I'm new to groovy and have this simple code snippet:
import java.util.concurrent.CountDownLatch
import java.util.concurrent.TimeUnit
CountDownLatch called = new CountDownLatch(1)
Timer timer = new Timer()
timer.schedule(new TimerTask() {
void run() {
called.countDown()
}
}, 0)
assert called.await(2, TimeUnit.SECONDS)
Upon running on command line, it just hang there, nothing happen. I expect that this program should quit at once. So where did I get wrong?
Actually, it's not await that leads to hanging in your case. It's just the Timer's thread is not a daemon. The JVM cannot terminate until all remaining running threads are daemons. As Thread#setDaemon() javadoc states it:
...The Java Virtual Machine exits when the only threads running are all daemon threads.
So in your case
You can just specify that the timer's thread is a daemon
CountDownLatch called = new CountDownLatch(1)
Timer timer = new Timer(true) //true means the underlying thread is a daemon
timer.schedule(new TimerTask() {
void run() {
called.countDown()
}
}, 0)
assert called.await(2, TimeUnit.SECONDS)
println("It's not await that leads to hanging")
Or if for some reason you don't want your timer's thread to be a daemon. E.g. you want the timer to handle all the scheduled tasks before the JVM terminates. In this case you can just cancel the timer at some appropriate moment
CountDownLatch called = new CountDownLatch(1)
Timer timer = new Timer() //now the underlying thread is NOT a daemon
timer.schedule(new TimerTask() {
void run() {
called.countDown()
}
}, 0)
assert called.await(2, TimeUnit.SECONDS)
println("It's not await that leads to hanging")
timer.cancel()//now we are done. All scheduled tasks will be cancelled. However, the running one will finish its job
P.S. If you want more flexible way of scheduling you can take a look at ScheduledThreadPoolExecutor. As Timers javadoc says:
...It is effectively a more versatile replacement for the Timer/TimerTask combination, as it allows multiple service threads, accepts various time units, and doesn't require subclassing TimerTask (just implement Runnable). Configuring ScheduledThreadPoolExecutor with one thread makes it equivalent to Timer.
I'm trying to understand how threads works in java. This is a simple database request that returns a ResultSet. I'm using JavaFx.
package application;
import java.sql.ResultSet;
import java.sql.SQLException;
import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
public class Controller{
#FXML
private Button getCourseBtn;
#FXML
private TextField courseId;
#FXML
private Label courseCodeLbl;
private ModelController mController;
private void requestCourseName(){
String courseName = "";
Course c = new Course();
c.setCCode(Integer.valueOf(courseId.getText()));
mController = new ModelController(c);
try {
ResultSet rs = mController.<Course>get();
if(rs.next()){
courseCodeLbl.setText(rs.getString(1));
}
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
// return courseName;
}
public void getCourseNameOnClick(){
try {
// courseCodeLbl.setText(requestCourseName());
Thread t = new Thread(new Runnable(){
public void run(){
requestCourseName();
}
}, "Thread A");
t.start();
} catch (NumberFormatException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
This returns an exception:
Exception in thread "Thread A" java.lang.IllegalStateException: Not on FX application thread; currentThread = Thread A
How do I correctly implement threading so that every database request is executed in a second thread instead of the main thread?
I've heard of implementing Runnable but then how do I invoke different methods in run method?
Never worked with threading before but I thought it's time for it.
Threading Rules for JavaFX
There are two basic rules for threads and JavaFX:
Any code that modifies or accesses the state of a node that is part of a scene graph must be executed on the JavaFX application thread. Certain other operations (e.g. creating new Stages) are also bound by this rule.
Any code that may take a long time to run should be executed on a background thread (i.e. not on the FX Application Thread).
The reason for the first rule is that, like most UI toolkits, the framework is written without any synchronization on the state of elements of the scene graph. Adding synchronization incurs a performance cost, and this turns out to be a prohibitive cost for UI toolkits. Thus only one thread can safely access this state. Since the UI thread (FX Application Thread for JavaFX) needs to access this state to render the scene, the FX Application Thread is the only thread on which you can access "live" scene graph state. In JavaFX 8 and later, most methods subject to this rule perform checks and throw runtime exceptions if the rule is violated. (This is in contrast to Swing, where you can write "illegal" code and it may appear to run fine, but is in fact prone to random and unpredictable failure at arbitrary time.) This is the cause of the IllegalStateException you are seeing: you are calling courseCodeLbl.setText(...) from a thread other than the FX Application Thread.
The reason for the second rule is that the FX Application Thread, as well as being responsible for processing user events, is also responsible for rendering the scene. Thus if you perform a long-running operation on that thread, the UI will not be rendered until that operation is complete, and will become unresponsive to user events. While this won't generate exceptions or cause corrupt object state (as violating rule 1 will), it (at best) creates a poor user experience.
Thus if you have a long-running operation (such as accessing a database) that needs to update the UI on completion, the basic plan is to perform the long-running operation in a background thread, returning the results of the operation when it is complete, and then schedule an update to the UI on the UI (FX Application) thread. All single-threaded UI toolkits have a mechanism to do this: in JavaFX you can do so by calling Platform.runLater(Runnable r) to execute r.run() on the FX Application Thread. (In Swing, you can call SwingUtilities.invokeLater(Runnable r) to execute r.run() on the AWT event dispatch thread.) JavaFX (see later in this answer) also provides some higher-level API for managing the communication back to the FX Application Thread.
General Good Practices for Multithreading
The best practice for working with multiple threads is to structure code that is to be executed on a "user-defined" thread as an object that is initialized with some fixed state, has a method to perform the operation, and on completion returns an object representing the result. Using immutable objects, in particular, a record, for the initialized state and computation result is highly desirable. The idea here is to eliminate the possibility of any mutable state being visible from multiple threads as far as possible. Accessing data from a database fits this idiom nicely: you can initialize your "worker" object with the parameters for the database access (search terms, etc). Perform the database query and get a result set, use the result set to populate a collection of domain objects, and return the collection at the end.
In some cases it will be necessary to share mutable state between multiple threads. When this absolutely has to be done, you need to carefully synchronize access to that state to avoid observing the state in an inconsistent state (there are other more subtle issues that need to be addressed, such as liveness of the state, etc). The strong recommendation when this is needed is to use a high-level library to manage these complexities for you.
Using the javafx.concurrent API
JavaFX provides a concurrency API that is designed for executing code in a background thread, with API specifically designed for updating the JavaFX UI on completion of (or during) the execution of that code. This API is designed to interact with the java.util.concurrent API, which provides general facilities for writing multithreaded code (but with no UI hooks). The key class in javafx.concurrent is Task, which represents a single, one-off, unit of work intended to be performed on a background thread. This class defines a single abstract method, call(), which takes no parameters, returns a result, and may throw checked exceptions. Task implements Runnable with its run() method simply invoking call(). Task also has a collection of methods which are guaranteed to update state on the FX Application Thread, such as updateProgress(...), updateMessage(...), etc. It defines some observable properties (e.g. state and value): listeners to these properties will be notified of changes on the FX Application Thread. Finally, there are some convenience methods to register handlers (setOnSucceeded(...), setOnFailed(...), etc); any handlers registered via these methods will also be invoked on the FX Application Thread.
So the general formula for retrieving data from a database is:
Create a Task to handle the call to the database.
Initialize the Task with any state that is needed to perform the database call.
Implement the task's call() method to perform the database call, returning the results of the call.
Register a handler with the task to send the results to the UI when it is complete.
Invoke the task on a background thread.
For database access, I strongly recommend encapsulating the actual database code in a separate class that knows nothing about the UI (Data Access Object design pattern). Then just have the task invoke the methods on the data access object.
So you might have a DAO class like this (note there is no UI code here):
public class WidgetDAO {
// In real life, you might want a connection pool here, though for
// desktop applications a single connection often suffices:
private Connection conn ;
public WidgetDAO() throws Exception {
conn = ... ; // initialize connection (or connection pool...)
}
public List<Widget> getWidgetsByType(String type) throws SQLException {
try (PreparedStatement pstmt = conn.prepareStatement("select * from widget where type = ?")) {
pstmt.setString(1, type);
ResultSet rs = pstmt.executeQuery();
List<Widget> widgets = new ArrayList<>();
while (rs.next()) {
Widget widget = new Widget();
widget.setName(rs.getString("name"));
widget.setNumberOfBigRedButtons(rs.getString("btnCount"));
// ...
widgets.add(widget);
}
return widgets ;
}
}
// ...
public void shutdown() throws Exception {
conn.close();
}
}
Retrieving a bunch of widgets might take a long time, so any calls from a UI class (e.g a controller class) should schedule this on a background thread. A controller class might look like this:
public class MyController {
private WidgetDAO widgetAccessor ;
// java.util.concurrent.Executor typically provides a pool of threads...
private Executor exec ;
#FXML
private TextField widgetTypeSearchField ;
#FXML
private TableView<Widget> widgetTable ;
public void initialize() throws Exception {
widgetAccessor = new WidgetDAO();
// create executor that uses daemon threads:
exec = Executors.newCachedThreadPool(runnable -> {
Thread t = new Thread(runnable);
t.setDaemon(true);
return t ;
});
}
// handle search button:
#FXML
public void searchWidgets() {
final String searchString = widgetTypeSearchField.getText();
Task<List<Widget>> widgetSearchTask = new Task<List<Widget>>() {
#Override
public List<Widget> call() throws Exception {
return widgetAccessor.getWidgetsByType(searchString);
}
};
widgetSearchTask.setOnFailed(e -> {
widgetSearchTask.getException().printStackTrace();
// inform user of error...
});
widgetSearchTask.setOnSucceeded(e ->
// Task.getValue() gives the value returned from call()...
widgetTable.getItems().setAll(widgetSearchTask.getValue()));
// run the task using a thread from the thread pool:
exec.execute(widgetSearchTask);
}
// ...
}
Notice how the call to the (potentially) long-running DAO method is wrapped in a Task which is run on a background thread (via the accessor) to prevent blocking the UI (rule 2 above). The update to the UI (widgetTable.setItems(...)) is actually executed back on the FX Application Thread, using the Task's convenience callback method setOnSucceeded(...) (satisfying rule 1).
In your case, the database access you are performing returns a single result, so you might have a method like
public class MyDAO {
private Connection conn ;
// constructor etc...
public Course getCourseByCode(int code) throws SQLException {
try (PreparedStatement pstmt = conn.prepareStatement("select * from course where c_code = ?")) {
pstmt.setInt(1, code);
ResultSet results = pstmt.executeQuery();
if (results.next()) {
Course course = new Course();
course.setName(results.getString("c_name"));
// etc...
return course ;
} else {
// maybe throw an exception if you want to insist course with given code exists
// or consider using Optional<Course>...
return null ;
}
}
}
// ...
}
And then your controller code would look like
final int courseCode = Integer.valueOf(courseId.getText());
Task<Course> courseTask = new Task<Course>() {
#Override
public Course call() throws Exception {
return myDAO.getCourseByCode(courseCode);
}
};
courseTask.setOnSucceeded(e -> {
Course course = courseTask.getCourse();
if (course != null) {
courseCodeLbl.setText(course.getName());
}
});
exec.execute(courseTask);
The API docs for Task have many more examples, including updating the progress property of the task (useful for progress bars..., etc.
Related
JavaFX - Background Thread for SQL Query
Sample for accessing a local database from JavaFX using concurrent tasks for database operations so that the UI remains responsive.
Exception in thread "Thread A" java.lang.IllegalStateException: Not on FX application thread; currentThread = Thread A
The exception is trying to tell you that you are trying to access JavaFX scene graph outside the JavaFX application thread. But where ??
courseCodeLbl.setText(rs.getString(1)); // <--- The culprit
If I can't do this how do I use a background thread?
The are different approaches which leads to similar solutions.
Wrap you Scene graph element with Platform.runLater
There easier and most simple way is to wrap the above line in Plaform.runLater, such that it gets executed on JavaFX Application thread.
Platform.runLater(() -> courseCodeLbl.setText(rs.getString(1)));
Use Task
The better approach to go with these scenarios is to use Task, which has specialized methods to send back updates. In the following example, I am using updateMessage to update the message. This property is bind to courseCodeLbl textProperty.
Task<Void> task = new Task<Void>() {
#Override
public Void call() {
String courseName = "";
Course c = new Course();
c.setCCode(Integer.valueOf(courseId.getText()));
mController = new ModelController(c);
try {
ResultSet rs = mController.<Course>get();
if(rs.next()) {
// update message property
updateMessage(rs.getString(1));
}
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null;
}
}
public void getCourseNameOnClick(){
try {
Thread t = new Thread(task);
// To update the label
courseCodeLbl.textProperty.bind(task.messageProperty());
t.setDaemon(true); // Imp! missing in your code
t.start();
} catch (NumberFormatException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
This has nothing to do with database. JavaFx, like pretty much all GUI libraries, requires that you only use the main UI thread to modify the GUI.
You need to pass the data from the database back to the main UI thread. Use Platform.runLater() to schedule a Runnable to be run in the main UI thread.
public void getCourseNameOnClick(){
new Thread(new Runnable(){
public void run(){
String courseName = requestCourseName();
Platform.runLater(new Runnable(){
courseCodeLbl.setText(courseName)
});
}
}, "Thread A").start();
}
Alternatively, you can use Task.
I have below code, the doTransaction method is getting invoked after 20 seconds, could any one tell me why its taking that long to invoke doTransaction method.
Its not happening always its happening very rarely. Any help on this is appreciated.
Thanks,
ExecutorService service = Executors.newSingleThreadExecutor();
TransactionTask task = new TransactionTask(object);
Future<Result> future = service.submit(task);
Result r = future.get(20000, TimeUnit.MILLISECONDS);
class TransactionTask implements Callable<Result> {
private Request req = null;
public TransactionTask(Request trx){
this.req = trx;
}
#Override
public Result call() throws Exception {
Result o = doTransaction(req);
return o;
}
}
The above behavior could be explained if you submit multiple tasks.
The ExecutorService you created is single threaded, so it could only execute only one task concurrently. Other tasks if submitted will be in queue and will start executing if the former task is terminated/completed
It executes only after 20 seconds because in future.get(20000, TimeUnit.MILLISECONDS); you cancel the already running task. Could you check that you are not getting CancellationException.
Edit:
Since you are using Tomcat which is concurrent in execution, I would suggest, you use multithread instead of single thread if not necessary:- Executors.newFixedThreadPool(N); for having N tasks executing concurrently and increase the timout
This is for my academic purpose only. Are the tasks that we add to the executor service are really executing in parallel. Well this is my example that raised this question
Runnable Class
public Tasks implement Runnable{
int taskCount;
public Tasks(int count){
this.taskCount = count;
}
public void run(){
System.out.println("In Task :"+taskcount +" run method");
}
}
Main Class
Class MyTest {
public static void main(String args[]){
ExecutorService service = Executors.newFixedThreadPool(10);
for(inti=0;i<10;i++){
Tasks taskObj = new Tasks(i);
service.submit(taskObj);
}
service.shutdown();
}
}
As soon as i submit a taskObj to the executor, the taskObj run() is invoked.
What if i have to something like this,
Add all the taskObj to the executor , the run() must not get invoked
Execute all the task objects at one shot. All the taskobj run() must be executed in parallel/concurrently
Please let me know
Thanks...V
If I understood you right, one way to solve this would be to use thread barriers. This might sound strange, but is actually implemented quite easy. You just take a variable (lets name it traffic-light) and make every thread loop on it. If you started enough threads (starting a new thread might consume some time) you just change it to green and all your threads will start execution at the same time.
For academic purposes we used to take an atomic-integer as counter (initialized with 0) and started n threads. The task of each threads was to increase the counter and then loop on it until it reached n. Like this you'll have all threads as parallel as possible.
If you still want to go with a thread pool system, you might have to implement your own thread system, where threads can wait upon a signal prior to grabbing work.
good luck
Questions about threads are in no shortage, I know, but I can't seem to find a "full" example of a thread doing http work and then coming back to update the UI.
I basically call a few web services upon app launch. I obviously don't want to freeze the UI so I would want to use a separate thread, right? I have found a bunch of examples online on how to get a new thread to perform some task. But I haven't yet found one that shows how to actually update the UI when the thread's task is done.
How do I know when the web service thread is done? Is there a callback method? Can I access the UI from this callback method if one exists.
Edit: (Here is some code)
//The activate method is called whenever my application gains focus.
public void activate(){
DoSomething wsThread = new DoSomething();
wsThread.start();
}
public void wsCallBack()
{
myTabScreen.add(new ButtonField("Callback called"));
}
public class DoSomething extends Thread
{
public void run()
{
try
{
wsCallBack();
}
catch(Exception e)
{
}
}
}
Very simple. But it never creates the button.
Any ideas?
Thanks a lot.
You can set up a "callback" system to notify the UI when the threads complete. Have a class that extends Thread and pass to it a reference of the class that should be called at the end. If you have a list of such classes that needs to be notified create a Vector on the Thread implementation to hold them. Override the run function and after doing everything you need to do simply call a method on the UI class (iterating through the vector if needed). So your classes may look like:
public class commThread extends Thread{
MyUIClass callbackObj;
public commThread(MyUIClass myUiClass){
callbackObj = myUiClass;
}
public void run(){
//do stuff
callbackObj.callback();
}
}
and your UI class:
public MyUIClass{
public void callback(){
//refresh the UI
}
}
Of course if you have multiple threads running at the same time and calling the same UI object make sure to synchronize the callback method.
Hope this helps!