How Can I Efficiently Wait for All Tasks to Complete in an ExecutorService?

ExecutorService: Await Task Completion

ExecutorService is a handy tool for efficient task execution, but pausing until all tasks are complete can be tricky. Let's dive into a common issue and its practical solution.

Error in wait()

The quoted code snippet, which employs a fixed-thread-pool ExecutorService, experiences an IllegalMonitorStateException when attempting to use es.wait(). This occurs because wait() is intended for use with specific thread synchronization idioms, not with ExecutorService.

Simplest Solution: invokeAll()

ExecutorService offers a simpler alternative for this purpose: invokeAll(). This method takes a collection of Callable tasks, executes them in parallel, and blocks until all tasks are finished. Here's how to use it:

ExecutorService es = Executors.newFixedThreadPool(2);
List<Callable<Object>> todo = new ArrayList<>(uniquePhrases.size());

for (DataTable singleTable: uniquePhrases) { 
    todo.add(Executors.callable(new ComputeDTask(singleTable))); 
}

List<Future<Object>> answers = es.invokeAll(todo);

invokeAll() returns a list of Future objects, each representing an individual task. Once all tasks are complete, accessing any Future's isDone() will return true.

Advantages of invokeAll()

• Blocks until all tasks are finished.
• Avoids manual shutdown and synchronization.
• Allows reuse of ExecutorService for multiple task cycles.

Other Methods

• shutdown() and awaitTermination() can also be used, but they require manual shutdown handling.
• submit(Callable) can be used for individual task submission, but it requires manual tracking of task completion.

Remember, invokeAll() requires your tasks to implement Callable. If ComputeDTask is a Runnable, wrap it as shown in the code snippet or utilize Executors.callable().

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