Custom thread pool configuration in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Thread pool behavior with fixed size

Given a Spring Boot application with a fixed thread pool of size 3, what happens when 5 tasks are submitted simultaneously?

Spring Boot

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;

@Configuration
public class ThreadPoolConfig {
    @Bean(name = "fixedThreadPool")
    public ThreadPoolTaskExecutor fixedThreadPool() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(3);
        executor.setMaxPoolSize(3);
        executor.setQueueCapacity(10);
        executor.initialize();
        return executor;
    }
}

A Only 3 tasks run, the other 2 are rejected with an exception

B All 5 tasks run immediately in parallel

C 3 tasks run immediately, 2 tasks wait in the queue until threads free up

D Tasks run sequentially one after another, ignoring thread pool size

💡 Hint

Think about how fixed thread pools handle tasks exceeding the core pool size with a queue.

A fixed thread pool with core and max size 3 can run 3 tasks concurrently. Additional tasks are placed in the queue (capacity 10 here) and wait until a thread is free.

📝 Syntax

intermediate

Correct syntax for custom thread pool bean

Which option correctly defines a Spring Boot ThreadPoolTaskExecutor bean with a core pool size of 5 and max pool size of 10?

A

executor.setCorePoolSize(5);
executor.setMaxPoolSize(10);

B

executor.setCorePoolSize = 5;
executor.setMaxPoolSize = 10;

C

executor.corePoolSize(5);
executor.maxPoolSize(10);

D

executor.setCorePool(5);
executor.setMaxPool(10);

💡 Hint

Look for the correct setter method names and syntax in Java.

The correct methods to set core and max pool sizes are setCorePoolSize(int) and setMaxPoolSize(int). They are called with parentheses and arguments.

🔧 Debug

advanced

Identify the cause of thread pool rejection

A Spring Boot app uses a ThreadPoolTaskExecutor with core size 2, max size 4, and queue capacity 2. When submitting 7 tasks, some are rejected. Why?

Spring Boot

ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(2);
executor.setMaxPoolSize(4);
executor.setQueueCapacity(2);
executor.initialize();

// 7 tasks submitted simultaneously

A Core pool size is too small causing immediate rejection

B Queue capacity is unlimited, so no rejection should occur

C Max pool size is ignored, causing rejection

D The queue is full and max threads are busy, so extra tasks are rejected

💡 Hint

Consider how tasks are handled when both threads and queue are at capacity.

When the queue is full (2 tasks) and all 4 threads (max pool size) are busy, additional tasks cannot be accepted and are rejected.

❓ state_output

advanced

Thread pool active count after task submission

After submitting 6 tasks to a ThreadPoolTaskExecutor with core size 3, max size 6, and queue capacity 0, what is the active thread count?

Spring Boot

ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(3);
executor.setMaxPoolSize(6);
executor.setQueueCapacity(0);
executor.initialize();

// Submit 6 tasks simultaneously
// Then call executor.getActiveCount()

A 0

B 6

C Depends on task duration

D 3

💡 Hint

With zero queue capacity, tasks must run immediately or be rejected.

With queue capacity 0, tasks cannot wait in queue. The pool expands up to max size 6 to run all tasks immediately, so active count is 6.

🧠 Conceptual

expert

Effect of allowing core thread timeout

What happens if you configure a ThreadPoolTaskExecutor with allowCoreThreadTimeOut(true) and a keepAliveSeconds of 10?

Spring Boot

ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(5);
executor.setMaxPoolSize(10);
executor.setKeepAliveSeconds(10);
executor.setAllowCoreThreadTimeOut(true);
executor.initialize();

A Core threads will terminate if idle for 10 seconds, reducing resource use

B Core threads never terminate regardless of idle time

C Max threads terminate after 10 seconds but core threads stay alive

D Threads terminate immediately after task completion

💡 Hint

Think about what allowing core thread timeout means for thread lifecycle.

Setting allowCoreThreadTimeOut(true) lets core threads terminate if idle longer than keepAliveSeconds, freeing resources when idle.

Practice

1. What is the main purpose of configuring a custom thread pool in a Spring Boot application?

easy

A. To change the database connection settings

B. To control the number of threads used for asynchronous tasks

C. To modify the HTTP request timeout

D. To customize the logging format

Solution

1. Step 1: Understand thread pools in Spring Boot

   Thread pools manage how many threads run tasks concurrently, improving performance and resource use.

2. Step 2: Identify the role of custom thread pools

   Custom thread pools let you set limits on thread count and behavior for asynchronous processing.

3. Final Answer:

   To control the number of threads used for asynchronous tasks -> Option B

4. Quick Check:

   Thread pool purpose = control threads [OK]

Hint: Thread pools manage threads for async tasks [OK]

Common Mistakes:

• Confusing thread pool with database or HTTP settings
• Thinking thread pools affect logging
• Assuming thread pools are for UI rendering

2. Which of the following is the correct way to define a custom thread pool bean in Spring Boot using Java configuration?

easy

A. @Bean public Executor taskExecutor() { return Executors.newFixedThreadPool(5); }

B. @Bean public Executor taskExecutor() { return new ThreadPoolExecutor(); }

C. @Bean public Executor taskExecutor() { return new ScheduledThreadPoolExecutor(10); }

D. @Bean public Executor taskExecutor() { return new ThreadPoolTaskExecutor(); }

Solution

1. Step 1: Recall Spring Boot thread pool bean creation

   Spring Boot uses ThreadPoolTaskExecutor for custom thread pools, which integrates with Spring's task execution.

2. Step 2: Analyze options

   @Bean public Executor taskExecutor() { return new ThreadPoolTaskExecutor(); } correctly returns a new ThreadPoolTaskExecutor instance, which is the recommended way.

3. Final Answer:

   @Bean public Executor taskExecutor() { return new ThreadPoolTaskExecutor(); } -> Option D

4. Quick Check:

   Spring thread pool bean = ThreadPoolTaskExecutor [OK]

Hint: Use ThreadPoolTaskExecutor for Spring thread pools [OK]

Common Mistakes:

• Using Executors.newFixedThreadPool directly without Spring wrapper
• Trying to instantiate ThreadPoolExecutor without parameters
• Confusing scheduled thread pools with normal thread pools

3. Given this Spring Boot configuration snippet, what will be the maximum number of threads allowed in the pool?

@Bean
public ThreadPoolTaskExecutor taskExecutor() {
  ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
  executor.setCorePoolSize(3);
  executor.setMaxPoolSize(6);
  executor.setQueueCapacity(10);
  executor.initialize();
  return executor;
}

medium

A. 6

B. 3

C. 10

D. 13

Solution

1. Step 1: Understand core and max pool size

   Core pool size is minimum threads kept alive; max pool size is the maximum threads allowed.

2. Step 2: Identify maxPoolSize value

   The code sets maxPoolSize to 6, so the pool can grow up to 6 threads.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Max threads = maxPoolSize = 6 [OK]

Hint: Max threads = maxPoolSize property [OK]

Common Mistakes:

• Confusing queue capacity with max threads
• Thinking corePoolSize is max threads
• Adding corePoolSize and queueCapacity incorrectly

4. What is wrong with this custom thread pool configuration in Spring Boot?

@Bean
public ThreadPoolTaskExecutor taskExecutor() {
  ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
  executor.setCorePoolSize(5);
  executor.setMaxPoolSize(3);
  executor.setQueueCapacity(10);
  executor.initialize();
  return executor;
}

medium

A. Missing @EnableAsync annotation on configuration class

B. Queue capacity cannot be set in ThreadPoolTaskExecutor

C. Core pool size is greater than max pool size, which is invalid

D. ThreadPoolTaskExecutor cannot be returned as a bean

Solution

1. Step 1: Check core and max pool size relationship

   Core pool size must be less than or equal to max pool size; here core is 5 but max is 3, which is invalid.

2. Step 2: Validate other settings

   Queue capacity is valid; @EnableAsync is optional but recommended; returning ThreadPoolTaskExecutor bean is correct.

3. Final Answer:

   Core pool size is greater than max pool size, which is invalid -> Option C

4. Quick Check:

   Core pool ≤ max pool required [OK]

Hint: Core pool size must not exceed max pool size [OK]

Common Mistakes:

• Setting core pool size larger than max pool size
• Assuming queue capacity is mandatory to set
• Forgetting to add @EnableAsync (not always required)

5. You want to create a custom thread pool in Spring Boot that has a core pool size of 4, max pool size of 8, and a queue capacity of 20. You also want to set a thread name prefix "MyApp-" for easier debugging. Which of the following code snippets correctly implements this configuration?

hard

A. @Bean
public ThreadPoolTaskExecutor taskExecutor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(4);
executor.setMaxPoolSize(8);
executor.setQueueCapacity(20);
executor.setThreadNamePrefix("MyApp-");
executor.initialize();
return executor;
}

B. @Bean
public Executor taskExecutor() {
return Executors.newFixedThreadPool(8);
}

C. @Bean
public ThreadPoolTaskExecutor taskExecutor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(8);
executor.setMaxPoolSize(4);
executor.setQueueCapacity(20);
executor.setThreadNamePrefix("MyApp-");
executor.initialize();
return executor;
}

D. @Bean
public ThreadPoolTaskExecutor taskExecutor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(4);
executor.setMaxPoolSize(8);
executor.setQueueCapacity(20);
executor.setThreadNamePrefix("App-");
executor.initialize();
return executor;
}

Solution

1. Step 1: Verify core, max pool sizes and queue capacity

   Core pool size 4, max pool size 8, and queue capacity 20 match the requirement in @Bean public ThreadPoolTaskExecutor taskExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(4); executor.setMaxPoolSize(8); executor.setQueueCapacity(20); executor.setThreadNamePrefix("MyApp-"); executor.initialize(); return executor; }.

2. Step 2: Check thread name prefix correctness

   @Bean public ThreadPoolTaskExecutor taskExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(4); executor.setMaxPoolSize(8); executor.setQueueCapacity(20); executor.setThreadNamePrefix("MyApp-"); executor.initialize(); return executor; } sets thread name prefix to "MyApp-" exactly as requested.

3. Step 3: Validate other options

   @Bean public Executor taskExecutor() { return Executors.newFixedThreadPool(8); } uses fixed thread pool without prefix; @Bean public ThreadPoolTaskExecutor taskExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(8); executor.setMaxPoolSize(4); executor.setQueueCapacity(20); executor.setThreadNamePrefix("MyApp-"); executor.initialize(); return executor; } reverses core and max sizes (invalid); @Bean public ThreadPoolTaskExecutor taskExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(4); executor.setMaxPoolSize(8); executor.setQueueCapacity(20); executor.setThreadNamePrefix("App-"); executor.initialize(); return executor; } uses wrong prefix.

4. Final Answer:

   Option A code snippet correctly implements the required configuration -> Option A

5. Quick Check:

   Correct sizes and prefix = @Bean public ThreadPoolTaskExecutor taskExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(4); executor.setMaxPoolSize(8); executor.setQueueCapacity(20); executor.setThreadNamePrefix("MyApp-"); executor.initialize(); return executor; } [OK]

Hint: Match core, max sizes and prefix exactly [OK]

Common Mistakes:

• Swapping core and max pool sizes
• Using wrong thread name prefix
• Using fixed thread pool instead of ThreadPoolTaskExecutor