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Can #Inject be used in a pojo

I am trying to use and understand CDI, when I use #Inject in a simple pojo class, it throws me NPE.
example
Greeting.java
public Class Greeting {
public String greet() {
System.out.println("Hello");
}
}
Test.java
import javax.inject.Inject;
public class Test {
#Inject
private Greeting greeting;
public void testGreet() {
greeting.testGreet();
}
}
When I call testGreet() it throws NPE, why is the greeting instance null. Does #Inject way of adding dependency only be used in container managed bean?
Note: jar is not the problem here.

TL;DR:
#Inject-annotated fields are only populated for container-instantiated beans.
Long version:
The CDI container provides you a lot of utilities for easily injecting dependencies to your beans, but it doesn't work by magic. The container can only populate the annotated fields of a client bean if the client bean itself was instantiated by the container. When the container is instantiating the object the sequence of events is as follows:
Your bean's constructor is called.
#Inject-annotated fields (and some other
annotations, #PersistenceContext and #EJB for instance) are
populated.
#PostConstruct-annotated no-args method is called.
Your bean is finished.
You're facing a classic bootstrapping problem, how to move from non-container-managed code into container-managed code. Your options are:
Get access to an instance of BeanManager from your JavaEE container via JNDI lookup. This is technical and a bit clumsy.
Use a CDI extension library such as Apache DeltaSpike. (Example: BeanProvider.getContextualReference(Test.class, false);)
Modify your application to start in a situation where you can inject your Test class rather than calling new Test();. This can be done for example by setting up a startup singleton ejb, which calls your test in it's #PostConstruct-annotated initialisation.
Hope this helps.

You need a JavaEE container, and than you need to define Greeting and Test as managed beans. After that you can inject one in another.
Try to take a look at:
https://docs.oracle.com/javaee/6/tutorial/doc/girch.html

Your class should be implemented from Serializable for being injectable as a "CDI Bean"

#Inject and #PostConstruct not working in singleton pattern

I have a class as below:
public class UserAuthenticator {
private static UserAuthenticator authenticator =
#Inject
private UserRepository userRepository;
#PostConstruct
public void init() {
List<User> allUsers = userRepository.findAll();
for (User user : allUsers) {
users.put(user.getEmail(), user.getPassword());
serviceKeys.put(user.getServiceKey(), user.getEmail());
}
}
public static UserAuthenticator getInstance() {
if (authenticator == null) {
authenticator = new UserAuthenticator();
}
return authenticator;
}
}
When I call
UserAuthenticator authenticator = UserAuthenticator.getInstance();
init() method isn't called and userRepository is null
My web application run in JBOSS EAP 6.3.
How is this caused and how can I solve it?

In a Java EE application, don't think in singletons. That's only recipe for trouble and confusion. Instead, think in "just create one". Tell the Java EE container to just create only one instance of the specified class, application wide, and obtain the instance via the facility offered by the Java EE container. Your concrete problem is caused because you're manually creating an instance of the class using new operator without manually performing the injection and post construct call like as the technical correct but conceptually wrong example below:
authenticator = new UserAuthenticator();
authenticator.userRepository = new UserRepository();
authenticator.init();
In other words, you incorrectly expected that the new operator magically recognizes the bean management and dependency injection related annotations.
The right approach depends on the one you'd like to point out as the responsible for creating and managing the instance of the specified class. If it's CDI, then just tell it to create only one managed bean instance of the backing bean class, application wide, using #Named #ApplicationScoped.
import javax.inject.Named;
import javax.enterprise.context.ApplicationScoped;
#Named
#ApplicationScoped
public class UserAuthenticator {}
It will be created just once and be available via #Inject as below in any other Java EE managed artifact (read: in any other class annotated with #Named, #Stateless, #ManagedBean, #WebServlet, #WebListener, #WebFilter, #Path, etc..):
#Inject
private UserAuthenticator userAuthenticator;
If you're absolutely positive that you need a static method to grab the current CDI managed bean instance of a given backing class, then you should be obtaining it via BeanManager as below instead of manually constructing the instance (assuming Java EE 7 / CDI 1.1 available):
#SuppressWarnings("unchecked")
public static <T> T getCurrentInstance(Class<T> beanClass) {
BeanManager beanManager = CDI.current().getBeanManager();
Bean<T> bean = (Bean<T>) beanManager.resolve(beanManager.getBeans(beanClass));
return (T) beanManager.getReference(bean, bean.getBeanClass(), beanManager.createCreationalContext(bean));
}
Usage:
UserAuthenticator userAuthenticator = YourCDIUtil.getCurrentInstance(UserAuthenticator.class);
// ...
See also:
Java singleton class vs JSF application scoped managed bean - differences?
Java EE 6 and Singletons

Well i think you shouldn't explictly call UserAuthenticator.getInstance() but to define the UserAuthenticator for example as #ApplicationScoped and get the instance via DI provided by your app server (#Inject).
UserAuthenticator should be then initialized properly.

The #PostConstruct method will not be invoked until you do some action on that class (ex: call some methods

Why Initialization is not required in EJB?

I am new to the Struts2 framework and to EJB as well. I have a class LoginDAO which implements checkUser method of an interface LoginDAOLocal. I don't understand why I see different behavior for the following scenarios:
If I use an EJB (LoginDAO is stateless session bean) as follows, method call works perfectly without any error.
#EJB
private LoginDAOLocal loginDao;
loginDao.checkUser(userName,password);
If I use Struts2 as follows, it gives a Null pointer exception for the method call.
public class LoginAction extends ActionSupport {
// Getters setters for userName and password)
private LoginDAOLocal loginDao;
loginDao.checkUser(this.userName,this.password);
}
If I use a simple Java application (no EJB or Struts2), the method call creates a compile time error saying loginDao is not initialized
public static void main(String[] args) {
LoginDAOLocal loginDao;
loginDao.checkUser(userName,password);
}
Can someone explain why this different behavior ?

Without getting too much into the Java EE spec: EJBs are managed by an EJB container that exists in J2EE servers (JBoss \ Websphere etc..). The container takes control of bean lifecycle and is responsible for creating \ destroying beans according to the application needs.
When running out of container (simple java application) your beans won't get initialized and you don't have a JNDI context to get beans from, even if you add #EJB annotation to the field member.

We can say that there are two ways to manage the beans, using the container (managed by the container), or by another component (managed by a servlet, listener or filter).
Using components managed by the container, the container injects the references. e.g.:
#WebServlet("/test")
public class MyServlet extends HttpServlet {
#Resource(lookup = "jdbc/TestDS")
private DataSource testDS;
}
By contrast, a component managed by a bean, e.g.:
#Namespace("/User")
#ResultPath(value = "/")
#Result(name = "success", location = "pages/login.jsp")
public class LoginAction extends ActionSupport {
}
is managed by the filter org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter. The latter should be responsible for performing dependency injection. Spring, for example, takes care of injecting all necessary dependencies.

Adding a dependency to an EJB

I want to add a dependency to an EJB. How do I do this using Spring? The dependent object is a general service object. Based on code below I want to wire myDependency without having to use 'new'.
The EJB runs in weblogic.
#Stateless(mappedName = "MyBean")
public class MyBean implements MyBeanRemote, MyBeanLocal {
#EJB(name = "MyOtherBean")
private MyOtherBean myOtherBean;
private MyDependency myDependency;
...
}

This is well described in the Spring documentation:
For EJB 3 Session Beans and Message-Driven Beans, Spring provides a
convenient interceptor that resolves Spring 2.5's #Autowired
annotation in the EJB component class:
org.springframework.ejb.interceptor.SpringBeanAutowiringInterceptor.
This interceptor can be applied through an #Interceptors annotation in
the EJB component class, or through an interceptor-binding XML element
in the EJB deployment descriptor.
#Stateless
#Interceptors(SpringBeanAutowiringInterceptor.class)
public class MyFacadeEJB implements MyFacadeLocal {
// automatically injected with a matching Spring bean
#Autowired
private MyComponent myComp;
// for business method, delegate to POJO service impl.
public String myFacadeMethod(...) {
return myComp.myMethod(...);
}
...
}
Stateless EJBs and Spring beans, however, offer more or less the same possibilities. Mixing them together seems like unnecessary complexity.

EJB 3.1 - Using #EJB inside an EJB - is it possible?

Is it possible to use #EJB inside another EJB? I'm trying to do this now, and my EJB is ending up null. I'll outline my problem in an example.
#Stateless
#LocalBean
#Local(LoginServiceLocal.class)
public class LoginService implements LoginServiceLocal {
public void createLogin(String email, String password) { ... }
}
#Stateless
#LocalBean
#Local(AccountServiceLocal.class)
public class AccountService implements AccountServiceLocal {
#PersistenceContext(unitName = "accounts")
private EntityManager accountEntityManager;
#EJB
private LoginServiceLocal loginService;
public void createAccount(Account account, String email, String password) {
accountEntityManager.persist(account);
loginService.createLogin(email, password);
}
}
Is this type of thing supposed to be possible? I should also mention that I'm using an embedded container (via EJBContainer), and I'm looking up the AccountService using JNDI, however when I try and call loginService.createLogin in the AccountService, the loginService is null (not being initialized by #EJB).
Is what I'm trying to do possible?
Thanks.

Yes, it's possible.
The #LocalBean annotation, enables an EJB to expose a no-interface client view, so that you won't need to define a Local interface.
On the other hand, the #Local annotation defines a bean's local client interface.
Choose one of the above configuration options not both.
If you choose to use the #LocalBean annotation, drop the #Local annotation, remove the implements keyword and inject the bean class name with the #EJB annotation.
If you choose to use the #Local annotation, drop both #Local and #LocalBean annotations and inject the bean with the #EJB annotation using the interface name.

Yes, I was just working on some of my code that does just that. It might be an issue with how you're creating the EJB. I have only done it using injection and not a jndi lookup.

If you are used EJB3.1 you can also use #Inject from CDI