Effective Java 68 Prefer executors and tasks to threads

Principle

The general mechanism for executing tasks is the executor service. If you think in terms of tasks and let an executor service execute them for you, you gain great flexibility in terms of selecting appropriate execution policies. In essence, the Executor Framework does for execution what the Collections Framework did for aggregation.

Exceutor Framework is a flexible interface-based task execution facility.

Creating work queue with Executor

ExecutorService executor = Executors.newSingleThreadExecutor();

executor.execute(runnable);

Here is how to tell the executor to terminate gracefully (if you fail to do this, it is likely that your VM will not exit):

executor.shutdown();

Usage of executor service

Usage	Utility
Wait for a particular task to complete.	"background thread SetObserver"
Wait for any or all of a collection of tasks to complete.	invokeAny or invokeAll methods
Wait for the executor service's graceful termination to complete.	awaitTermination method
Retrive the results of tasks one by one as they complete.	ExecutorCompletionService
Control every aspect of a thread pool's operation	ThreadPoolExecutor class
Small program or lightly loaded server (submitted tasks are not queued but immediately handed off to a thread for execution. If no threads are available, a new one is created. )	Executors.newCachedThreadPool
Heavily loaded production server	Executors.newFixedThreadPool ThreadPoolExecutor

Thread pool (Fixed or variable service number of threads) .

Task - The key abstraction is the unit of work.

Task category	return value
Runnable	N
callable	Y

	Multithread	Time accuracy for long running tasks	Grace recover on uncaught exception
java.util.Timer	N	N	N
ScheduledThreadPoolExecutor	Y	Y	Y