Background Tasks Jobs API in Java

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Background Tasks Jobs API
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Long-running operations should be executed in the background so that the UI stays responsive One solution is to fork a lower-priority thread to perform the operation rather than performing the operation in the UI thread But, how do you keep the user informed as to the progress of the background operation Eclipse provides a Jobs API for creating, managing, and displaying background operations In the Favorites product, you want to periodically check for the availability of a newer version Rather than interrupt the user, you want to have this check performed in the background and provide the user with nonintrusive progress information as the operation proceeds To accomplish this, create NewVersionCheckJob The goal is to exercise the Jobs API, not Internet access, so NewVersionCheckJob only simulates a version check
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218 Background Tasks Jobs API
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package comqualityeclipsefavoritesjobs; import public class NewVersionCheckJob extends Job { private NewVersionCheckJob(String name) { super(name); } protected IStatus run(IProgressMonitor monitor) { // Simulate check for new version monitorbeginTask("check for new version", 20); for (int i = 20; i > 0; --i) { monitorsubTask("seconds left = " + i); try { Threadsleep(1000); } catch (InterruptedException e) { // Ignored } monitorworked(1); } monitordone(); // Reschedule job to execute in 2 minutes schedule(120000); return StatusOK_STATUS; } }
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The user will control this operation via a new checkbox on the Favorites preference page, so first add a new constant to the PreferenceConstants (see Section 1224, Favorites preference page, on page 495)
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public static final String FAVORITES_NEW_VERSION_CHECK_PREF = "favoritesnewVersionCheck";
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Next, expose this new preference on the Favorites preference page by adding a new checkbox This entails a new field plus additional code at the end of the createFieldEditors() method (see Section 1224, Favorites preference page, on page 495)
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private BooleanFieldEditor newVersionCheckEditor; public void createFieldEditors() { original code here newVersionCheckEditor = new BooleanFieldEditor( PreferenceConstantsFAVORITES_NEW_VERSION_CHECK_PREF, "Periodically check for new version" + " of Favorites product (simulated)", getFieldEditorParent()); addField(newVersionCheckEditor); }
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Now you want to tie the new version check job to this preference by adding a preference listener to NewVersionCheckJob The preference listener either schedules or cancels the job depending on the preference setting as specified by the user
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private static final String JOB_NAME = "Favorites check for new version"; private static NewVersionCheckJob job = null; public boolean shouldSchedule() { return equals(job); } private static final Preferences preferences = FavoritesActivatorgetDefault()getPluginPreferences(); private static final PreferencesIPropertyChangeListener propertyListener = new PreferencesIPropertyChangeListener() { public void propertyChange(PropertyChangeEvent event) { update(); } }; private static void update() { if (preferencesgetBoolean( PreferenceConstantsFAVORITES_NEW_VERSION_CHECK_PREF)) { if (job == null) { job = new NewVersionCheckJob(JOB_NAME); // setUser true to show progress dialog // or false for system job // jobsetUser(true); jobschedule(); } } else { if (job != null) { jobcancel(); job = null; } } }
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Next, create additional methods that are called by FavoritesActivator when the plug-in starts up and shuts down
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public static void startup() { preferencesaddPropertyChangeListener(propertyListener); update(); } public static void shutdown() { preferencesremovePropertyChangeListener(propertyListener); }
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219 Plug-in ClassLoaders
When all this is in place, selecting the Periodically check for new version of Favorites product (simulated) checkbox on the Favorites preference page will cause the new version check operation to be performed periodically Feedback to the user is automatically provided as part of the Jobs API through the Progress view (see Figure 21 5) The % done shown in the Progress view is based on the total work specified in the beginTask() method and the number of units worked is based on calls to the worked() method The seconds left = n is specified by calling the subTask() method (see Section 941, IProgressMonitor, on page 415)
Figure 21 5
Progress view for a background operation
Typically, jobs are executed in the background, but the IProgressService provides the showInDialog() method and the UIJob class for executing them in the foreground (see Section 944, IProgressService, on page 419) In addition, if setUser(true) is called after the job is instantiated but before it is scheduled, and if the user has the General > Always run in background preference unchecked, then it will execute in the foreground
job = new NewVersionCheckJob(JOB_NAME); jobsetUser(true); jobschedule();