I have an event queue to process. A thread adds events to the queue.
I have created a runnable Task that in the run method does all which is necessary to process the event.
I have declared an Executors.newCachedThreadPool(); and I execute each Task.
public class EventHandler {
private static final ExecutorService handlers = Executors.newCachedThreadPool();
public void handleNextEvent(AnEvent event){
handlers.execute(new Task(evt));
}
public class Task implements Runnable{
#Override
public void run() {
//Event processing
}
}
public AnotherClass{
public void passEvent(AnEvent evt)//This is called by another thread
{
EventHandler.handleNextEvent(evt);
}
}
My problem is that if I call execute of the executor, my code will get the next event and run next runnable via the executor.
My purpose is to process next event from queue only after previous task has ended.
How would I know that the previous task has finished or not so that I know I can call handleNextEvent again?
Is having some status field updated by the Task a good idea?
Thanks
Executors.newCachedThreadPool() will create new threads on demand, so it's not what you want. You want something like Executors.newSingleThreadExecutor(), which will process the events one at a time, and queue up the rest.
See javadoc:
Creates an Executor that uses a single worker thread operating off an unbounded queue. (Note however that if this single thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.) Tasks are guaranteed to execute sequentially, and no more than one task will be active at any given time.
I think Executors.newSingleThreadExecutor() and the submit() Method are the solution to your problem: http://download.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html
Related
I have an application in which there are multiple threads. I want them to execute in order.so i choose executorService for multi-threading. if any one of thread(run method) is in error , I want to move on to net thread so that by the end i can come to know how many thread are completed successfully (count needed).My sample code:
The Main class:
public class MySampleClass{
ExecutorService executor = Executors.newSingleThreadExecutor();
for(int i=0; i<=100;i++){
executor.submit(new ThreadClass());
}
//After all threads executed now to shutdown executor
executor.shutdown()
executor.awaitForTermination(1,Time.MILLISECONDS);
My Sample Thread Class :
public class ThreadClass implements Runnable{
#override
public void run(){
boolean isCompleted= doAction();
if(!isCompleted){
// I want here to stop this thread only..what to do ?
//executor.shutdown will stop all other threads
}
}
}
Any Suggestion what to do ?? Am i doing it wrong way ?
Thread.currentThread().interrupt();
You shouldn't stop a thread. There is a reason Thread.stop is deprecated. Instead you can interrupt the current thread.
You can use Callable instead of Runnable. If you do that, submit method returns a Future (http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Future.html) instance on which you can verify if the callable do it´s work in the right way. The documentation explains it:
http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html#submit(java.util.concurrent.Callable)
Hope i explained in the right way.
I have a ScheduledThreadPoolExecutor which has one thread and runs for every 30 seconds.
Now, if the current executing thread throws some exception, then I need to make sure that the next thread do not run and the the ScheduledThreadPoolExecutor is down.
How do I achieve this?
Catch the exception call shutdown/shutdownNow API in ExecutorService
shutdown()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down.
This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.
shutdownNow()
Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.
This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that.
There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so any task that fails to respond to interrupts may never terminate.
Refer to these post for more details with working code.
How to forcefully shutdown java ExecutorService
As a clean way, you can simply use a static accessed class to set/check the execution availability.
import java.util.concurrent.atomic.AtomicBoolean;
class ThreadManager
{
private static AtomicBoolean shouldStop = new AtomicBoolean(false);
public static void setExceptionThrown(boolean val)
{
shouldStop.set(val);
}
public boolean shouldExecuteTask()
{
return !shouldStop.get();
}
}
And a custom runnable implementation that allows you to check for the possibility to execute the task
abstract class ModdedRunnable implements Runnable
{
#Override
public void run()
{
if(ThreadManager.shouldExecuteTask())
{
try
{
runImpl();
}
catch(Exception t)
{
ThreadManager.setExceptionThrown(true);
}
}
}
public abstract void runImpl() throws Exception;
}
I am trying to implement a simple synchronization strategy in android.
A service instantiates class A and calls it's method sendToServer() for every iteration of a loop. This results in multiple Async tasks being started and the service ends immediately. The service may run again anytime and repeat the process.
So, to prevent two Async tasks from taking the same input, i store the Ids in a synchronized list and check the list before i start the async task.
But i am confused which piece of code i need to put in a synchronized block? Do i define the entire method isAlreadyRunning() as synchronized? Or do i not need to define any synchronized block of code at all?
Here is my class :
public class A{
private static List<Integer> idList = Collections.synchronizedList(new ArrayList<Integer>());
private boolean isAlreadyRunning(id){
//iterate through the list and return true if the id is already present
....
}
private class sendToServerAsyncTask extends AsyncTask<Void, Void, Boolean>{
#Override
protected Boolean doInBackground(Void... params) {
//send http request
}
#Override
protected void onPostExecute(Boolean result){
idList.remove(id);
}
}
public void sendToServer(int id) {
if(isAlreadyRunning(id)){
// an async task is already running for this id.
//,so dont start the async task again, just exit
return;
else {
idList.add(id);
new sendToServerAsyncTask(id).execute();
}
}
}
As per Android's documentation
ASYNC TASK's ORDER OF EXECUTION
When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.
The instances of Asynctask are already placed in a queue maintained by the framework and they are executed sequentially i.e. only after one task finishes the other will start so there is no chance of issue due to parallel execution because it doesn't exist.
So you need not do anything and the framework will take care of it for you.
I have a series of tasks (i.e. Runnables) to be executed by an Executor.
Each task requires a certain condition to be valid in order to proceed. I would be interested to know if there is a way to somehow configure Executor to move tasks in the end of the queue and try to execute them later when the condition would be valid and the task be able to execute and finish.
So the behavior be something like:
Thread-1 take tasks from queue and run is called
Inside run the condition is not yet valid
Task stops and Thread-1 places task in the end of the queue and
gets next task to execute
Later on Thread-X (from thread pool) picks task again from queue condition is valid
and task is being executed
In Java 6, the ThreadPoolExecutor constructor takes a BlockingQueue<Runnable>, which is used to store the queued tasks. You can implement such a blocking queue which overrides the poll() so that if an attempt is made to remove and execute a "ready" job, then poll proceeds as normal. Otherwise the runnable is place at the back of the queue and you attempt to poll again, possibly after a short timeout.
Unless you have to have busy waiting, you can add a repeating task to a ScheduledExecutorService with an appropriate polling interval which you cancel or kill after it is "valid" to run.
ScheduleExecutorService ses = ...
ses.scheduleAtFixedRate(new Runnable() {
public void run() {
if (!isValid()) return;
preformTask();
throw new RuntimeException("Last run");
}
}, PERIOD, PERIOD, TimeUnit.MILLI_SECONDS);
Create the executor first.
You have several possibilites.
If I suppose that your tasks implement a simple interface to query their status (something like an enum with 'NeedReschedule' or 'Completed'), then implement a wrapper (implementing Runnable) for your tasks which will take the task and the executor as instanciation parameters. This wrapper will run the task it is bound to, check its status afterwards, and if necessary reschedule a copy of itself in the executor before terminating.
Alternatively, you could use an execption mechanism to signal the wrapper that the task must be rescheduled.
This solution is simpler, in the sense that it doesn't require a particular interface for you task, so that simple Runnable could be thrown in the system without trouble. However, exceptions incur more computation time (object construction, stack trace etc.).
Here's a possible implementation of the wrapper using the exception signaling mechanism.
You need to implement the RescheduleException class extending Throwable, which may be fired by the wrapped runnable (no need for a more specific interface for the task in this setup). You could also use a simple RuntimeException as proposed in another answer, but you will have to test the message string to know if this is the exception you are waiting for.
public class TaskWrapper implements Runnable {
private final ExecutorService executor;
private final Runnable task;
public TaskWrapper(ExecutorService e, Runnable t){
executor = e;
task = t;
}
#Override
public void run() {
try {
task.run();
}
catch (RescheduleException e) {
executor.execute(this);
}
}
Here's a very simple application firing up 200 wrapped tasks randomly asking a reschedule.
class Task implements Runnable {
#Override
public void run(){
if (Maths.random() > 0.5)
throw new RescheduleException();
}
}
public class Main {
public static void main(String[] args){
ExecutorService executor = Executors.newFixedThreadPool(10);
int i = 200;
while(i--)
executor.execute(new TaskWrapper(executor, new Task());
}
}
You could also have a dedicated thread to monitor the other threads results (using a message queue) and reschedule if necessary, but you lose one thread, compared to the other solution.
I recently began working with Threads and I am trying to complete a Java implementation of the Looper class in Android. Basically I am making a Java class that puts threads into a queue that will then be executed by the Looper class. I have the code completed for the most part but have an issue with the enqueuing of tasks.
In the Looper class I have the queue declared and my enqueue method:
List<Runnable> queue;
public synchronized void enqueue(Runnable runnable) {
queue.add(runnable);
notify(); // signal a waiting thread
}
I then created another class called TaskManager to add Tasks into the queue. I receive the error when I call:
loop.enqueue(new Task());
Where Task() implements runnable and just adds two integers together in its run() method...this is just a test.
The error I receive is:
Exception in thread "Thread-0" java.lang.NullPointerException
at Looper.enqueue(Looper.java:20) (this is the queue.add(runnable))
at TaskMaker.run(TaskMaker.java:16) (this is the loop.enqueue(new Task())
I'm obviously doing something wrong and not implementing this right...how should I go about this? Is the way I am enqueuing the task right? Thanks for any help it is much appreciated!
Are you initializing the queue variable? like:
List<Runnable> queue = new ArrayList<Runnable>();