Eclipse工具条中添加下拉按钮

<extension
point="org.eclipse.ui.commands">
<command
defaultHandler="a.handlers.Handler"
id="a.command"
name="Hello">
</command>
<command
defaultHandler="a.handlers.Handler"
id="referenceToMenuId"
name="referenceToMenu">
</command>
</extension>
<extension
point="org.eclipse.ui.menus">
<menuContribution
locationURI="toolbar:org.eclipse.ui.main.toolbar">
<toolbar
id="a.toolbar6">
<command
commandId="referenceToMenuId"
label="pulldown menu"
style="pulldown">
</command>
</toolbar>
</menuContribution>
<menuContribution
allPopups="false"
locationURI="menu:referenceToMenuId">
<command
commandId="a.command"
id="a.toobar.cmd3"
label="hello1"
style="push">
</command>
<command
commandId="a.command"
label="hello 2"
style="push">
</command>
<menu
label="menu1">
<menu
label="label">
<command
commandId="a.command15"
style="push">
</command>
</menu>
</menu>
</menuContribution>
</extension>