我在开发一款eclipse插件，需要在package explorer、project explorer、navigator视图的右键菜单中添加一个菜单项，遇到一个问题是在package explorer和navigator视图中菜单项能显示出来并正常使用，但是在project explorer视图里却显示不出来。请问各位大神是什么原因？
action类的定义：
public class DfbdExportFileAction extends Action implements IViewActionDelegate, IActionDelegate, IWorkbenchWindowActionDelegate
plugin.xml中扩展点定义：
<!-- contexts -->
<extension
point="org.eclipse.ui.popupMenus">
<viewerContribution
id="com.dfbd.exportfiles.contextmenu.PackageExplorer"
targetID="org.eclipse.jdt.ui.PackageExplorer">
<menu
id="org.eclipse.jdt.ui.PackageExplorer.DfbdExportFile"
label="aa">
</menu>
<action
icon="icons/icon-1.png"
label="DFBD Export Files"
name="DFBD Export Files"
menubarPath="org.eclipse.jdt.ui.PackageExplorer.DfbdExportFile"
class="com.dfbd.exportfiles.contextmenu.DfbdExportFileAction"
id="com.dfbd.exportfiles.contextmenu.DfbdExportFileAction.PackageExplorer"
enablesFor="+">
</action>
</viewerContribution>
<viewerContribution
id="com.dfbd.exportfiles.contextmenu.ProjectExplorer"
targetID="org.eclipse.ui.navigator.ProjectExplorer">
<menu
id="org.eclipse.ui.navigator.ProjectExplorer.DfbdExportFile"
label="bb">
</menu>
<action
icon="icons/icon-1.png"
label="DFBD Export Files"
name="DFBD Export Files"
menubarPath="org.eclipse.ui.navigator.ProjectExplorer.DfbdExportFile"
class="com.dfbd.exportfiles.contextmenu.DfbdExportFileAction"
id="com.dfbd.exportfiles.contextmenu.DfbdExportFileAction.ProjectExplorer">
</action>
</viewerContribution>
<!-- 这个也能正确显示 -->
<viewerContribution
id="com.dfbd.exportfiles.contextmenu.ResourceNavigator"
targetID="org.eclipse.ui.views.ResourceNavigator">
<menu
id="org.eclipse.ui.views.ResourceNavigator.DfbdExportFile"
label="cc">
</menu>
<action
icon="icons/icon-1.png"
label="DFBD Export Files"
name="DFBD Export Files"
menubarPath="org.eclipse.ui.views.ResourceNavigator.DfbdExportFile"
class="com.dfbd.exportfiles.contextmenu.DfbdExportFileAction"
id="com.dfbd.exportfiles.contextmenu.DfbdExportFileAction.ResourceNavigator"
enablesFor="+">
</action>
</viewerContribution>
</extension>
