@PostConstruct and @PreDestroy in Spring Boot - Step-by-Step Execution

Concept Flow - @PostConstruct and @PreDestroy

Bean Created

↓

@PostConstruct Method Called

↓

Bean Ready to Use

↓

Application Context Closing

↓

@PreDestroy Method Called

↓

Bean Destroyed

Shows the lifecycle of a Spring bean: after creation, @PostConstruct runs once before use; before destruction, @PreDestroy runs once.

Execution Sample

Spring Boot

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;

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

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

Defines a Spring bean with methods that run after creation and before destruction.

Execution Table

Step	Event	Method Called	Output	Bean State
1	Bean instance created	None	None	Created, not initialized
2	@PostConstruct triggered	init()	"Bean initialized"	Initialized, ready
3	Bean used in application	None	None	In use
4	Application context closing	None	None	Preparing to destroy
5	@PreDestroy triggered	cleanup()	"Bean destroyed"	Destroyed
6	Bean removed from context	None	None	Removed

💡 Bean lifecycle ends after @PreDestroy method runs and bean is removed.

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
Bean State	Not created	Initialized	Destroyed	Removed

Key Moments - 2 Insights

Why does the @PostConstruct method run only once?

Because it runs right after the bean is created and before it is used, as shown in step 2 of the execution_table.

When exactly is the @PreDestroy method called?

It is called just before the bean is destroyed when the application context is closing, as shown in step 5 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the bean state after step 2?

A Initialized, ready

B Created, not initialized

C Destroyed

D Removed

💡 Hint

Check the 'Bean State' column at step 2 in the execution_table.

At which step does the @PreDestroy method run?

A Step 2

B Step 5

C Step 4

D Step 6

💡 Hint

Look for '@PreDestroy triggered' in the 'Event' column of the execution_table.

If the application context never closes, which method will NOT run?

A @PostConstruct

B Bean constructor

C @PreDestroy

D Bean usage

💡 Hint

Refer to the execution_table steps related to application context closing and @PreDestroy.

Concept Snapshot

@PostConstruct and @PreDestroy in Spring Boot:
- @PostConstruct runs once after bean creation, before use.
- @PreDestroy runs once before bean destruction.
- Used for setup and cleanup tasks.
- Methods must be void, no args.
- Lifecycle tied to Spring application context.

Full Transcript

In Spring Boot, beans have a lifecycle. When a bean is created, the method annotated with @PostConstruct runs once to initialize it. This happens before the bean is used in the application. Later, when the application is shutting down, the method annotated with @PreDestroy runs once to clean up resources before the bean is destroyed. This ensures proper setup and cleanup. The execution table shows these steps clearly: creation, initialization, usage, context closing, cleanup, and removal. Understanding this helps manage resources safely in Spring applications.

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