Adding Logic to MVC Commands

You can completely override MVC commands, or any OSGi service for that matter, but adding logic to the commands is the better option. Discarding necessary logic is bad. Conversely any logic you copy from the original might not work in new versions of the portlet. Adding custom logic while continuing to invoke the original logic decouples the custom class from the original implementation. Keeping the new logic separate form the original logic keeps the code clean, maintainable, and easy to understand.

Here are the steps for adding logic to MVC commands:

  1. Implement the interface
  2. Publish as a component
  3. Refer to the original implementation
  4. Add the logic, and call the original

Step 1: Implement the interface

Implement the respective MVC Command interface either directly or by extending an existing base class that implements it. Extending a base class for the interface relieves you from implementing logic that should typically be a part of most command implementations. For example, to add logic to the Blogs portlet’s EditEntryMVCActionCommand, you would extend base class BaseMVCActionCommand.

public class CustomBlogsMVCActionCommand extends BaseMVCActionCommand {...}

Check the MVC command interfaces for existing base classes:

Next make your class a service component.

Step 2: Publish as a component

The Declarative Services @Component annotation facilitates customizing MVC commands. All the customization options require publishing your MVC command class as a component. For example, this @Component annotation declares an MVCActionCommand service.

    immediate = true,
    property = { 
        "" + BlogsPortletKeys.BLOGS_ADMIN, 
    service = MVCActionCommand.class
public class CustomBlogsMVCActionCommand extends BaseMVCActionCommand {

It publishes CustomBlogsMVCActionCommand as a service component for the MVCActionCommand class. Upon resolving, it’s activated immediately because immediate = true. The component is invoked in the Blogs Admin portlet by the command URL /blogs/edit_entry. Its service ranking of 100 prioritizes it ahead of the original service component, whose ranking is 0.

Here’s what you need to specify in an @Component annotation for your custom MVC command:

  • for each portlet you want the customization to affect. JSPs in these portlets can invoke the MVC command via applicable command URL tags. You can specify the same portlets as the original MVC command or a subset of those portlets.

  • this property declares the command URL that maps to this custom MVC command component.

  • service.ranking:Integer: set this property to a higher integer than the original service implementation’s ranking. The ranking tells the OSGi runtime which service to use, in cases where multiple components register the same service, with the same properties. The higher the integer you specify here, the more weight your component carries. Liferay’s service implementations typically have a 0 ranking.

  • service: this attribute specifies the service (interface) to override.

  • immediate: set this attribute to true to activate your component immediately upon resolution.

You can refer back to this list as you add @Component annotations to your custom MVC commands.

Next reference the original implementation.

Step 3: Refer to the original implementation

Use a field annotated with @Reference to fetch a reference to the original MVC command component. If there are no additional customizations on the original component, this reference will be for the original MVC command type. For example, this field references the original MVC command component EditEntryMVCActionCommand.

    target = "(")
protected MVCActionCommand mvcActionCommand;

Here’s how to add the reference:

  1. Declare the field as the MVC command interface type that it is. For example, the mvcActionCommand field is type MVCActionCommand.

  2. Add the @Reference annotation.

  3. In the annotation, define a target attribute that filters on a equal to the default service implementation class’s fully qualified name.

When your custom component resolves, the OSGi runtime assigns the targeted service to your field. It’s time to add your custom logic.

Step 4: Add the logic

Adding the logic involves overriding the primary method of the base class you’re extending or the interface you’re implementing. In your method override, add your new logic AND then invoke the original implementation. For example, the following method overrides BaseMVCActionCommand’s method doProcessAction.

protected void doProcessAction(
	ActionRequest actionRequest, ActionResponse actionResponse)
throws Exception {
    // Add custom logic here 
    // Call the original service implementation 
    mvcActionCommand.processAction(actionRequest, actionResponse);

The method above defines custom logic and then invokes the original service it referenced in the previous step.

If you use this approach, your extension will continue to work with new versions of the original portlet, because no coupling exists between the original portlet logic and your customization. The command implementation class can change. Make sure to keep your reference updated to the name of the current implementation class.

Congratulations on adding logic to your existing MVC command.

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