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've seen so many questions similar to mine, but no answers that quite seem to apply to my situation.
My ASP.NET MVC app with EF 6 Code first and Unity has a web service that adds something to the database, then fires off another thread that adds more stuff to the database. The reason for using the other thread is to return the original request as quickly as possible. The context class is obtained using the Unity container RegisterType().
I've got lots of repository classes all using the same context, so to make sure they get the same instance I could use the PerRequestLifetimeManager in my Unity container, and that's fine for the http request threads but that the other threads can't use the context returned by the PerRequestLifetimeManager because this is only valid on the original http request thread.
So, I can use the PerThreadLifetimeManager. This is great because now the main request thread and the other thread it kicks off get the same instance of the context returned by Unity. The trouble is that so do other requests if they get given the same thread, so this is no good either.
So how can I configure things so that the request threads get their own PerRequest Lifetime Manager created context, and other threads get a different context?
The issue is made a little more difficult by the fact that the new thread calls other classes that need to use a context instance. However, these other classes can be used from the main request thread or the new thread, so grabbing a context instance when the thread is started and then passing it around will be tricky.
Thanks in advance
No takers then...
I'm going to have a go at answering my own question, but could do with some thoughts on my approach.
So I can't use the PerRequestLifetimeManager because worker threads can't use the context that this returns, but I can't use the PerThreadLifetimeManager because the context can last the lifetime of several HTTP requests. This class attempts to provide the best of both worlds.
/// <summary>
/// For the context class the PerRequestLifetimeManager is the most suitable lifetime manager,
/// but this doesn't work when a new worker thread is started as this needs to access the context.
/// The PerThreadLifetimeManager is no good either as the context can last for more than on request.
/// This class attempts to give the best of both worlds: per request lifetime management for HTTP requests
/// and thread storage for worker threads.
/// </summary>
public class PerRequestOrThreadLifetimeManager : PerRequestLifetimeManager, IDisposable
{
private const string threadDataSlotName = "PerRequestOrThreadLifetimeManager";
public override object GetValue()
{
if (System.Web.HttpContext.Current != null)
{
return base.GetValue();
}
else
{
return getManagedObject();
}
}
public override void RemoveValue()
{
throw new NotImplementedException();
}
public override void SetValue(object newValue)
{
if (System.Web.HttpContext.Current != null)
{
base.SetValue(newValue);
}
else
{
Thread.SetData(Thread.GetNamedDataSlot(threadDataSlotName), newValue);
}
}
private object getManagedObject()
{
return Thread.GetData(Thread.GetNamedDataSlot(threadDataSlotName));
}
public void Dispose()
{
try
{
IDisposable obj = getManagedObject() as IDisposable;
if (obj != null)
{
obj.Dispose();
obj = null;
}
}
catch { }
}
}
I am having an eclipse View. Inside the view I added a Table. Now I am calling a thread from run method of the view using asyncExec.
My View class is like -
public class SampleViewAction implements IWorkbenchWindowActionDelegate{
Thread t;
int Count;
#Override
public void run(IAction arg0) {
}
}
Now I added a thread like this -
PlatformUI.getWorkbench().getDisplay().asyncExec(new Runnable() {
#Override
public void run() {
new UDPReadThread();
}
});
Where UDPReadThread is a class extends a thread where in UDPReadThread 's constructor I started the thread.
But I am getting invalid Thread exception.
How to resolve the issue.
Similar to AWT and the EventDispatchThread, SWT must process everything in the UI thread.
Your SampleViewAction is run on the UI thread already, in response to a menu or tool item selection.
It looks like your problem comes from then using an asyncExec(*) which will post the runnable to be run on the UI thread (which delays it), and starting a new thread from that asyncExec Runnable. You may as well simply start your thread, and get rid of that asyncExec.
Your UDPReadThread is not the UI thread. If you need to update UI widgets from UDPReadThread, that's the code that needs the asyncExec:
display.asyncExec(
new Runnable() {
public void run(){
label.setText(text);
}
});
Just as an aside, you should not subclass Thread unless you really are extending threads capabilities. The normal pattern when you just want to start another thread:
UDPReadRunnable udpRunnable = ....;
Thread thread = new Thread(udpRunnable);
thread.start();
You can get more information on the display thread from http://www.eclipse.org/swt/faq.php#uithread
Could you let me know the reason for this error in SWT
"org.eclipse.swt.SWTException" Invalid Thread access ?
And How to fix such errors.
It happens when you try to act upon an interface item from a thread that's not the UI thread.
To run a code on the UI thread you have to use a Runnable and ask the display thread to run it. This way:
Display.getDefault().syncExec( new Runnable() {
#Override
public void run() {
// Do your job here
}
} );
As stated by the syncExec method javadoc,
the thread which calls this method is suspended until the runnable completes.
Also, you might check the asyncExec method.
In SWT you can access GUI resources only from the display thread. For example when setting the text in a org.eclipse.swt.widgets.Text control you must already be in the display thread or call
final Text text = ...;
Display.getCurrent().syncExec(new Runnable() {
#Override
public void run() {
text.setText("test");
}
});
As my user changes the CurrentItem of a dataForm, I need to go the server to get addtional data. It's quite likely that the user could scroll through several items before finding the desired one. I would like to sleep for 500ms before going to get the data.
Is there a component already in the SDK or toolkit like a background worker that would assist in getting back to the UI thread to make my WCF async call once the 500ms sleep is done? It seems that if I don't do that, and try instead to call the WCF async method on the sleeper thread then the Completed event fires on the sleeper thread and not the UI thread, which of course is not good.
I think you might be a little off-track in your thinking. I'm not sure why you feel you need to get back to the UI thread in order to make the asych call. Generally you do as much work as you can on a BG thread and only marshal back to the UI thread when you have the results (by way of the Dispatcher).
I typically use a System.Threading.Timer for this purpose:
public class MyViewModel
{
private readonly Timer refreshTimer;
public MyViewModel()
{
this.refreshTimer = new Timer(this.DoRefresh);
}
public object CurrentItem
{
get { ... }
set
{
...
Invalidate();
}
}
// anything that should invalidate the data should wind up calling this, such as when the user selects a different item
private void Invalidate()
{
// 1 second delay
this.refreshTimer.Change(1000, Timeout.Infinite);
}
private void DoRefresh()
{
// make the async call here, with a callback of DoRefreshComplete
}
private void DoRefreshComplete()
{
// update the UI here by way of the Dispatcher
}
}