My Eclipse cannot show the editor window. Error: Could not open the editor: The

Question

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Editorial Team

Asked: May 23, 20262026-05-23T07:10:10+00:00 2026-05-23T07:10:10+00:00

My Eclipse cannot show the editor window. Error: Could not open the editor: The

0

My Eclipse cannot show the editor window.

Error:

Could not open the editor: The editor class could not be instantiated.
This usually indicates a missing no-arg constructor or that the
editor’s class name was mistyped in plugin.xml.

Notes:

I’m using different Eclipse (and not plug-ins) for C++ and Python – and they work flawlessly, it’s just the java eclipse that is causing troubles.
I’ve already tried re-installing eclipse, and even reinstalling java. any idea?
It has no troubles displaying the project file, it’s “just” the *.java and *.class files that cannot be opened.

Stack trace

org.eclipse.core.runtime.internal.adaptor.EclipseLazyStarter$TerminatingClassNotFoundException: An error occurred while automatically activating bundle org.eclipse.jdt.ui (150).
    at org.eclipse.core.runtime.internal.adaptor.EclipseLazyStarter.postFindLocalClass(EclipseLazyStarter.java:121)
    at org.eclipse.osgi.baseadaptor.loader.ClasspathManager.findLocalClass(ClasspathManager.java:453)
    at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.findLocalClass(DefaultClassLoader.java:216)
    at org.eclipse.osgi.internal.loader.BundleLoader.findLocalClass(BundleLoader.java:393)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:469)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:422)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:410)
    at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.loadClass(DefaultClassLoader.java:107)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at org.eclipse.osgi.internal.loader.BundleLoader.loadClass(BundleLoader.java:338)
    at org.eclipse.osgi.framework.internal.core.BundleHost.loadClass(BundleHost.java:232)
    at org.eclipse.osgi.framework.internal.core.AbstractBundle.loadClass(AbstractBundle.java:1197)
    at org.eclipse.core.internal.registry.osgi.RegistryStrategyOSGI.createExecutableExtension(RegistryStrategyOSGI.java:174)
    at org.eclipse.core.internal.registry.ExtensionRegistry.createExecutableExtension(ExtensionRegistry.java:904)
    at org.eclipse.core.internal.registry.ConfigurationElement.createExecutableExtension(ConfigurationElement.java:243)
    at org.eclipse.core.internal.registry.ConfigurationElementHandle.createExecutableExtension(ConfigurationElementHandle.java:55)
    at org.eclipse.ui.internal.WorkbenchPlugin$1.run(WorkbenchPlugin.java:267)
    at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
    at org.eclipse.ui.internal.WorkbenchPlugin.createExtension(WorkbenchPlugin.java:263)
    at org.eclipse.ui.internal.registry.EditorDescriptor.createEditor(EditorDescriptor.java:235)
    at org.eclipse.ui.internal.EditorManager.createPart(EditorManager.java:845)
    at org.eclipse.ui.internal.EditorReference.createPartHelper(EditorReference.java:609)
    at org.eclipse.ui.internal.EditorReference.createPart(EditorReference.java:465)
    at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:595)
    at org.eclipse.ui.internal.EditorAreaHelper.setVisibleEditor(EditorAreaHelper.java:271)
    at org.eclipse.ui.internal.EditorManager.setVisibleEditor(EditorManager.java:1429)
    at org.eclipse.ui.internal.EditorManager$5.runWithException(EditorManager.java:942)
    at org.eclipse.ui.internal.StartupThreading$StartupRunnable.run(StartupThreading.java:31)
    at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
    at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:134)
    at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4041)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3660)
    at org.eclipse.ui.application.WorkbenchAdvisor.openWindows(WorkbenchAdvisor.java:803)
    at org.eclipse.ui.internal.Workbench$31.runWithException(Workbench.java:1567)
    at org.eclipse.ui.internal.StartupThreading$StartupRunnable.run(StartupThreading.java:31)
    at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
    at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:134)
    at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4041)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3660)
    at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2548)
    at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2438)
    at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:671)
    at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
    at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:664)
    at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
    at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:115)
    at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
    at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
    at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
    at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:369)
    at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:620)
    at org.eclipse.equinox.launcher.Main.basicRun(Main.java:575)
    at org.eclipse.equinox.launcher.Main.run(Main.java:1408)
Caused by: org.osgi.framework.BundleException: The activator org.eclipse.jdt.internal.ui.JavaPlugin for bundle org.eclipse.jdt.ui is invalid
    at org.eclipse.osgi.framework.internal.core.AbstractBundle.loadBundleActivator(AbstractBundle.java:156)
    at org.eclipse.osgi.framework.internal.core.BundleContextImpl.start(BundleContextImpl.java:751)
    at org.eclipse.osgi.framework.internal.core.BundleHost.startWorker(BundleHost.java:370)
    at org.eclipse.osgi.framework.internal.core.AbstractBundle.start(AbstractBundle.java:284)
    at org.eclipse.osgi.framework.util.SecureAction.start(SecureAction.java:417)
    at org.eclipse.osgi.internal.loader.BundleLoader.setLazyTrigger(BundleLoader.java:265)
    at org.eclipse.core.runtime.internal.adaptor.EclipseLazyStarter.postFindLocalClass(EclipseLazyStarter.java:106)
    ... 57 more
Caused by: java.lang.NoClassDefFoundError: org/eclipse/jdt/core/IBufferFactory
    at java.lang.Class.getDeclaredConstructors0(Native Method)
    at java.lang.Class.privateGetDeclaredConstructors(Unknown Source)
    at java.lang.Class.getConstructor0(Unknown Source)
    at java.lang.Class.newInstance0(Unknown Source)
    at java.lang.Class.newInstance(Unknown Source)
    at org.eclipse.osgi.framework.internal.core.AbstractBundle.loadBundleActivator(AbstractBundle.java:151)
    ... 63 more
Caused by: org.eclipse.core.runtime.internal.adaptor.EclipseLazyStarter$TerminatingClassNotFoundException: An error occurred while automatically activating bundle org.eclipse.jdt.core (140).
    at org.eclipse.core.runtime.internal.adaptor.EclipseLazyStarter.postFindLocalClass(EclipseLazyStarter.java:121)
    at org.eclipse.osgi.baseadaptor.loader.ClasspathManager.findLocalClass(ClasspathManager.java:453)
    at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.findLocalClass(DefaultClassLoader.java:216)
    at org.eclipse.osgi.internal.loader.BundleLoader.findLocalClass(BundleLoader.java:393)
    at org.eclipse.osgi.internal.loader.SingleSourcePackage.loadClass(SingleSourcePackage.java:33)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:466)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:422)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:410)
    at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.loadClass(DefaultClassLoader.java:107)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 69 more
Caused by: org.osgi.framework.BundleException: Exception in org.eclipse.jdt.core.JavaCore.start() of bundle org.eclipse.jdt.core.
    at org.eclipse.osgi.framework.internal.core.BundleContextImpl.startActivator(BundleContextImpl.java:806)
    at org.eclipse.osgi.framework.internal.core.BundleContextImpl.start(BundleContextImpl.java:755)
    at org.eclipse.osgi.framework.internal.core.BundleHost.startWorker(BundleHost.java:370)
    at org.eclipse.osgi.framework.internal.core.AbstractBundle.start(AbstractBundle.java:284)
    at org.eclipse.osgi.framework.util.SecureAction.start(SecureAction.java:417)
    at org.eclipse.osgi.internal.loader.BundleLoader.setLazyTrigger(BundleLoader.java:265)
    at org.eclipse.core.runtime.internal.adaptor.EclipseLazyStarter.postFindLocalClass(EclipseLazyStarter.java:106)
    ... 78 more
Caused by: java.lang.NullPointerException
    at org.eclipse.core.runtime.Path.<init>(Path.java:183)
    at org.eclipse.core.internal.resources.WorkspaceRoot.getProject(WorkspaceRoot.java:181)
    at org.eclipse.jdt.internal.core.ExternalFoldersManager.isExternalFolderPath(ExternalFoldersManager.java:90)
    at org.eclipse.jdt.internal.core.JavaProject.addToResult(JavaProject.java:2738)
    at org.eclipse.jdt.internal.core.JavaProject.resolveClasspath(JavaProject.java:2688)
    at org.eclipse.jdt.internal.core.JavaProject.resolveClasspath(JavaProject.java:2783)
    at org.eclipse.jdt.internal.core.DeltaProcessingState.getRootInfos(DeltaProcessingState.java:301)
    at org.eclipse.jdt.internal.core.DeltaProcessingState.initializeRootsWithPreviousSession(DeltaProcessingState.java:271)
    at org.eclipse.jdt.internal.core.JavaModelManager.startup(JavaModelManager.java:4712)
    at org.eclipse.jdt.core.JavaCore.start(JavaCore.java:5048)
    at org.eclipse.osgi.framework.internal.core.BundleContextImpl$1.run(BundleContextImpl.java:783)
    at java.security.AccessController.doPrivileged(Native Method)
    at org.eclipse.osgi.framework.internal.core.BundleContextImpl.startActivator(BundleContextImpl.java:774)
    ... 84 more

EDIT:

eclipse version: 3.6.2

eclipse.ini file:

-startup
plugins/org.eclipse.equinox.launcher_1.1.1.R36x_v20101122_1400.jar
--launcher.library
plugins/org.eclipse.equinox.launcher.win32.win32.x86_1.1.2.R36x_v20101222
-product
org.eclipse.epp.package.java.product
--launcher.defaultAction
openFile
--launcher.XXMaxPermSize
256M
-showsplash
org.eclipse.platform
--launcher.XXMaxPermSize
256m
--launcher.defaultAction
openFile
-vmargs
-Dosgi.requiredJavaVersion=1.5
-Xms40m
-Xmx384m

EDIT 2:

I changed the workspace and it fixed the problem (for the new workspace at least) but still – no idea why it happened. (though I am still curious, so I leave this question open).

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1 Answer

Editorial Team · Answer 1 · 2026-05-23T07:10:11+00:00

This is often a problem with temporary files saved by eclipse plugins.
Check YOUR_WORKSPACE/.metadata/.plugins/org.eclipse.jdt.core/.
There are a lot of index files. Sometimes it helps to delete them.
Also, start your eclipse with -clean.

But I think your problem is slightly different.
Did you have a reference to a library(jar-file) somewhere in your filesystem?
If you deleted it or moved it, eclipse can’t build your projects.

Did you refresh all your projects?

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