AsyncTask enables the proper and easy use of the UI thread. This class allows you to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers. This is the definition given for AsyncTask in android documentation. Now let us see how this class works internally.
AsyncTask uses Handler, Thread, Executor, FutureTask, Callable to achieve its functionality, which is to perform the operation in background and post the status and result of the operation onto the main thread. Whenever a new AsyncTask object is created, a new Callable and FutureTask instance are created in the constructor.
Every AsyncTask created will have a common THREAD_POOL_EXECUTOR and SERIAL_EXECUTOR at the process level to execute the tasks in a new thread.
SERIAL_EXECUTOR - This is the default executor used in AsyncTask, which executed the AsyncTask in a sequential order, using a Queue implementation. However, SERIAL_EXECUTOR used THREAD_POOL_EXECUTOR at the end to execute the task in a new thread.
THREAD_POOL_EXECUTOR - This executor is used when the AsyncTask is executed using executeOnExecutor() method. This executed uses Java’s ThreadPoolExecutor to maintain a pool of threads and execute all the asynctasks using threads from that pool.
So in order to execute multiple AsyncTask in parallel, we have to use THREAD_POOL_EXECUTOR and execute the AsyncTasks.
Below is the sample of running multiple AsyncTasks
for (Request req : allPendingRequest) {
new BatchTask().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, req);
}What the above code does is, for all the parallel requests to be made, it creates a new AsyncTask and instead of executing it using SERIAL_EXECUTOR(by using execute() method), we pass in the AsyncTask.THREAD_POOL_EXECUTOR to be used as executor for executing tasks in parallel.
This AsyncTask.THREAD_POOL_EXECUTOR will queue up all the task and execute it 5 tasks at a time and when any of running task is completed, the next on from the queue is executed. This is repeated until queue becomes empty. AsyncTask.THREAD_POOL_EXECUTOR cannot queue more that 128 tasks at a time. So if we submit more than that, an exception will be thrown.