Mockito - mock ApplicationContext

I have a Springboot application that looks up the bean from the ApplicationContext at runtime based on the input parameter passed by the user. For this method, I am trying to write Mockito test cases but it is not working and throws NullPointerException.

The class which bootstraps the application:

@SpringBootApplication
public class MyApplication {

	private static ApplicationContext appContext;
	
	public static void main(String[] args) {
		appContext = SpringApplication.run(MyApplication.class, args);
	}
	
	public static ApplicationContext getApplicationContext() {
		return appContext;
	}
	
}

Class for which I am trying to write the test cases:

@Service
public class Mailbox {

	@Autowired
	MailProcessor processor;
	
	public void processUserInput(Envelope object) {
	
		processor.setCommand(MyApplication.getApplicationContext().getBean(object.getAction(), Command.class));
		processor.allocateEnvelopes(object);
		
	}
} 

And my test case is as below:

@RunWith(MockitoJUnitRunner.class)
@SpringBootTest
@ActiveProfiles("test")
public class MailboxTest {

	@Mock
	MailProcessor processor;
	
	@InjectMocks
	Mailbox mailbox;
	
	@Test
	public void testProcessUserInput() {
		
		Envelope message = new Envelope();
		message.setAction("userAction");
		message.setValue("userInput");
		
		doNothing().when(processor).setCommand(any());
		doNothing().when(processor).allocateEnvelopes(any());
		
		mailbox.processUserInput(message);
		
		Mockito.verify(processor).allocateEnvelopes(any());
		
	}
	
	
}

Whenever I run the test cases it gives the NullPointerException at processor.setCommand(MyApplication.getApplicationContext().getBean(object.getAction(), Command.class)); in Mailbox class. How can I mock the ApplicationContext lookup? Am I missing any mocking step?

Can't say for sure without debugging but it looks like MyApplication.getApplicationContext() is returning null.

Instead of storing it in a static variable you should try injecting the ApplicationContext in your @Service class where you need it:

@Autowired
private ApplicationContext appContext;

Spring wise your code doesn't look good, and in particular is not unit testable. I'll explain:

1. Your Mailbox service should not be aware of MyApplication at any level. It is an entry point of spring boot application and your business logic should not depend on that.
   Its true that you can inject the application context directly into the class. See an example below. Another (more "old-school") option here is using ApplicationContextAware interface in the Mailbox service (see this example). However, its still a bad code IMO:

@Service
public class Mailbox {
 private final ApplicationContext ctx;
 ...
 public Mailbox(ApplicationContext ctx) {
     this.ctx = ctx;
 }
 ...
}

2. Even if you resolve it, in general its not a good idea to depend on the ApplicationContext as well. Because this way you become spring dependent and there is no reason to do that in the Mailbox class. The class will become unit testable though.

3. In terms of resolution:

In spring you can inject a Map<String, Command> into the mailbox (Its a built-in feature in spring) so that the key of the map will be a bean name, exactly an action of your envelop.
So here is the solution (simplified in places not relevant to injection, just to illustrate the idea):

public interface Command {
 void execute();
}

@Component("delete") // note this "delete" word - it will be a key in the map in the Mailbox
public class DeleteMailCommand implements Command {
    @Override
    public void execute() {
        System.out.println("Deleting email");
    }
}

@Component("send")
public class SendMailCommand implements Command{
    @Override
    public void execute() {
        System.out.println("Sending Mail");
    }
}

Note, that all the commands must be driven by spring (which seems to be your case anyway).
Now, the Mailbox will look like this:

@Service
public class Mailbox {
    private final Map<String, Command> allCommands;
    private final MailProcessor processor;
    // Note this map: it will be ["delete" -> <bean of type DeleteMailCommand>, "send" -> <bean of type SendMailCommand>]
    public Mailbox(Map<String, Command> allCommands, MailProcessor mailProcessor) {
        this.allCommands = allCommands;
        this.processor = mailProcessor;
    }

    public void processUserInput(Envelope envelope) {
        Command cmd = allCommands.get(envelope.getAction());
        processor.executeCommand(cmd);

    }
}

This solution is easily unit testable, because you can populate the map with mock commands if you wish and there is no need to deal with the application context.

Update

I took a look on your test now, and it's also not really good, sorry
@RunWith(MockitoJUnitRunner.class) is used to run unit tests (without spring at all). There is no point in placing this annotation in conjunction with @SpringBootTest which runs a full-fledged system test: starts the whole spring boot application, loads configurations and so forth.

So make sure what kind of tests you want to run and use the appropriate annotations.

Try initializing mailbox object by injecting processor before first test.

mailbox = new Mailbox(processor);