Executors class has factory methods that act on a pool of threads, rather than on a single thread. A thread pool is a group of pre-instantiated reusable threads that are available to perform a set of arbitrary tasks. The following table describes methods for creating pools :
| Method Name | Return Type | Description |
| newSingleThreadExecutor() | ExecutorService | Creates a single-threaded executor that uses a single worker thread operating off an unbounded queue. Results are processed sequentially in the order in which they are submitted. |
|
newSingleThread ScheduledExecutor() |
ScheduledExecutor Service |
Creates a single-threaded executor that can schedule commands to run after a given delay or to execute periodically |
| newCachedThreadPool() | ExecutorService | Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available |
| newFixedThreadPool(int nThreads) | ExecutorService | Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue |
| newScheduledThreadPool(int nThreads) |
ScheduledExecutor Service |
Creates a thread pool that can schedule commands to run after a given delay or to execute periodically |
There are also overloaded versions of each of the methods in above table that create threads using a ThreadFactory input parameter.
The difference between a single-thread and a pooled-thread executor is what happens when a task is already running. While a single-thread executor will wait for an available thread to become available before running the next task, a pooled-thread executor can execute the next task concurrently. If the pool runs out of available threads, the task will be queued by the thread executor and wait to be completed.
