README.md

Spring Beans Pt. 1

In this exercise, you'll be doing a series of different tasks. Pay attention to each and try to achieve the final result.

In this exercise, you'll be required to create your own Spring Boot Application for each task!

The simplest you to do that is to head to https://start.spring.io/. No extra dependencies are needed to any task.

Before each task, create a Spring Boot application and work your way on it. Download the zip file (if done with Spring Initializr), open it with your IDE and load the maven/gradle file. Then everything should be good to go.

Task 1 - Getting beans from the ApplicationContext

Create a package called task1. This task will be done inside this package.

For this task, you'll be playing with the ApplicationContext. Create a class inside a package called mybean. You can give it any name.

Make it visible to Spring! annotate it with the @Component annotation.

Now, make your bean be aware of the ApplicationContext! Make it implement the ApplicationContextAware interface.

This interface requires you to implement a single method:

void setApplicationContext(ApplicationContext applicationContext)throws BeansException;

So create a field in your class of type ApplicationContext and set its value using this method. Go ahead and also create a get method for the ApplicationContext! So we can use it using our bean.

Now, create three different classes. They can also be located in the mybean package, or you can just create them in another package. You can just call them Bean1, Bean2 and Bean3, or whichever name you prefer.

Annotate each one of them with the @Component annotation, so they will be registered as beans in our application context.

Now, head to the test class of your application. You'll find it inside the test/java/ folder.

Initially, your test class should look like this:

Note that it uses the @SpringBootTest annotation, which runs the application in a test context. That means the IoC container (the application context) is created and the beans initialized, the same way as it does when running the application. The advantage here is that we can inject beans to this class the same way we do in a normal context.

You are to create a couple tests. Remember to always mark your tests with the @Test annotation.

Test 1: void appContextNotNull()

This is a very simple test. You are to inject your ApplicationContextAware bean here to the test class. You can achieve that with the @Autowired, injecting directly to the field.

Once it's wired, the test should do a simple assert: make sure the injected bean is not null.

Hint: You should use the Assertions.assertNotNull(myBean) here.

Test 2: void appContext_getByName(Bean1 bean1)

In this test, you should try to get the bean by name from the application context, and assert that the expected result is the same as the one that is wired to other components.

See how the test method receives an instance of Bean1? You should wire the bean to the method, using the @Autowired annotation.

Once it's wired get the application context from your ApplicationContextAware bean using the get method you put in it. Now, make sure the bean you get from the application context is the same as the one you're injecting in this test method. For this test, you should get the bean from the application context by its name.

Hint: If not given a name, the bean created with the @Component annotation will have the class' name, but uncapitalized.

Bean1 class >> bean1 bean name.

You should have an assertion similar to the following:

Assertions.assertEquals(bean1,applicationContext.getBean("bean1"));

Test 3: void appContext_getByType(Bean2 bean2)

This test should follow the same principle as the one above, but you'll be wiring a bean of type Bean2 instead.

Besides that, you should get the Bean2 from the application context by its type:

Assertions.assertEquals(bean2,applicationContext.getBean(Bean2.class));

Test 4: void appContext_getByNameAndType(Bean3 bean3)

Finally, the last test should follow the same principle as the ones before, but you'll be wiring a bean of type Bean3 here.

You should also get the Bean3 from the application context by its name and type:

Assertions.assertEquals(bean3,applicationContext.getBean("bean3",Bean3.class));

If you did everything right, you should be getting the following result:

---

Task 2 - Using different scopes

Create a package called task2. The code for this task should go here.

In this task, we'll be checking the two main scopes a bean can have: singleton and prototype.

Task 2.1 - Singleton Scope

In the first part of this task, you should define a bean with the singleton scope.

Create a class called Person inside a beans package. It should have a single field called name , and use only the default constructor. Create also a getter and setter for this field.

Create also a class called Task2Config inside a config package. The class should be marked with the @Configuration annotation.

Inside this class, define a bean of type Person. You do that by creating a method that returns an instance of Person, and by marking the method with the @Beans annotation.

Also explicitly mark the bean as a singleton, by using the @Scope annotation.

Hint: The scope can be defined in two different ways: Either by writing down the value as a String, or by using the constant that contains the String value (thus being typing error proof):

• @Scope("singleton")
• @Scope(ConfigurableBeanFactory.SCOPE_SINGLETON)

Test 2.1 - Testing Singleton Scope

Now it's time to write the test for the singleton scope. Create a new test class called Task2Tests. It should follow the same principle of the previous test class (be annotated with @SpringBootTest).

Autowire the application context by setting a field in the test class of ApplicationContext and annotating it with the @Autowired annotation.

Also create two String constants, containing two different names. It can be any name. I.e.:

private static final String NAME_1="Jack James";
private static final String NAME_2="Hill Downhill";

Now, create a test to test the singleton bean. The test should:

1. Create two variables of type Person (i.e. person1 and person2) and assign to them the bean captured from the applicationContext. To not conflict with the next test, make sure to get the bean using both the name and the type;
2. Using the setter, assign the first name (NAME_1) to only one of the variables, i.e. to person1;
3. Assert that the actual value of both person1.getName() and person2.getName() should be the value of NAME_1;
4. Now, assign the value of NAME_2 to only one of the variables, i.e. to `person2 ;
5. Assert again that both person1.getName() and person2.getName() should be the value of NAME_2.

Task 2.2 - Prototype Scope

Now we'll be using the prototype scope. Go to the Task2Config file and declare another bean of type Person, the same way you did before, but now declare the scope as "prototype".

Hint: Alternatively, just like before, you can use the constant value instead to declare the scope: @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)

Test 2.2 - Testing Prototype Scope

The test should be similar:

1. Create two variables of type Person (i.e. person1 and person2) and assign to them the bean captured from the applicationContext. To not conflict with the next test, make sure to get the bean using both the name and the type;
2. Using the setter, assign the first name (NAME_1) to person1;
3. Now, assert that person1.getName() should be NAME_1, and person2.getName() should be null ;
4. Set the name of person2 to NAME_2;
5. Assert that person1.getName() should be NAME_1 and person2.getName() should be NAME_2.

If you did everything right, you should be getting the following result:

---

Task 3 - Bean Lifecycle

Create a package called task3. The code for this task should go here.

In this task, we'll write a Spring bean to see the execution order of each phase of the lifecycle.

Task 3.1 - The Lifecycle Bean

Inside this package, create a new bean class. Create a field called message in it and create its getter and setter.

Implement a couple aware interfaces, like BeanNameAware, ApplicationContextAware and BeanFactoryAware.

Make it also implement the lifecycle callback interfaces InitializingBean and DisposableBean.

For each of the methods presented in this bean, you should simply print a message to the console saying which method is being executed. I.e:

  @Override
public void setBeanName(String name){
    System.out.println("--- setBeanName executed ---");
    }

After implementing all the aware and callback interfaces, go ahead and also create two new methods, using the JSR-250 annotations, @PostConstruct and @PreDestroy. Implement these methods in the same way as the others.

Now also create the methods that will be defined in the @Bean declaration, an initMethod and a destroyMethod.

Last but not least, create the BeanPostProcessor class to hook into the before and after initialization phases. Implement the BeanPostProcessor interface and manually override the postProcessBeforeInitialization and postProcessAfterInitialization methods.

In these implementations, since the BeanPostProcessor is not bean specific but application specific, you need to make sure we're dealing with our bean before printing out a message:

  @Override
public Object postProcessBeforeInitialization(Object bean,String beanName)
    throws BeansException{
    if(bean instanceof MySpringBean){
    System.out.println("--- postProcessBeforeInitialization executed ---");
    }
    return bean;
    }

Task 3.2 - The Configuration Class

Once all the hooks of our bean have been defined, we need to create a @Configuration class, where we should define both our lifecycle bean and our BeanPostProcessor bean. Don't forget to declare the initMethod and the destroyMethod!

Task 3.3 - The Test Class

Finally, we write a @SpringBootTest class to run our Spring context.

In our test class, you should autowire your lifecycle bean.

The class should contain a single test, where you should set the message of your bean using the bean.setMessage().

After that, assert that the message is equals to the message you just set. Print the message on the console afterwards.

Watch the console output. You should have a similar output to this one:

Happy Coding!