CODEWITHSUNDEEP
javascheduled-tasks
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Reputation: 1611

Is ScheduledThreadPoolExecutor ok for doing multiple tasks at same time?

The docs for ScheduledThreadPoolExecutor says that - Tasks scheduled for exactly the same execution time are enabled in first-in-first-out (FIFO) order of submission.

Does this mean that the tasks which SHOULD be done at the same time are never done at the same time. Instead they are executed in FIFO order ?

If that is true then which class do I use which is better than Timer and also does not have this FIFO problem ?

Upvotes: 6

Views: 4615

Answers (3)

creinig
creinig

Reputation: 1580

Please note that the docs talk about "enabling" the tasks and that we are talking about a threadpool executor. :-)

That means the tasks will wait until the designated time, then they are treated as if put into a normal ThreadPoolExecutor. If there are enough threads available in the pool all these tasks will be run in parallel.

Only if you have more tasks becoming active than available threads in the pool some tasks will have to wait.

Upvotes: 1

Taylor
Taylor

Reputation: 4086

The way a ScheduledThreadPoolExecutor works is there is a single "scheduling" or master thread which checks for tasks to execute.

If it finds a task, it delegates it to a "worker" thread from the pool.

If multiple tasks are ready to be executed, they are "kicked off" one at a time, though once "kicked off", subsequent processing is concurrent, per Java's definition.

If you have two tasks that are both scheduled through the executor for the same time, the order in which they complete could vary from run to run and unless you put in specific controls such as locks, waits, etc... to handle this, it's up to java's thread scheduling (how java allots time to threads on a core) to determine how and when what gets processed. Please note that setting up such locks, waits, etc... is a deceptively complex task prone to race conditions leading to unexpected deadlocks, live locks, etc...

Upvotes: 6

Peter Bratton
Peter Bratton

Reputation: 6408

It depends on the size of your thread pool. If you schedule 1000 tasks to fire at midnight, and you only have 25 threads, then only 25 can be executed initially, while the rest must wait for available threads. FIFO here refers to the order in which the executor will hand tasks off to the execution threads.

Upvotes: 1

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