Bean concept in Spring in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Bean in Spring Framework?

A Bean is an object that is managed by the Spring container. It is created, configured, and assembled by Spring to be used in the application.

beginner

How does Spring know which classes to manage as Beans?

Spring uses annotations like @Component, @Service, @Repository, or XML configuration to identify and manage classes as Beans.

intermediate

What is the default scope of a Bean in Spring?

The default scope is 'singleton', meaning Spring creates only one instance of the Bean per container.

intermediate

Explain the difference between singleton and prototype Bean scopes.

Singleton scope creates one shared instance for the whole application. Prototype scope creates a new instance every time the Bean is requested.

beginner

What is Dependency Injection in the context of Spring Beans?

Dependency Injection is when Spring automatically provides the required Beans to other Beans, so they don’t have to create or find them manually.

Which annotation is commonly used to mark a class as a Spring Bean?

A @Component

B @Bean

C @Autowired

D @ServiceProvider

@Component is used to mark a class as a Spring-managed Bean.

What is the default scope of a Spring Bean?

A prototype

B singleton

C request

D session

By default, Spring Beans are singleton scoped.

Which of the following is NOT a valid Spring Bean scope?

A transient

B prototype

C singleton

D request

'transient' is not a Spring Bean scope.

What does Dependency Injection help with in Spring?

A Writing SQL queries

B Manually creating objects

C Automatically providing required Beans

D Managing database connections

Dependency Injection automatically provides Beans where needed.

Which annotation is used to inject a Bean into another Bean?

A @Component

B @Repository

C @Service

D @Autowired

@Autowired is used to inject Beans into other Beans.

Describe what a Spring Bean is and how Spring manages it.

Think about how Spring controls the objects your app uses.

Explain the difference between singleton and prototype Bean scopes in Spring.

Consider how many copies of the Bean exist in each scope.

Practice

1. What is a Bean in the Spring framework?

easy

A. A type of Java exception

B. A database connection

C. An object managed by the Spring container

D. A user interface component

Solution

1. Step 1: Understand Spring's role

   Spring manages objects to help organize and connect parts of an application.

2. Step 2: Define what a Bean is

   A Bean is any object that Spring creates and manages in its container.

3. Final Answer:

   An object managed by the Spring container -> Option C

4. Quick Check:

   Bean = Managed object [OK]

Hint: Beans are Spring-managed objects, not UI or exceptions [OK]

Common Mistakes:

• Confusing beans with database connections
• Thinking beans are UI elements
• Assuming beans are exceptions

2. Which annotation is used to declare a class as a Spring Bean?

easy

A. @Qualifier

B. @Component

C. @Autowired

D. @RequestMapping

Solution

1. Step 1: Identify annotations for beans

   @Component marks a class as a Spring-managed bean.

2. Step 2: Differentiate from other annotations

   @Qualifier specifies which bean to inject, @Autowired injects beans, @RequestMapping maps web requests.

3. Final Answer:

   @Component -> Option B

4. Quick Check:

   @Component = Bean declaration [OK]

Hint: Use @Component to declare beans; others serve different roles [OK]

Common Mistakes:

• Confusing @Autowired with bean declaration
• Using @RequestMapping for beans
• Thinking @Qualifier declares beans

3. Given the code below, what will be printed (assuming the @Autowired field and System.out.println are inside a Spring-managed class)?

@Component
public class MyBean {
  public String greet() { return "Hello Spring"; }
}

@Autowired
MyBean myBean;

System.out.println(myBean.greet());

medium

A. Hello Spring

B. null

C. Compilation error

D. No output

Solution

1. Step 1: Understand @Component and @Autowired

   @Component creates the bean, @Autowired injects it into the field.

2. Step 2: Check method call output

   Calling greet() returns "Hello Spring", so it prints that string.

3. Final Answer:

   Hello Spring -> Option A

4. Quick Check:

   Injected bean method returns greeting [OK]

Hint: Autowired injects bean; method returns greeting string [OK]

Common Mistakes:

• Assuming null because of missing initialization
• Thinking it causes compile error
• Expecting no output without System.out.println

4. What is wrong with this Spring bean declaration?

@Component
public class MyService {
  @Autowired
  private MyRepository repo;

  public void doWork() {
    repo.save();
  }
}

Assuming MyRepository is not annotated as a bean.

medium

A. No @Autowired on MyService class

B. Missing @Service annotation on MyService

C. Method doWork() should return a value

D. MyRepository is not a Spring bean, so injection fails

Solution

1. Step 1: Check bean injection requirements

   Spring can only inject beans that are managed by the container.

2. Step 2: Identify missing bean annotation

   MyRepository lacks @Component or similar, so Spring cannot inject it.

3. Final Answer:

   MyRepository is not a Spring bean, so injection fails -> Option D

4. Quick Check:

   Injection requires bean annotation [OK]

Hint: Injected classes must be beans too [OK]

Common Mistakes:

• Thinking @Service is mandatory for injection
• Assuming method must return value
• Confusing @Autowired placement

5. You want to create a bean that returns a customized object. Which method correctly defines a bean using Java configuration?

hard

A. @Bean
public MyBean myBean() {
return new MyBean("custom");
}

B. @Component
public MyBean myBean() {
return new MyBean("custom");
}

C. public MyBean myBean() {
return new MyBean("custom");
}

D. @Autowired
public MyBean myBean() {
return new MyBean("custom");
}

Solution

1. Step 1: Understand @Bean usage

   @Bean annotation on a method inside a @Configuration class defines a bean returned by that method.

2. Step 2: Differentiate from other annotations

   @Component is for classes, not methods; @Autowired injects dependencies, not defines beans.

3. Final Answer:

   @Bean public MyBean myBean() { return new MyBean("custom"); } -> Option A

4. Quick Check:

   @Bean method returns custom bean [OK]

Hint: Use @Bean on methods inside @Configuration classes [OK]

Common Mistakes:

• Using @Component on methods instead of classes
• Omitting @Bean annotation
• Confusing @Autowired with bean creation