Concurrency

Tomcat Threads vs ThreadPoolTaskExecutor

Aspect	Tomcat Threads	ThreadPoolTaskExecutor
Scope	Web server level (HTTP request handling)	Application level (task execution)
Primary Use	Process incoming HTTP requests	Execute asynchronous/background tasks
Configuration	Tomcat server.xml or properties	Spring configuration (Java/XML)
Thread Management	Managed by Tomcat	Managed by Spring/application
Task Type	Synchronous HTTP request processing	Asynchronous or parallel tasks
Queueing	Limited queueing (depends on acceptCount)	Explicit task queue with configurable size
Control	Limited application control	Full application control
Use Case	Handling client HTTP requests	Offloading long-running/non-HTTP tasks
Example	Processing servlet/controller requests	Running @Async methods, batch jobs

Key Concept

• Core Pool Size: The minimum number of threads kept alive in the pool, even when idle
• Max Pool Size: The maximum number of threads allowed in the pool when the task queue is full
• Queue Capacity: The number of tasks that can be queued when all core threads are busy (before scaling up to maxPoolSize)

Example: To analyze the first 200 requests that arrive to the server in just 0.02 seconds(20ms), processing time of each task: 10ms

• Configuration
  • Core Pool Size: 10
  • Max Pool Size: 20
  • Queue Capacity: 25
• Processing
  1. Initialization: 10 threads are available and queue is empty(0/25)
  2. First 10 requests(1-10): 10 threads are processing the first 10 requests, queue is empty(0/25)
  3. Next 25 requests(11-35): with all 10 core threads busy, the next 25 requests are queued(25/25)
  4. Next 10 requests(36-45):
     • The queue is full, so the executor creates additional threads up to the max pool size(20)
     • Requests 36–45 are assigned to 10 new threads(total 20 threads: 10 core + 10 additional)
  5. Request 46-200:
     • The queue is full (25 tasks), and the max pool size is reached(20 threads)
     • Any additional tasks (requests 46–200) cannot be processed or queued
     • The default AbortPolicy kicks in, throwing a RejectedExecutionException for each of these 155 requests