13 Concurrent Programming using Tasks and Services

Transcription

1 60 Advanced Java for Bioinformatics, WS 17/18, D. Huson, January 18, Concurrent Programming using Tasks and Services Any programs that you write will be multi-threaded. Modern computers have multiple cores. The Java runtime enviroment uses many threads. How to explicitly use multiple threads? User iteractions are handled in the JavaFX Application Thread. Any computations that you have done in this thread must be very quick, on the order of milliseconds. Any longer calculations must be performed in a separate thread so as to prevent freezing the UI. Example: If you have a multiple alignment of n sequences of length k and you want to compute all n n pairwise distances between them, then this will take O(n 2 k) time, which can be on the order of seconds, for usual problem sizes, and so this should not be done in the JavaFX application thread. We want to be able to: Setup a calculation using the UI Run it in a separate thread Monitor progress of the calculation and be notified when it has completed Retrieve the result and present in the UI Calculation: Atlantic-1.1 cggccttctatctctcttcttctagctcacctagttccgcgttttgtttcacgattttcc Atlantic-1.2 cggccttctatctctcttcttctagcttacctagttccgcgttttgttccacgattttcc Atlantic-1.3 cggccttctatctctcttcttctagctcacctagttccgcgttttctttcacgattttcc Atlantic-1.4 cggccttctatctctcttcttctagttcacctagttctgcgttttgtttcacgatttttc Atlantic-1.5 cggccttctatctctcttcttctagcttacctagttccacgttttgttccacgattttcc Atlantic-2.1 cggccttctatctctcttcttttagcccgtctggttccgcgttttgtttcacgattttcc Atlantic-2.2 cggccttctatctctcttcttttagctcacctagttccgcgttttgttttacgattttcc Atlantic-3 cggccttttatctctcctctcctagcccgtctggttccgcgttttgtttcacgattttcc Atlantic-4 cggccttctgtctctcttcttctagcccacccggttctgcgctttgtttcacgattctcc Atlantic-5 tggccttctgtctctcttcttctagcccacccggttctgcgctttgtttcacgattctcc Atlantic-6 cggccttctatctcttttcttctagcccacctggttccgcgttctgtttcacaatttccc Atlantic-7 cggccttctatctctcctttcctagcccgtctggttccgcgttttgtttcacgattttcc Example: Connect to a webservice, request a service, capture the response and present to user. More concretely, for example, contact the PDB web service, request a list of all available PDB files, receive the list and present list to user. Use can then select a PDB file, have its content downloaded and then displayed in a text area:

2 Advanced Java for Bioinformatics, WS 17/18, D. Huson, January 18, Need to: Contact webservice, request data, download result, all in a separate thread show progress bar and allow cancel enable and disable controls 13.1 The Worker interface The Worker interface allows one to do work in a separate worker thread. It has a state property with the values: WorkerState.READY, SCHEDULED, RUNNING, CANCELED, SUCCEEDED and FAILED, indicting the state of the task. Other properties include: totalwork, workdone and progress. The progress property comunicates the amount of work completed, between 0 and 1, or 1, if unknown. Usually will bind a progress bar to this, thus: ProgressBar bar=new ProgressBar(); bar.progressproperty().bind(worker.progressproperty()); Additional properties: message: messages reporting on progress title: title of the task exception: exception that was thrown during calculation, if the state is FAILED value: value returned by task 13.2 The Task class The JavaFX class Task<V> implements Worker. It has one method that you must override: V call () This method should implement that calculation that you want to do in a separate thread and it should return a value of type V.

3 62 Advanced Java for Bioinformatics, WS 17/18, D. Huson, January 18, 2018 The return value can be retrieved using the method V get(), which blocks until the computation has been completed. The method call() will run in a separate thread, so you cannot directly modify the scene graph from it. Task<V> provides a methods for updating the above mentioned task specific properties from within the thread in which the calculation is running: updatemessage(string message) updateprogress(double workdone, double totalwork) updatetitle(string title) updatevalue(v value) 13.3 Example: prime numbers The following algorithm computes all prime numbers upto n (using the sieve of Eratosthenes ): public s t a t i c BitSet apply ( int n ) { BitSet a=new BitSet ( ) ; a. s e t ( 2, n+1); for ( int i =2; i<=math. s q r t ( n ) ; i ++) { i f ( a. get ( i ) ) { int i 2=i i ; for ( int j=i 2 ; j<=n ; j+=i ) { a. s e t ( j, f a l s e ) ; return a ; Rewrite as task: public class SieveOfEratosthenesTask extends Task<BitSet> { private f i n a l int n ; public SieveOfEratosthenesTask ( int n ) { this. n=n public BitSet c a l l ( ) { BitSet a=new BitSet ( ) ; a. s e t ( 2, n+1); double nsqrt=math. s q r t ( n ) ; updateprogress ( 0, nsqrt ) ; for ( int i =2; i<=nsqrt ; i ++) { i f ( a. get ( i ) ) { int i 2=i i ; for ( int j=i 2 ; j<=n ; j+=i ) { a. s e t ( j, f a l s e ) ; i f ( i s C a n c e l e d ( ) ) return null ; updateprogress ( i, nsqrt ) ; return a ; The task can be run like this:

4 Advanced Java for Bioinformatics, WS 17/18, D. Huson, January 18, SieveOfEratosthenesTask task=new SieveOfEratosthenesTask ( n ) ; Executors. newsinglethreadexecutor ( ). submit ( task ) ; How to monitor progress, update the state of the UI and capture the result during execution of the task? 13.4 Task event handlers A task provides a number of event handlers that correspond to the different worker states: task.setonsucceeded((workerstateevent e)->{...); task.setonfailed(...) task.setonscheduled(...) task.setonrunning(...) task.setoncanceled(...) These methods are called in the JavaFX application thread and so can be used to update the UI. For example, like this: task. setonscheduled ( ( v) >{ outputtextarea. c l e a r ( ) ; progressbar. s e t V i s i b l e ( true ) ; runbutton. s e t D i s a b l e ( true ) ; ) ; task. setonsucceeded ( ( v) >{ S t r i n g B u i l d e r buf=new S t r i n g B u i l d e r ( ) ; for ( int i=task. getvalue ( ). nextsetbit ( 2 ) ; i!= 1; i=task. getvalue ( ). nextsetbit ( i +1)) { i f ( buf. l e n g t h () >0) buf. append (, ) ; buf. append ( i ) ; outputtextarea. settext ( buf. t o S t r i n g ( ) ) ; progressbar. s e t V i s i b l e ( f a l s e ) ; runbutton. d i s a b l e P r o p e r t y ( ). s e t D i s a b l e ( f a l s e ) ; ) ; All this code is placed inside runbutton.setonaction(...) Service class A Task object can only be run once. For running tasks multiple times, JavaFX provides the Service class. The Service class implements the Worker interface and methods refer to the currently running task. It provides a number of additional methods: reset() - reset the service, prepare for running a new task, if state not RUNNING start - start new task, only if in state READY restart() - cancel task, if one is currently running, then start new task

5 64 Advanced Java for Bioinformatics, WS 17/18, D. Huson, January 18, 2018 To use a Service, you need only implement a method that creates a new task: protected Task<V> createtask() Like this: public class S i e v e O f E r a t o s t h e n e s S e r v i c e extends S e r v i c e <BitSet> { private int protected Task<BitSet> createtask ( ) { return new SieveOfEratosthenesTask ( n ) ; public int getn ( ) { return n ; public void setn ( int n ) { this. n = n ; As we saw above, without a service, we have to set the state change event handlers for the task each time a task object is created. This happens inside the onaction() method of the run button. When using a service, we set these handlers once for the service and then the service monitors the tasks for us, e.g.: S i e v e O f E r a t o s t h e n e s S e r v i c e s e r v i c e = new S i e v e O f E r a t o s t h e n e s S e r v i c e ( ) ; progressbar. v i s i b l e P r o p e r t y ( ). bind ( s e r v i c e. runningproperty ( ) ) ; progressbar. p r o g r e s s P r o p e r t y ( ). bind ( s e r v i c e. p r o g r e s s P r o p e r t y ( ) ) ; runbutton. d i s a b l e P r o p e r t y ( ). bind ( inputtextfield. textproperty ( ). isempty ( ). or ( s e r v i c e. runningproperty ( ) ) ) ; s e r v i c e. setonscheduled ( ( v ) > { outputtextarea. c l e a r ( ) ; ) ; s e r v i c e. setonsucceeded ( ( v ) > { f i n a l S t r i n g B u i l d e r buf = new S t r i n g B u i l d e r ( ) ;... The code for the run button needs now only launch the task and does not have to deal with setting up state change handlers and bindings: runbutton. setonaction ( ( e ) > { int n = I n t e g e r. p a r s e I n t ( inputtextfield. gettext ( ) ) ; s e r v i c e. setn ( n ) ; s e r v i c e. r e s t a r t ( ) ; ) ; 13.6 Summary Lengthy computations or connections with large files or remote locations should take place in a separate thread. For this, design a class that extends Service. It will have a number of methods that allow you to configure a task and it will implement the method createtask so as to create new task objects using the set configurations. In your task implementation, you use updateprogress() to allow monitoring of progress. Also, you check for iscanceled() and abort if this returns true. Also, you might use updatemessage() to report on the status, e.g. contacting webservice or waiting for response. Monitor the status of the service so as to enable and disable control items appropriately. For example, disable launching of a task while one is currently being executed:

6 Advanced Java for Bioinformatics, WS 17/18, D. Huson, January 18, runbutton.disableproperty().bind(inputtextfield.textproperty().isempty().or(service.runningproperty()));