"Invalid thread access" exception in a previously opened Hop web page after opening the same Hop web page in another tab

[haoruiqian]

I open the Hop web page in two browser tabs, and when I double-click the Relational Database Connection to create a new metadata in the previously opened tab, it throws an exception about the "Invalid thread access" error. No exception when doing the same operation on the newly opened tab.

The error details message is

org.eclipse.swt.SWTException: Invalid thread access
	at org.eclipse.swt.SWT.error(SWT.java:3753)
	at org.eclipse.swt.SWT.error(SWT.java:3676)
	at org.eclipse.swt.SWT.error(SWT.java:3647)
	at org.eclipse.swt.widgets.Widget.error(Widget.java:1018)
	at org.eclipse.swt.widgets.Widget.checkWidget(Widget.java:956)
	at org.eclipse.swt.custom.CTabFolder.getItemCount(CTabFolder.java:304)
	at org.eclipse.swt.custom.CTabItem.<init>(CTabItem.java:99)
	at org.apache.hop.ui.hopgui.perspective.metadata.MetadataPerspective.addEditor(MetadataPerspective.java:379)
	at org.apache.hop.ui.core.metadata.MetadataManager.newMetadataWithEditor(MetadataManager.java:473)
	at org.apache.hop.ui.hopgui.perspective.metadata.MetadataPerspective.onNewMetadata(MetadataPerspective.java:542)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:569)
	at org.apache.hop.ui.core.gui.BaseGuiWidgets.lambda$getListener$1(BaseGuiWidgets.java:213)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:109)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:687)
	at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:594)
	at org.eclipse.swt.widgets.Display.executeNextEvent(Display.java:1216)
	at org.eclipse.swt.widgets.Display.runPendingMessages(Display.java:1197)
	at org.eclipse.swt.widgets.Display.safeReadAndDispatch(Display.java:1180)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:1172)
	at org.eclipse.rap.rwt.application.AbstractEntryPoint.createUI(AbstractEntryPoint.java:69)
	at org.eclipse.rap.rwt.internal.lifecycle.RWTLifeCycle.createUI(RWTLifeCycle.java:177)
	at org.eclipse.rap.rwt.internal.lifecycle.RWTLifeCycle$UIThreadController.run(RWTLifeCycle.java:290)
	at java.base/java.lang.Thread.run(Thread.java:840)
	at org.eclipse.rap.rwt.internal.lifecycle.UIThread.run(UIThread.java:107)

Any idea about why it happened? I also tried with two different browsers and can reproduce it too.

[haoruiqian]

Update:
I've found the root cause. MetadataPerspective instance is static and shared with all the UI sessions. Each time a new UI session is initiated, a new MetadataPerspective is constructed and set to the static instance variable, see https://github.com/apache/hop/blob/release/2.13.0/ui/src/main/java/org/apache/hop/ui/hopgui/perspective/metadata/MetadataPerspective.java#L138. Then, all other UI sessions will use this new MetadataPerspective instance and will cause the issue.

I can store the MetadataPerspective instance in a ThreadLocal variable so each UI session thread will have its own instance. Does anyone see any potential issues with this solution?

[hansva]

The issue is the same for all our perspectives, The application was never written as a multi-session application; it is a single-user desktop application. It could be a considerable amount of work to get this done. I am not against it so feel free to do a proposal or create pull requests.