ThreadPoolExecutor - Core and maximum pool sizes [duplicate]

问题

This question already has an answer here:

• What is the difference between corePoolSize and maxPoolSize in the Spring ThreadPoolTaskExecutor 5 answers

When a new task is submitted in method execute(java.lang.Runnable),and fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle.

1) Why there is a need to create a new thread to handle the request if there are idle threads?

If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full.

2) I don't understand the difference between corePoolSize and maximumPoolSize here. Secondly, how can a queue be full when threads are less than maximumPoolSize? Queue can only be full if threads are equal to or more than maximumPoolSize. Isn't it?

回答1:

Here are Sun’s rules for thread creation in simple terms:

1. If the number of threads is less than the corePoolSize, create a new Thread to run a new task.
2. If the number of threads is equal (or greater than) the corePoolSize, put the task into the queue.
3. If the queue is full, and the number of threads is less than the maxPoolSize, create a new thread to run tasks in.
4. If the queue is full, and the number of threads is greater than or equal to maxPoolSize, reject the task.

Full article

回答2:

You can find the definition of the terms corepoolsize and maxpoolsize in the javadoc. http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ThreadPoolExecutor.html

The link above has the answer to your question. However, just to make it clear. The application will creating threads until it reaches the corePoolSize. It means that these number of threads should be sufficient to handle the inflow of tasks. After that the tasks will be queued. Once the queue is full the executor will start creating new threads. It is kind of balancing. What it essentially means is that the inflow of tasks is more than the processing capacity. So, Executor will start creating new threads again till it reaches Max number of threads. Again, a new threads will be created if and only if the queue is full.

回答3:

Core and maximum pool sizes

A ThreadPoolExecutor will automatically adjust the pool size according to the bounds set by corePoolSize and maximumPoolSize .

When a new task is submitted in method execute(java.lang.Runnable), and fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle. If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full.By setting corePoolSize and maximumPoolSize the same, you create a fixed-size thread pool.

By setting maximumPoolSize to an essentially unbounded value such as Integer.MAX_VALUE, you allow the pool to accommodate an arbitrary number of concurrent tasks. Most typically, core and maximum pool sizes are set only upon construction, but they may also be changed dynamically using setCorePoolSize(int) and setMaximumPoolSize(int). link

来源：https://stackoverflow.com/questions/17660048/threadpoolexecutor-core-and-maximum-pool-sizes