27.2. Hello world as an extension

27.2.1. Extending multiple extension points with a single extension

We will use the classical Hello world as the first simple example of an extension. This extension will add a new menu item in the menu which displays a popup with the text "Hello world!" when selected. Copy the XML code below and save it to a file in the /WEB-INF/extensions directory. The filename must end with .xml. If you have enabled automatic installation just wait a few seconds and the extension will be installed automatically. Otherwise you may have to do a manual scan: (ExtensionsManual scan…).

When the extension has been installed you should have a new menu item: ExtensionsHello world! which pops up a message in a Javascript window.

[Note] Note

You may have to logout and login again to see the new menu item.

<?xml version="1.0" encoding="UTF-8" ?>
<extensions xmlns="http://base.thep.lu.se/extensions.xsd">
         <name>Hello world</name>
            The very first extensions example. Adds a "Hello world"
            menu item that displays "Hello world" in a javascript
            popup when selected.
            <title>Hello world!</title>
            <tooltip>This is to test the extensions system</tooltip>
            <onClick>alert('Hello world!')</onClick>

The <extensions> tag is the root tag and is needed to set up the namespace and schema validation.

The <extension> defines a new extension. It must have an id attribute that is unique among all installed extensions and an extends attribute which id the ID of the extension point. For the id attribute we recommend using the same naming conventions as for java packages. See Java naming conventions from Sun.

The <about> tag is optional and can be used to provide meta information about the extension. We recommend that all extensions are given at least a <name>. Other supported subtags are:

[Tip] Global about tag

<about> tag can also be specified as a first-level tag (eq. as a child to <extensions>). This can be useful when an XML file defines more than one extension and you don't want to repeat the same information for every extension. You can still override the information for specific extensions by including new values in the extension's <about> tag.

The <action-factory> tag is required and so is the <factory-class> subtag. It tells the extension system which factory to use for creating actions. The FixedMenuItemFactory is a very simple factory that is shipped with BASE. This factory always creates the same menu item, no matter what. Another factory for menu items is the PermissionMenuItemFactory which can create menu items that depends on the logged in user's permissions. It is for example, possible to hide or disable the menu item if the user doesn't have enough permissions. If none of the supplied factories suits you it is possible to supply your own implementation. More about this later.

The <parameters> subtag is used to provide initialisation parameters to the factory. Different factories supports different parameters and you will have to check the javadoc documentation for each factory to get information about which parameters that are supported.

[Tip] Tip

In case the factory is poorly documented you can always assume that public methods the start with set and take a single String as an argument can be used as a parameter. The parameter tag to use should be the same as the method name, minus the set prefix and with the first letter in lowercase. For example, the method setIcon(String icon) corresponds to the <icon> parameter.

27.2.1. Extending multiple extension points with a single extension

A single extension can extend multiple extension points as long as their action classes are compatible. This is for, for example, the case when you want to add a button to more than one toolbar. To do this use the <extends> tag with multiple <ref> tags. You can skip the extends attribute in the main tag.

      <ref index="2">net.sf.basedb.clients.web.tabcontrol.edit.sample</ref>
      <ref index="2">net.sf.basedb.clients.web.tabcontrol.edit.extract</ref>

This is a feature of the XML format only. Behind the scenes two extensions will be created (one for each extension point). The extensions will share the same action and renderer factory instances. Since the id for an extension must be unique a new id will be generated by combining the original id with the parts of the id's from the extension points.