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.