Eclipse editor won't open

Question

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).

Answer 1

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?