Java Language Executor, ExecutorService and Thread pools ThreadPoolExecutor


A common Executor used is the ThreadPoolExecutor, which takes care of Thread handling. You can configure the minimal amount of Threads the executor always has to maintain when there's not much to do (it's called core size) and a maximal Thread size to which the Pool can grow, if there is more work to do. Once the workload declines, the Pool slowly reduces the Thread count again until it reaches min size.

ThreadPoolExecutor pool = new ThreadPoolExecutor(
    1,                                     // keep at least one thread ready, 
                                           // even if no Runnables are executed
    5,                                     // at most five Runnables/Threads
                                           // executed in parallel
    1, TimeUnit.MINUTES,                   // idle Threads terminated after one
                                           // minute, when min Pool size exceeded
    new ArrayBlockingQueue<Runnable>(10)); // outstanding Runnables are kept here

pool.execute(new Runnable() {
    @Override public void run() {
        //code to run

Note If you configure the ThreadPoolExecutor with an unbounded queue, then the thread count will not exceed corePoolSize since new threads are only created if the queue is full:

ThreadPoolExecutor with all parameters:

ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, 
TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory,
RejectedExecutionHandler handler)

from JavaDoc

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


  1. BlockingQueue size can be controlled and out-of-memory scenarios can be avoided. Application performance won't be degraded with limited bounded queue size.

  2. You can use existing or create new Rejection Handler policies.

    1. In the default ThreadPoolExecutor.AbortPolicy, the handler throws a runtime RejectedExecutionException upon rejection.

    2. In ThreadPoolExecutor.CallerRunsPolicy, the thread that invokes execute itself runs the task. This provides a simple feedback control mechanism that will slow down the rate that new tasks are submitted.

    3. In ThreadPoolExecutor.DiscardPolicy, a task that cannot be executed is simply dropped.

    4. In ThreadPoolExecutor.DiscardOldestPolicy, if the executor is not shut down, the task at the head of the work queue is dropped, and then execution is retried (which can fail again, causing this to be repeated.)

  3. Custom ThreadFactory can be configured, which is useful :

    1. To set a more descriptive thread name
    2. To set thread daemon status
    3. To set thread priority

Here is a example of how to use ThreadPoolExecutor