How is a thread pool able to re-use threads?

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Our current course assignment specifies that we are supposed to create a manager for a thread pool using the "Object Pool Manager" design pattern which spawns a set amount of threads. The ownership of these threads shall be transferred to the client and then back to the pool after the client has finished using it. If no thread exists in the pool then the client has to wait.

My confusion comes from the fact that a thread is supposedly not reusable, which defeats the purpose of pooling them. Have I understood the assignment incorrectly?

multithreading java
2 Answers

Threads are reusable as long as they have not ended. A pool of threads generally involves threads that do work as it is given to them, and then wait for more work. Thus, they never end until explicitly told to do so. The trick is designing them in a way such that the work they are given ends, but the thread itself does not. Thread pools are useful because it is often relatively expensive to create/destroy threads.

user_1233780

@Kaliatech has already explained the concept behind re-use of threads. Also "The ownership of these threads shall be transferred to the client" is slightly misleading as the ownership of threads generally remain with the thread-pool/object-pool as it is the manager of this pool and the client should simply submits the task to the pool which can either complete successfully or fail. The thread continues to run ready to pick the next task submitted to the pool. As a design too the separation of task object ( Runnable/Callable) and the object representing thread execution (Thread) are designed to be different. Should the need arise the thread-pool is responsible for ramping up/down the number of threads as they are expensive to create and manage. Java ThreadPoolExecutor will be a good example to refer to how typically such a thread pool works.

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