Creating a Model Listener
Model listeners listen for persistence method calls that signal changes to a specified model (such as update
or add
methods). Most of the methods model listeners use are called from DXP’s BasePersistenceImpl
class. You can define model listeners for out-of-the-box entities (like JournalArticle
or AssetEntry
), or for your own entities.
To add a model listener, you implement the ModelListener
interface.
Examine a Running Model Listener
Start a new Liferay instance by running
docker run -it -m 8g -p 8080:8080 liferay/portal:7.4.3.112-ga112
Sign in to Liferay at http://localhost:8080. Use the email address test@liferay.com and the password test. When prompted, change the password to learn.
Then, follow these steps to deploy an example model listener for the JournalArticle
model:
-
Download and unzip
Acme Model Listener
.curl https://resources.learn.liferay.com/dxp/latest/en/liferay-development/liferay-internals/extending-liferay/liferay-n4g6.zip -O
unzip liferay-n4g6.zip
-
Build and deploy the example.
./gradlew deploy -Ddeploy.docker.container.id=$(docker ps -lq)
NoteThis command is the same as copying the deployed jars to /opt/liferay/osgi/modules on the Docker container.
-
Confirm the deployment in the Docker container console.
STARTED com.acme.n4g6.impl_1.0.0
-
Verify that the example model listener was added by viewing the added log message. Open your browser to
https://localhost:8080
and navigate to the Site menu → Content & Data → Web Content.Click the add
button, then click Basic Web Content to add a new article. Fill out a title and some content; then click Publish. A warning message appears in the console:
2020-03-17 23:14:56.301 WARN [http-nio-8080-exec-5][N4G6ModelListener:23] Added journal article 20478.
Congratulations! You’ve successfully built and deployed a new model listener that implements ModelListener
.
As you can see, Model Listeners listen for an event on a particular model. For this listener, the event is onAfterCreate
. When the content is created, the listener “hears” the event, and the action is fired when the event happens.
Now you’ll modify the example to listen for a different event.
Identify a Model Class and Event
Model classes in Liferay DXP are generated by Service Builder. Model interfaces can be found in the -api
module of any application. For example, to find the model interface for a Message Boards message, look in the modules/apps/message-boards/message-boards-api
project in Liferay DXP’s source code.
The exception to this rule is for core models. If you want to create a Model Listener for a core class such as User
, you can find its interface in the portal-kernel
folder of Liferay DXP’s source code.
The model listener you’ll create is for a Message Boards message. You’ll trigger a message that prints in the log a report when a message is deleted. For a list of possible events, see the Javadoc for BaseModelListener.
Model Listener Behavior
Model listeners implement the ModelListener
interface for specific entities. Model listeners can have code to execute before or after those entities are created, updated, or removed. All of these methods are called from the BasePersistenceImpl
class; the code for created or updated entities is called from the update
method in BasePersistenceImpl
, and the code for removed entities is called from the remove
method in BasePersistenceImpl
.
Model listeners can also have code to execute before or after other kinds of related entities are added or removed. These methods are called from the _addTableMapping
method in TableMapperImpl
.
Now you’ll modify the project so it operates on the MBMessage
class and the onBeforeRemove
event.
Declare the Model
-
Open the
N4G6ModelListener
class in your text editor or IDE. -
Find the class declaration:
@Component(service = ModelListener.class) public class N4G6ModelListener extends BaseModelListener<JournalArticle> {
When extending
BaseModelListener
, you define the model class where your listener listens for events (in this example,JournalArticle
). -
Modify the model class to
MBMessage
:@Component(service = ModelListener.class) public class N4G6ModelListener extends BaseModelListener<MBMessage> {
When this model listener is registered, it listens to events for the model defined. The model can be an out-of-the-box entity or a custom entity. Extending the
BaseModelListener
class gives a default, empty implementation for each ofModelListener
’s methods, so your code stays clean and contains overrides for only the events you need.
Declare the Event
Next, override the implementation for the event you want:
-
Find the
onAfterCreate
method:public void onAfterCreate(JournalArticle journalArticle)
-
Change the method so it overrides
onBeforeRemove
and passes theMBMessage
as a parameter calledmodel
:public void onBeforeRemove(MBMessage model)
Implement Your Business Logic
Triggering a particular action is a typical reason to listen for a particular model event. This example keeps things simple: when a Message Boards message is deleted, we want to report the message’s subject in the logs.
-
In your new
onBeforeRemove
method, replace theif
statement with this one:if (_log.isWarnEnabled()) { _log.warn("Warning! Message " + model.getSubject() + " was just removed."); }
-
Add the new import for
MBMessage
to your imports section at the top of the file:import com.liferay.message.boards.model.MBMessage;
Remove the unused import for
JournalArticle
. -
Save your new model listener.
Deploy and Test
You can build and deploy the model listener as you did above:
./gradlew deploy -Ddeploy.docker.container.id=$(docker ps -lq)
Test your listener by adding and then deleting a message boards message:
-
Go to Product Menu → Content & Data → Message Boards.
-
Click the add
button, type a Subject and a Body, and click Publish.
-
Click Message Boards from the menu again to see your message. Click the Action
button and choose Delete. Notice that you don’t see your message in the logs yet because the message has only been recycled.
-
Click Recycle Bin from the Product Menu, and you’ll see your message.
-
Click the Action
button and select Delete. Confirm the deletion.
-
Check your log. Your message appears:
2020-04-17 21:10:31.080 WARN [http-nio-8080-exec-5][N4G6ModelListener:19] Warning! Message This is a Test Message was just removed.
Conclusion
Congratulations! You now know how to implement the ModelListener
interface, and have added a new model listener to Liferay DXP.