Thursday, September 8, 2016

Best Practice: Always check for Control disposed in Display.asyncExec()

Display.getDefault().asyncExec(new Runnable()
public void run()
//Always check for whether the control is disposed.
if (!progressComposite.isDisposed())
//perform UI operation.

Reason being, when we say asyncExec thread - it will execute on the next reasonable opportunity. But by that time the control would have been disposed.

Wednesday, September 7, 2016

HttpURLConnection setConnectTimeout issues

HttpURLConnection connection = (HttpURLConnection) url.openConnection();
// Give it a 4 seconds delay before deciding that it's a dead connection
connection.setRequestMethod("HEAD"); // Don't ask for content

Problem here is, application can hung for more than 10 sec even after configuring the timeout for connection.

How that can happen?

connection.setConnectTimeout() is only for configuring timeout to accept the connection, but after accepting the request server might take a long time to respond to you due to various issues at the server end. 

As per the doc - “timeout for opening a communications link to the resource referenced by this URLConnection”.

This kind of things will lead to application hung if you’re waiting for the response.

To resolve this - we should also set the connection.setReadTimeout(5000). 

As per doc - “timeout when reading from Input stream when a connection is established to a resource.”

Thursday, September 1, 2016

How to set default language for eclipse or for your RCP product

Before you launch an eclipse, configure  -Duser.language=en property in eclipse.ini file.

In the context of mac os.

  1. Go to /Applications/Eclipse/
  2. Open eclispe.ini file
  3. Append -Duser.language=en to the end of the file
  4. Restart eclipse

In windows, you can find eclipse.ini file in the eclipse root directory.

Thursday, August 25, 2016

SWT - Single UI threaded model

The backbone of any UI toolkits are threads. The most basic aspect of UI toolkits is to provide a nice event driven architecture, as events are the most integral part of any UI.
The underlying operating system maintains a queue of application events and if that event is long running and is blocked it will freeze your UI.
In SWT when a user presses a mouse a mouse pressed event is generated. Determining which window is supposed to receive the event and placing it in an event queue is done by the underlying OS.
SWT has a single UI thread which creates the display and from which all other widgets are created. When writing a SWT app the most common piece of code existing in all programs is
while (!shell.isDisposed())
if (!display.readAndDispatch())
display in SWT is the class which interacts with the underlying operating system. In the above code readAndDispatch() is the call which reads events from the event queue and dispatches them to the correct place.
Since there is only one UI thread, if we try and process events in the same thread the UI will freeze. This is where threads come to our rescue. But again when working with UI’s the most tricky and dangerous thing to do would be access UI components from non UI threads. If you spawn a new thread and try to update UI components from that non UI thread you will be in for a bummer. When your UI toolkit is SWT, it will nicely point out to you your chivalry and throw an SWTException.

Friday, August 19, 2016

SWT Threading issues

Threading issues
When working with a widget toolkit, it is important to understand the underlying thread model that is used for reading and dispatching platform GUI events. The implementation of the UI thread affects the rules that applications must follow when using Java threads in their code.

Native event dispatching

Underneath any GUI application, regardless of its language or UI toolkit, the OS platform detects GUI events and places them in application event queues. Although the mechanics are slightly different on different OS platforms, the basics are similar. As the user clicks the mouse, types characters, or surfaces windows, the OS generates application GUI events, such as mouse clicks, keystrokes, or window paint events. It determines which window and application should receive each event and places it in the application's event queue.
The underlying structure for any windowed GUI application is an event loop. Applications initialize and then start a loop which simply reads the GUI events from the queue and reacts accordingly. Any work that is done while handling one of these events must happen quickly in order to keep the GUI system responsive to the user.
Long operations triggered by UI events should be performed in a separate thread in order to allow the event loop thread to return quickly and fetch the next event from the application's queue. However, access to the widgets and platform API from other threads must be controlled with explicit locking and serialization. An application that fails to follow the rules can cause an OS call to fail, or worse, lock up the entire GUI system.

SWT UI thread

SWT follows the threading model supported directly by the platforms. The application program runs the event loop in its main thread and dispatches events directly from this thread. The UI thread is the thread in which the Display was created. All other widgets must be created in the UI thread.
Since all event code is triggered from the application's UI thread, application code that handles events can freely access the widgets and make graphics calls without any special techniques. However, the application is responsible for forking computational threads when performing long operations in response to an event.
Note: SWT will trigger an SWTException for any calls made from a non-UI thread that must be made from the UI thread.
The main thread, including the event loop, for an SWT application has the following structure:
   public static void main (String [] args) {
      Display display = new Display ();
      Shell shell = new Shell (display); ();
      // start the event loop. We stop when the user has done
      // something to dispose our window.
      while (!shell.isDisposed ()) {
         if (!display.readAndDispatch ())
            display.sleep ();
      display.dispose ();
Once the widgets are created and the shell is opened, the application reads and dispatches events from the OS queue until the shell window is disposed. If there are no events available for us in the queue, we tell the display to sleep to give other applications a chance to run.
SWT provides special access methods for calling widget and graphics code from a background thread.

Executing code from a non-UI thread

Applications that wish to call UI code from a non-UI thread must provide a Runnable that calls the UI code. The methodssyncExec(Runnable) and asyncExec(Runnable) in the Display class are used to execute these runnables in the UI thread during the event loop.
  • syncExec(Runnable) should be used when the application code in the non-UI thread depends on the return value from the UI code or otherwise needs to ensure that the runnable is run to completion before returning to the thread. SWT will block the calling thread until the runnable has been run from the application's UI thread. For example, a background thread that is computing something based on a window's current size would want to synchronously run the code to get the window's size and then continue with its computations.
  • asyncExec(Runnable) should be used when the application needs to perform some UI operations, but is not dependent upon the operations being completed before continuing. For example, a background thread that updates a progress indicator or redraws a window could request the update asynchronously and continue with its processing. In this case, there is no guaranteed relationship between the timing of the background thread and the execution of the runnable.
The following code snippet demonstrates the pattern for using these methods:
   // do time-intensive computations
   // now update the UI. We don't depend on the result,
   // so use async.
   display.asyncExec (new Runnable () {
      public void run () {
         if (!myWindow.isDisposed())
            myWindow.redraw ();
   // now do more computations
It is good practice to check if your widget is disposed from within the runnable when using asyncExec. Since other things can happen in the UI thread between the call to asyncExec and the execution of your runnable, you can never be sure what state your widgets are in by the time your runnable executes.

The workbench and threads

The threading rules are very clear when you are implementing an SWT application from the ground up since you control the creation of the event loop and the decision to fork computational threads in your application.
Things get a bit more complicated when you are contributing plug-in code to the workbench. The following rules can be considered "rules of engagement" when using platform UI classes, although from release to release there may be exceptions to these rules:
  • In general, any workbench UI extensions you add to the platform will be executing in the workbench's UI thread, unless they are specifically related to threads or background jobs (such as background job progress indication).
  • If you receive an event from the workbench, it is not guaranteed that it is executing in the UI thread of the workbench. Consult the javadoc for the particular class that defines the listener or event. If there is no specific documentation discussing threading, and the class is clearly a UI-related class, you may expect that the event arrives in the UI thread of the workbench.
  • Likewise, a platform UI library should not be considered thread-safe unless it is specifically documented as such. Note that most platform UI classes dispatch listeners from the calling thread that triggered the event. Workbench and JFace API calls do not check that the caller is executing in the UI thread.This means that your plug-in may introduce a problem if you call a method that triggers an event from a non-UI thread. SWT triggers an SWTException for all API calls made from a non-UI thread. In general, avoid calling platform UI code from another thread unless the javadoc specifically allows it.
  • If your plug-in forks a computational thread or uses a workbench Job, it must use the Display asyncExec(Runnable) orsyncExec(Runnable) methods when calling any API for the workbench, JFace, or SWT, unless the API specifically allows call-in from a background thread.
  • If your plug-in uses the JFace IRunnableContext interface to invoke a progress monitor and run an operation, it supplies an argument to specify whether a computational thread is forked for running the operation.

Thursday, July 21, 2016

Eclipse in full screen mode - programmatically

Eclipse 4.4/4.5

 active contribution item identifier:

The active contribution location URI:

Thursday, July 14, 2016

Recovering staged lost files during git reset --hard

$ git fsck --cache --unreachable $(git for-each-ref --format="%(objectname)")   $(egrep commit all | cut -d ' ' -f 3)
Checking object directories: 100% (256/256), done.
Checking objects: 100% (542/542), done.
unreachable blob 6900c6418b5df26f6471b7d4853822d99fcf2534
unreachable blob 945019391229d1304b47c7d1dc112ff45bbca360
unreachable blob 0ed3acde384de116f4b194bfd226055074a9f52d
unreachable blob c0234fc886423ff40a96b4c3e7ba67a4deb944c0
unreachable blob c9754aed2e981cdf678159ab41f7bfea89c93cbd
unreachable blob 8168f4a5a91997641211ce0d30b01e8bf8920792
unreachable blob 8c9cc9beddbc89a0ef46a1631265f79551a78e8a

unreachable blob 758e9c59bc04d38bd676c3c98e6d092b698defa4

Your file should be one of those unreachable blob from the above list. To verify that, do git show for every blob and see.

$ git show 6900c6418b5df26f6471b7d4853822d99fcf2534