that the CDI spec does not mention @Singleton at all.

[seto]

How can I access the field of client proxy? Is setter & getter the only way to do that?

[mkouba]

Yes, a method is the only way to access a field of a bean instance when you're working with a client proxy.