@PostConstruct and @PreDestroy in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

When is a method annotated with @PostConstruct called?

In a Spring Boot application, what is the correct time when a method annotated with @PostConstruct is executed?

A Right after the bean is fully initialized and dependencies are injected, before the bean is ready to use.

B Only when the application context is closed.

C After the bean is destroyed and removed from the context.

D Before the bean is created and dependencies are injected.

💡 Hint

Think about when you want to run setup code after all dependencies are ready.

The @PostConstruct method runs once the bean is fully created and all dependencies are injected. It is used to perform initialization tasks before the bean is used.

❓ component_behavior

intermediate

What happens when a method is annotated with @PreDestroy?

In a Spring Boot bean, when is a method annotated with @PreDestroy executed?

A After the bean is fully initialized and ready to use.

B When the bean is created and dependencies are injected.

C When the application starts.

D Right before the bean is destroyed and removed from the Spring context.

💡 Hint

Think about cleanup tasks before the bean disappears.

The @PreDestroy method runs just before the bean is destroyed by the Spring container. It is used to release resources or perform cleanup.

📝 Syntax

advanced

Identify the correct usage of @PostConstruct in a Spring Boot bean

Which of the following code snippets correctly uses @PostConstruct in a Spring Boot component?

A

public class MyBean {
  @PostConstruct
  public void init(String param) {
    System.out.println(param);
  }
}

B

public class MyBean {
  @PostConstruct
  private int init() {
    return 1;
  }
}

C

public class MyBean {
  @PostConstruct
  public void init() {
    System.out.println("Bean initialized");
  }
}

D

public class MyBean {
  @PostConstruct
  public static void init() {
    System.out.println("Static init");
  }
}

💡 Hint

Remember the method must be void, no parameters, and non-static.

The @PostConstruct method must be void, have no parameters, and not be static. Option C meets all these requirements.

🔧 Debug

advanced

Why does the @PreDestroy method not run in this Spring Boot application?

Consider this Spring Boot bean: public class MyService { @PreDestroy public void cleanup() { System.out.println("Cleaning up resources"); } } Why might the cleanup method never be called when the application stops?

A The bean is not managed by Spring because it lacks a stereotype annotation like @Component.

B @PreDestroy methods only run on singleton beans, and this bean is prototype scoped.

C The application context is not closed properly, so @PreDestroy is skipped.

D The method is public and void, so it should run without issues.

💡 Hint

Check if Spring knows about this bean to manage its lifecycle.

If a class is not registered as a Spring bean (e.g., missing @Component), Spring does not manage its lifecycle, so @PreDestroy methods won't be called.

🧠 Conceptual

expert

What is the effect of multiple @PostConstruct methods in a single Spring bean?

If a Spring bean class has two methods both annotated with @PostConstruct, what will happen when the bean is initialized?

A Only the first declared @PostConstruct method will be called; the second is ignored.

B Both methods will be called in an undefined order during bean initialization.

C Spring will throw a runtime exception due to multiple @PostConstruct methods.

D Neither method will be called because of the conflict.

💡 Hint

Think about how Java handles multiple lifecycle callbacks in one class.

Spring calls all methods annotated with @PostConstruct in a bean, but the order is not guaranteed. Both methods will run during initialization.

Practice

1. What is the main purpose of the @PostConstruct annotation in a Spring Boot application?

easy

A. To run a method immediately after the bean is created and dependencies are injected

B. To mark a method that should run before the bean is destroyed

C. To define a method that runs only when the application starts

D. To schedule a method to run repeatedly at fixed intervals

Solution

1. Step 1: Understand the role of @PostConstruct

   @PostConstruct is used to run initialization code after the Spring bean is fully created and dependencies are set.

2. Step 2: Compare with other lifecycle annotations

   @PreDestroy runs before bean removal, and scheduling annotations run repeatedly, so they don't match @PostConstruct's purpose.

3. Final Answer:

   To run a method immediately after the bean is created and dependencies are injected -> Option A

4. Quick Check:

   @PostConstruct = run after bean creation [OK]

Hint: Remember: PostConstruct runs right after bean setup [OK]

Common Mistakes:

• Confusing @PostConstruct with @PreDestroy
• Thinking @PostConstruct runs before bean creation
• Assuming it schedules repeated tasks

2. Which of the following is the correct way to use @PreDestroy in a Spring Boot bean?

easy

A. Place @PreDestroy on a static method to run after application shutdown

B. Annotate a method with @PreDestroy to run cleanup code before bean destruction

C. Use @PreDestroy on a constructor to initialize resources

D. Annotate the class with @PreDestroy to mark it for destruction

Solution

1. Step 1: Identify correct usage of @PreDestroy

   @PreDestroy is placed on a method to run cleanup code before the bean is destroyed.

2. Step 2: Eliminate incorrect options

   @PreDestroy cannot annotate a class or constructor, and static methods are not lifecycle callbacks.

3. Final Answer:

   Annotate a method with @PreDestroy to run cleanup code before bean destruction -> Option B

4. Quick Check:

   @PreDestroy = method cleanup before bean removal [OK]

Hint: Use @PreDestroy only on instance methods for cleanup [OK]

Common Mistakes:

• Annotating the class instead of a method
• Using @PreDestroy on constructors
• Marking static methods with @PreDestroy

3. Consider the following Spring Boot bean code snippet:

public class MyBean {

    @PostConstruct
    public void init() {
        System.out.println("Bean is initialized");
    }

    @PreDestroy
    public void cleanup() {
        System.out.println("Bean is about to be destroyed");
    }

}

What will be printed when the application context starts and then shuts down?

medium

A. Bean is about to be destroyedBean is initialized

B. Only Bean is initialized

C. Bean is initializedBean is about to be destroyed

D. Only Bean is about to be destroyed

Solution

1. Step 1: Understand @PostConstruct behavior on startup

   The init() method runs after bean creation, printing "Bean is initialized" when the app context starts.

2. Step 2: Understand @PreDestroy behavior on shutdown

   The cleanup() method runs before bean destruction, printing "Bean is about to be destroyed" when the app context shuts down.

3. Final Answer:

   Bean is initialized
   Bean is about to be destroyed -> Option C

4. Quick Check:

   Startup prints init, shutdown prints cleanup [OK]

Hint: PostConstruct runs on start, PreDestroy runs on shutdown [OK]

Common Mistakes:

• Mixing order of printed messages
• Expecting only one message to print
• Thinking @PreDestroy runs on startup

4. Given this Spring Boot bean code:

public class ResourceBean {

    @PostConstruct
    public void setup() {
        System.out.println("Setup resources");
    }

    @PreDestroy
    public void cleanup() {
        System.out.println("Cleanup resources");
    }

    public void start() {
        System.out.println("Start method called");
    }

}

When running the application, the developer notices that "Cleanup resources" is never printed on shutdown. What is the most likely cause?

medium

A. The @PostConstruct method prevents @PreDestroy from running

B. The @PreDestroy method must be static to run correctly

C. The cleanup() method must be called manually to run

D. The bean is not managed by Spring, so lifecycle annotations are ignored

Solution

1. Step 1: Check if bean is managed by Spring

   @PostConstruct and @PreDestroy only work if Spring manages the bean lifecycle (e.g., via @Component or @Bean).

2. Step 2: Understand why cleanup isn't called

   If the bean is created manually (new keyword), Spring won't call lifecycle methods, so cleanup() won't run automatically.

3. Final Answer:

   The bean is not managed by Spring, so lifecycle annotations are ignored -> Option D

4. Quick Check:

   Lifecycle annotations need Spring-managed beans [OK]

Hint: Lifecycle methods run only on Spring-managed beans [OK]

Common Mistakes:

• Thinking @PreDestroy must be static
• Assuming @PostConstruct blocks @PreDestroy
• Believing cleanup() runs without Spring management

5. You want to ensure a Spring Boot bean opens a database connection after creation and closes it before destruction. Which code snippet correctly uses @PostConstruct and @PreDestroy to achieve this?

hard

A. public class DbBean {

private Connection conn;

@PostConstruct
public void open() {
conn = DriverManager.getConnection("url");
System.out.println("Connection opened");
}

@PreDestroy
public void close() {
conn.close();
System.out.println("Connection closed");
}

}

B. public class DbBean {

private Connection conn;

public void open() {
conn = DriverManager.getConnection("url");
System.out.println("Connection opened");
}

public void close() {
conn.close();
System.out.println("Connection closed");
}

}

C. public class DbBean {

private Connection conn;

@PreDestroy
public void open() {
conn = DriverManager.getConnection("url");
System.out.println("Connection opened");
}

@PostConstruct
public void close() {
conn.close();
System.out.println("Connection closed");
}

}

D. public class DbBean {

private Connection conn;

@PostConstruct
public void open() {
System.out.println("Connection opened");
}

@PreDestroy
public void close() {
System.out.println("Connection closed");
}

}

Solution

1. Step 1: Identify correct lifecycle method usage

   @PostConstruct should open the connection after bean creation; @PreDestroy should close it before destruction.

2. Step 2: Check code correctness

   public class DbBean { private Connection conn; @PostConstruct public void open() { conn = DriverManager.getConnection("url"); System.out.println("Connection opened"); } @PreDestroy public void close() { conn.close(); System.out.println("Connection closed"); } } correctly uses @PostConstruct to open and @PreDestroy to close the connection with proper method calls.

3. Final Answer:

   public class DbBean { private Connection conn; @PostConstruct public void open() { conn = DriverManager.getConnection("url"); System.out.println("Connection opened"); } @PreDestroy public void close() { conn.close(); System.out.println("Connection closed"); } } -> Option A

4. Quick Check:

   Open after creation, close before destruction [OK]

Hint: Open in @PostConstruct, close in @PreDestroy for resource management [OK]

Common Mistakes:

• Swapping @PostConstruct and @PreDestroy methods
• Not calling connection methods inside lifecycle methods
• Missing lifecycle annotations on resource methods