It is necessary to know how many resources are allocated in order to know if the leak has been plugged or not. Fortunately, SWT provides a mechanism to do this via the Display and the DeviceData classes. Normally, this is done by a separate plug-in, but in this example ClockView will be modified to show this behavior.
- At the start of the
ClockViewclass'createPartControl()method, add a call to obtain the number of allocatedObjects, viaDeviceDataof theDisplayclass:public void createPartControl(Composite parent) { Object[] oo=parent.getDisplay().getDeviceData().objects; - Iterate through the allocated
objectscounting how many are instances ofColor:int c = 0; for (int i = 0; i < oo.length; i++) if (oo[i] instanceof Color) c++; - Print the count to the standard error stream:
System.err.println("There are " + c + " Color instances"); - Now run the code in debug mode and show the Clock View. The following will be displayed in the host Eclipse Console View:
There are 0 Color instances There are 0 Color instances There are 0 Color instances
Note
For efficiency, SWT doesn't log all the allocated resources all the time. Instead, it's an option which is enabled at startup through an
optionsfile, which is a text properties file withname=valuepairs. This can be passed to an Eclipse instance at launch via the-debugflag.Fortunately, it is easy to set within Eclipse from the launch configuration's tracing tab.
- Close the target Eclipse application, if it is running.
- Go to the launch configuration via the Debug | Debug Configurations menu.
- Select the Eclipse Application (if it's not selected already) and go to the Tracing tab. Enable the tracing option, and select the
org.eclipse.uiplugin. Select both thedebug(at the top) and thetrace/graphicsoptions:
- Now launch the application by hitting Debug, and open and close the Clock View a few times:
There are 87 Color instances There are 92 Color instances There are 95 Color instances There are 98 Color instances
Clearly, something is leaking three Color instances each time the Clock View is opened. Not surprisingly, three instances of the Color object are allocated in the three instances of ClockWidget. This suggests that there is a resource leak in ClockView or ClockWidget.
When SWT is running in trace mode, it will keep a list of previously allocated resources in a global list, which is accessible through the DeviceData object. When the resource is disposed, it will be removed from the allocated list. This allows the monitoring of the state of resources at play in the Eclipse workbench and discover leaks, typically through repeated actions and noting an increase each time in the resource count.
Other object types are also stored in this list (for example, Fonts and Images) so it's important to filter by type when looking for a resource set. It's also important to note that Eclipse has its own runtime resources, which are used and so when tracing these are included in the list as well.
By learning how to enable tracing and how to programmatically detect what objects are allocated, it will be possible to discover such leaks, or verify whether they have been fixed afterwards.