Exactly five years ago, in one of this blog’s first articles, I wrote an article explaining how to trap the standard Java Swing events in Matlab, in order to achieve much more extensive behavior customizability than Matlab’s standard uicontrols. Three years ago I wrote a related article showing how to trap any Java events as simple Matlab callbacks, even those from custom Java classes. This worked very well over the years, until now.
Over the past few weeks, several people have commented and emailed me about a problem that occurs in R2014a but not in earlier releases: Whereas Java objects in R2013b and earlier automatically exposed their public events as Matlab callbacks, they no longer do so in R2014a.
As a simple example to illustrate the point, let’s take a simple Java JButton and modify its label when the mouse moves over it, using the mouseEntered, mouseExited events. In R2013b and earlier this was pretty easy:
% Create and display a simple JButton jButton = javax.swing.JButton('click me'); javacomponent(jButton, [50,50,80,20], gcf); % Set the Matlab callbacks to the JButton's events set(jButton, 'MouseEnteredCallback', @(h,e)set(jButton,'Text','NOW !!!')) set(jButton, 'MouseExitedCallback', @(h,e)jButton.setText('click me')) |
(Note how I used two distinct variants to set the button’s text, using either the Matlabized Text property with the set function, or the standard JButton’s setText() method. We can use either of these variants: they are equivalent and largely a matter of personal taste, although the Java method is generally preferable.)
Unfortunately, in R2014a we get an error, since the
jButton object no longer exposes its events as Matlab callbacks:
>> set(jButton, 'MouseEnteredCallback', @(h,e)set(jButton,'Text','NOW !!!')) The name 'MouseEnteredCallback' is not an accessible property for an instance of class 'javax.swing.JButton'.
The solution – wrap the Java reference in a Matlab handle
Do not set Matlab callbacks on the so-called “naked” Java reference. Instead, wrap the reference with a Matlab handle:
>> jButton = handle(jButton, 'CallbackProperties'); hjButton = javahandle_withcallbacks.javax.swing.JButton % we can now set the callbacks on jButton as before |
The simple act of using the handle wrapper rather than the naked Java reference also prevents some memory leaks, as I explained here. While you’re at it, also wrap (or more precisely, auto-delegate) the Java reference with a javaObjectEDT, if it is a Swing GUI component, in order to avoid EDT timing issues. Using these wrappers form two of my six rules for safe Java programming in Matlab, that I outlined in section 1.5 of my Matlab-Java programming book. Anyone who followed that advice would not need to modify their code for R2014a. Even if you didn’t, the solution is quite painless: simply add the wrapper line for any Java reference whose events you wish to trap.
Note that the explanation above relates to all Java objects that expose events, not just for GUI ones. I used JButton merely to illustrate the point.
This change “enforces” using callbacks in a way which prevents memory leaks. But since this was essentially undocumented all along this change opens up a more interesting question. Is this change part of under the hood reorganizations in preparation for HG2? Or possibly a movement towards aligning java support with MCOS and away from UDD?
Maybe both, I don’t know…
For the record, I miss UDD’s ability to dynamically control/add class properties (schema.prop et al) in run time. It’s causing me a lot of headaches migrating some of my code to HG2.
I’ve just run the above example and it works without the update; Have MATLAB patched R2014a. my version reports 8.3.0.532 (R2014a)?
Also the EventTest example does not work/without the update:
Any pointers?
@Sean – as I explained here, you need to place the relevant class folder/JAR in Matlab’s static Java classpath.
Thank you, using the static path worked (but I had to use “javaclasspath.txt” in the prefdir() instead). The callback now sets, but the value looks wrong:
Your example showed:
We are nearly there.
PS: I do have a copy of your book which has been very helpful.
This is due to the difference between running platforms and/or Matlab releases. If you’d like my assistance in helping you on your specific platform, contact me offline for a consulting proposal.
Running Matlab 2014b, I had the same problem the Sean Lynch posted:
But trying something like
demonstrates that it is working. One step at a time.
Thank you! This article helped a lot as well as the previous article: http://undocumentedmatlab.com/blog/matlab-callbacks-for-java-events/.
I ran into a problem from using java 7 that took a while to discover so I thought I’d share it. Hopefully it helps those with the same problem in the future.
The problem is java 7 has changed the interface to the component and it is no longer a state change event but an adjustment event.
(see http://www.java2s.com/Code/Java/Swing-JFC/ListeningforScrollbarValueChangesinaJScrollPaneContainer.htm). It is corrected by using
hjScroll.AdjustmentValueChangedCallbackin this example. Be on the lookout for these types of changes in the java interface.Does anyone know how to get the window events like window closing using this method?
Is there a clean way to do it from when your loading a .jar file?
For example I have a .jar file called test.jar
I call it like jFrame=test(); after adding it to the dynamic Java classpath
but that jFrame doesnt have properties to getWindow or JavaFrame?
JAR files contain Java classes, not Matlab figures. The entire blog here discusses Matlab.
How do I get delimiter of the date format ?
example :
system foramt – 1/23/2017
Get Output from your script – mm dd,yyyy
I need output like – dd/mm/yyyy
Thanks in Advance
You can either use the Java SimpleDateFormat class to convert the date format into your desired output format (read the Java docs online to see how to do this), or use Matlab’s datenum and datestr functions with the appropriate format specifiers (read the corresponding Matlab docs to see how to do this).
This looks elegant. Thank you. One issue I am not able to sort out – on maximizing the figure window the location of “javax.swing.JSlider” object does not scale properly?Is there a way to specify the ‘Units’,’normalize’ property like we do in uicontrol() to maintain the scaling and location.
The javacomponent function returns 2 objects: a Java reference and a Matlab container. Simply set the value of the Units property of the Matlab container to ‘normalized’: